GaN Multiple Quantum Wells Research Articles

PROPOSAL OF NOVEL STRUCTURE LIGHT EMITTING DEVICES CONSISTING OF InN/GaN MQWs WITH ULTRATHIN InN WELLS IN GaN MATRIX

Novel structure light emitting diodes (LEDs) made of InN / GaN multiple quantum wells (MQWs) are proposed and demonstrated. The MQWs consisted of very fine and narrow 1 monolayer (ML)-thick InN wells embedded in GaN matrix, which were successfully fabricated by radio-frequency molecular beam epitaxy. The thickness of InN wells can be fractional ML and/or two MLs depending on the growth conditions, resulting in different wavelength light emissions from deep violet to blue. Epitaxy processes for the MQWs fabrication are very unique on the basis of the self-ordering and coherent growth mode for atomically flat ~1 ML InN well deposition on GaN template. It is shown that the epitaxy temperature for 1 ML InN wells can be much higher than the highest epitaxy temperature of thick InN layer due to the effects of GaN matrix. Bright electroluminescence (EL) emission is observed at 418 nm at room temperature in LEDs fabricated by the MQWs. Further it is confirmed that the quantum confined Stark effect (QCSE) in InN wells is remarkably reduced due to the effects with using ultimately thin InN wells as active layers, resulting an extremely small blue shift in the EL peak wavelengths for two orders different injection current levels.

Enhanced luminescence efficiency due to carrier localization in InGaN∕GaN heterostructures grown on nanoporous GaN templates

Low defect density GaN was achieved through dislocation annihilation by regrowing GaN on strain relaxed nanoporous GaN template formed by UV-enhanced electrochemical etching. The InGaN∕GaN single and multiple quantum wells grown on this nanoporous GaN template show enhanced indium incorporation due to strain relaxation. The step edges of regrown GaN on these nanoporous GaN act as effective nucleation sites for impinging indium atoms during growth. Evidence shows fluctuation in the quantum well width caused by indium segregation leading to carrier localization. A higher luminescence efficiency of InGaN∕GaN quantum wells is achieved through a combination of excitons localization, higher energy barrier for nonradiative recombination of carriers with dislocations and the reduction in defect density of the materials grown on the nanoporous GaN template.

High quality ultraviolet AlGaN∕GaN multiple quantum wells with atomic layer deposition grown AlGaN barriers

Low dislocation density ultraviolet (UV) AlGaN∕GaN multiple quantum well (MQW) structure was grown using atomic layer deposition (ALD) technique. The AlGaN∕GaN MQW grown on the sapphire substrate consisted of three GaN QWs and four AlGaN barriers formed by ALD grown AlN∕GaN superlattices. The as-grown sample showed smooth surface morphology with a root-mean-square roughness value of only 0.35nm, and no surface cracks were found. The dislocation density was estimated to be as low as 3.3×107cm−2. X-ray and transmission electron microscope data showed the MQW had sharp interfaces with good periodicity. The sample had an UV photoluminescence emission at 334nm (3.71eV) with a very narrow linewidth of 47meV at 13K. The cathodoluminescence image revealed a fairly uniform luminescence pattern at room temperature. The AlGaN∕GaN MQW grown by ALD technique should be useful for providing high crystalline quality for fabrication of various optical devices.

Strain relaxation induced microphotoluminescence characteristics of a single InGaN-based nanopillar fabricated by focused ion beam milling

A freestanding nanopillar with a diameter of 300nm and a height of 2μm is demonstrated by focused ion beam milling. The measured microphotoluminescence (μ-PL) from the embedded InGaN∕GaN multiple quantum wells shows a blueshift of 68meV in energy with a broadened full width at half maximum, ∼200meV. Calculations based on the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the observed energy shift and spectrum broadening. Moreover, the power-dependent μ-PL measurement suggests that the strain-relaxed emission region exhibits a higher radiative recombination rate than that of the strained region, indicating potential for realizing high-efficiency nanodevices in the UV/blue wavelength range.

Optical detection of deoxyribonucleic acid hybridization with InGaN∕GaN multiple quantum wells

Based on the high surface sensitivity of piezoelectric polarization of strained nitride semiconductors, surface functionalized nitride light emitting devices (LEDs) provide an excellent opportunity for the development of biological sensors. To demonstrate our working principle, a probe chip based on In0.22Ga0.78N∕GaN multiple quantum wells has been constructed and exposed to target DNA solutions, matched and/or mismatched, with different concentrations. The pronounced changes of photoluminescence spectra as well as Raman scattering A1(LO) spectra in matched target DNA clearly illustrate the feasibility of our proposed mechanism. The results shown here open up a new possibility for the application of nitride LEDs in biosensor engineering.

Cool white III-nitride light emitting diodes based on phosphor-free indium-rich InGaN nanostructures

Phosphor-free cool white emitting light emitting diodes (LEDs) have been fabricated using a dual stacked InGaN∕GaN multiple quantum wells (MQWs) comprising of a lower set of MQWs emitting yellow and an upper set of MQWs emitting blue. The lower set of MQWs incorporates indium-rich InGaN connected-dot nanostructures with a height of ∼1.0nm in the well. The well is first grown with an InGaN layer serving as the wetting layer, then treated with trimethylindium (TMIn) to initiate nanostructure growth of another InGaN layer to complete the well layer. This gives a broadened yellow emission peak. With the combination of emission from the upper blue emitting InGaN∕GaN MQWs subsequently grown, cool white light emission is achieved. The In-rich nanostructures formed during TMIn treatment enhance indium incorporation in InGaN well and also act as effective radiative recombination sites for carriers at the lower set of MQWs.

Growth and characterization of crack-free semipolar {1-101}InGaN∕GaN multiple-quantum well on V-grooved (001)Si substrates

This work elucidates the two-stage growth of GaN on V-grooved (001)Si substrates using metal-organic chemical vapor deposition. The first growth stage proceeds on the {111}Si sidewalls until GaN fills the V grooves. Then the second stage continues and leads to a semipolar surface with the {1-101}GaN facets. GaN films with thickness of over 1μm can be obtained without cracks by this two stage-growth. Excitation-power-dependent and time-resolved photoluminescence measurements confirm that the internal electric field in the InGaN∕GaN multiple-quantum well (MQW) grown on this GaN template is indeed smaller than that of the MQW grown on (0001)GaN.

Characterization of a-plane InGaN multiple-quantum wells grown on maskless lateral epitaxially overgrown a-plane GaN

We investigated the properties of nonpolar a-plane InGaN∕GaN multiple-quantum wells (MQWs) grown on maskless lateral epitaxial overgrowth (LEO) a-plane GaN∕r-sapphire. Many surface defects with asymmetric V-shape were observed on a-plane InGaN MQWs grown on the defective regions which were seed and coalescence regions. In the low defect regions, the surface defect density of a-plane InGaN MQWs was ∼1.0×107∕cm2, which was higher than that of conventional c-plane LEO-GaN, by measuring atomic force microscope and scanning tunneling electron microscope. The cathode luminescence intensity distribution of a-plane InGaN MQWs was significantly dependent on the distribution of surface asymmetric V-defect. Therefore, we suggest that the optical properties of a-plane InGaN active layer were affected by the asymmetric V-defects which were generated by interaction between the epitaxial defects and the limit of InGaN growth kinetics.

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN∕GaN multiple quantum wells

Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.

Quantized level transitions and modification in InGaN∕GaN multiple quantum wells

A detailed study of emission mechanism is performed in undoped and Mg-doped InGaN∕GaN multiple quantum wells (MQWs) by means of injection-current- and temperature-dependent electroluminescence measurements. Two emission peaks corresponding to the recombination in InGaN quantum well are observed at high injection-current level in both MQWs. According to the emission behaviors with increasing injection current and decreasing temperature, in conjunction with the numerical calculations, these two peaks are tentatively assigned to be the interband transitions from the first quantized electron level to the first and second quantized heavy-hole levels (1e-1hh and 1e-2hh), respectively. Moreover, the energy separation of the interband transitions is reduced from 200to130meV by Mg dopant, which indicates that the quantized levels have been modified as a result of weakening of the polarization field.

Phosphor-free white light-emitting diode with laterally distributed multiple quantum wells

A phosphor-free white light-emitting diode (LED) was fabricated with laterally distributed blue and green InGaN∕GaN multiple quantum wells (MQWs) grown by a selective area growth method. Photoluminescence and electroluminescence (EL) spectra of the LED showed emission peaks corresponding to the individual blue and green MQWs. The integrated EL intensity ratio of green to blue emission varied from 2.5 to 6.5 with the injection current below 300mA, but remained constant at high injection currents above 300mA. The stability of the emission color at high currents is attributed to parallel carrier injection into both MQWs.

Phosphor-Free GaN-Based Transverse Junction White-Light Light-Emitting Diodes With Regrown n-Type Regions

In this study, we demonstrate a GaN-based phosphor-free white-light light-emitting diode (LED), which is composed of GaN-based dual-wavelength (blue and yellow-green) multiple-quantum-wells (MQWs) and a transverse p-n junction. The device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped MQWs. The problems related to the bias-dependent shape of the electroluminescence spectra that occur in traditional phosphor-free white-light LEDs (with vertical p-n junctions) are greatly minimized. The current-voltage performance of our device is comparable to that of the commercially available phosphor white-light LEDs. In addition, the dynamic measurement results indicate that we can attain a much higher modulation bandwidth (22 versus 3 MHz) with this device than with the currently available commercial ones.

Investigation of the V-pit related morphological and optical properties of InGaN∕GaN multiple quantum wells

In this work, the effects of large V-pits on the morphological and optical properties of InGaN∕GaN multiple quantum wells (MQWs) were studied using scanning electron microscopy, transmission electron microscopy, and photoluminescence. InGaN∕GaN MQWs with high-density large V-pits were grown by metal organic chemical vapor deposition. In addition to the regular c-plane MQWs, the MQWs grown on the {101¯1} faceted sidewalls of the V-pits were also observed, which gave much higher emission energies than those of the c-plane MQWs. Furthermore, when the low-temperature GaN buffer was very thin, the {112¯m} (m⩾2) faceted sidewalls of the V-pits were observed. It was then found that MQWs grown on such sidewalls had emission energies between those of the c-plane MQWs and those of the {101¯1} faceted sidewall MQWs.

Investigation of Near Field Intensity in GAN MQW in 300–375 Nanometer Wavelength Ranges

Analysis of optical confinement and investigation of near field intensity in multiple quantum well (MQW) structure of GaN/AlGaN has been carried out in 300–375 nanometer wavelength ranges. Spread of near field intensity has been deduced in terms of full width at half maximum (FWHM) as a function of wavelength, Aluminum composition in the barrier and temperature. FWHM was found to be decreasing with increase of Al mole fraction and temperature and FWHM was increasing with wavelength. Confinement factor has been explored for varying values of Aluminum mole fraction, temperature and wavelength. It is inferred from our analysis that confinement of near field increases with increase in Aluminum composition of barrier and temperature while it was found to be decreasing with increase in wavelength.

Origin of efficiency droop in GaN-based light-emitting diodes

The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Optical properties of GaN/AlGaN QW nanostructures with different well and barrierwidths

Optical properties of wurtzite AlGaN/GaN quantum well (QW) structures grown bymolecular-beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) onc-plane sapphire substrates have been investigated by means of photoluminescence(PL) and time-resolved photoluminescence (TRPL) at low temperature. ThePL spectra exhibit a blue-shifted emission of GaN/AlGaN QW nanostructuresby decreasing the barrier width, in contrast to the arsenide system (Pabla A Set al 1993 Appl. Phys. Lett. 63 752). This behavior is attributed to a redistributionacross the samples of the huge built-in electric field (several hundreds ofkV cm−1) induced by the polarization difference between wells and barriers. The trend of the barrierwidth dependence of the internal polarization field is reproduced by using simpleelectrostatic arguments. In addition, the effect of well width variation on the opticaltransition and decay time of GaN multiple quantum wells (MQWs) have been investigated,and it has been shown that the screening of the piezoelectric field and the electron–holeseparation are strongly dependent on the well thickness and have a profound effect onthe optical properties of the GaN/AlGaN MQWs. The time-resolved PL spectraof 3 nm well MQWs reveal that the spectral peak position shifts toward lowerenergies as the decay time increases and becomes red-shifted at longer decay times.

Radiative and nonradiative lifetimes in nonpolar m-plane InxGa1−xN∕GaN multiple quantum wells grown on GaN templates prepared by lateral epitaxial overgrowth

Different from the case for polar (0001) InxGa1−xN multiple quantum wells (MQWs), the effective radiative lifetimes (τR,eff) at 8K of violet (V: 3.15eV), purple (P: 3.00eV), and blue (B: 2.83eV) emission peaks in nonpolar (11¯00) InxGa1−xN∕GaN MQWs fabricated on various GaN templates were found to be nearly independent of InN molar fraction x being approximately 1ns. The result indicates the absence of polarization fields parallel to the MQW normal. For each luminescence peak, the effective nonradiative lifetime (τNR,eff) at room temperature of the MQWs grown on “Ga-polar” wings of the GaN template prepared by lateral epitaxial overgrowth (LEO) was longer than that for the MQWs grown on “N-polar” wings, windows, or on conventional GaN templates, which had high density basal plane stacking faults and threading dislocations. Since τR,eff was little affected by the presence of defects, the increase in τNR,eff brought fivefold improvement in the equivalent internal quantum efficiency (ηinteq) of V peak. Because B peak was mostly generated from defective areas, the increase in ηinteq was not so remarkable (29%–36%). However, these values are approximately twice that reported for (112¯0) InGaN∕GaN MQWs grown on LEO GaN templates [Onuma et al., Appl. Phys. Lett. 86, 151918 (2005)].

Polarization of edge emission from III-nitride light emitting diodes of emission wavelength from 395to455nm

Polarization-resolved edge-emitting electroluminescence of InGaN∕GaN multiple quantum well (MQW) light emitting diodes (LEDs) from 395to455nm was measured. Polarization ratio decreased from 3.2 of near-ultraviolet LEDs (395nm) to 1.9 of blue LEDs (455nm). Based on TE mode dominant emissions in InGaN∕GaN MQWs, compressive strain in well region favors TE mode, indium induced quantum-dot-like behavior leads to an increased TM component. As wavelength increased, indium enhanced quantum-dot-like behavior became obvious and E‖C electroluminescence signal increased thus lower polarization ratio. Electroluminescence spectrum shifts confirmed that quantum dotlike behaviors rather than strain might be dominant in modifying luminescence mode of InGaN∕GaN MQWs from near ultraviolet to blue.

Carrier relaxation dynamics and steady-state charge distributions in coupled InGaN∕GaN multiple and single quantum wells

We have examined the carrier capture dynamics and excitation dependent charge distributions of coupled InGaN∕GaN multiple quantum well samples. We measured the temporal evolution of time-delayed cathodoluminescence (CL) spectra to study the temperature- and excitation-dependent transfer of carriers from a surrounding confinement region into a coupled single quantum well. Samples possessing two different structures for the confinement region [i.e., number of quantum wells (QWs) and varying widths] were examined with CL. In order to study state filling of the SQW and QWs in the confinement region, we calculated the quasi-Fermi levels and carrier densities by utilizing a model that involves self-consistent solutions of the nonlinear Poisson-Schrödinger equation for wurtzite QWs including strain, deformation potentials, and polarization fields. Band-edge and effective mass parameters were first obtained from a strain- and In composition-dependent k⋅p calculation for wurtzite InxGa1−xN, using a 6×6 k⋅p Hamiltonian in the {0001} representation. The model shows that the difference in the quasi-Fermi levels between the confinement and SQW regions decreases with increasing excitation and temperature. Likewise, a reversal in the relative magnitude of the carrier densities between these two regions occurs at a certain temperature and excitation. Furthermore, the results for the model describing the steady-state excitation are consistent with those for the transient excitation in time-resolved CL, which also exhibit a marked increase in the rate of carrier transfer to the SQW region as the temperature increases.

Optical characteristics of a-plane InGaN∕GaN multiple quantum wells with different well widths

a -plane InGaN∕GaN multiple quantum wells of different widths ranging from 3to12nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3to12nm. Low temperature (9K) time-resolved PL study shows that the sample with 3-nm-thick wells has a better optical property with a fast exciton decay time of 0.57ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify the more uniform and stronger luminescence intensity distribution observed for the samples of thinner quantum wells, indicating that the important growth parameters for a-plane InGaN∕GaN multiple quantum wells could be dominated by the In fluctuation and crystal quality during the epitaxial growth.