Influence of barrier thickness on the structural and optical properties of InGaN/GaN multiple quantum wells

The structural and optical properties of InGaN/GaN multiple quantum wells （MQWs） with different barrier thick-nesses are studied by means of high resolution X-ray diffraction （HRXRD）, a cross-sectional transmission electron mi-croscope （TEM）, and temperature-dependent photoluminescence （PL） measurements. HRXRD and cross-sectional TEM measurements show that the interfaces between wells and barriers are abrupt and the entire MQW region has good periodic- ity for all three samples. As the barrier thickness is increased, the temperature of the turning point from blueshift to redshift of the S-shaped temperature-dependent PL peak energy increases monotonously, which indicates that the localization po- tentials due to In-rich clusters is deeper. From the Arrhenius plot of the normalized integrated PL intensity, it is found that there are two kinds of nonradiative recombination processes accounting for the thermal quenching of photoluminescence, and the corresponding activation energy （or the localization potential） increases with the increase of the barrier thickness. The dependence on barrier thickness is attributed to the redistribution of In-rich clusters during the growth of barrier layers, i.e., clusters with lower In contents aggregate into clusters with higher In contents.

Direct Observation of Carrier Transportation Process in InGaAs/GaAs Multiple Quantum Wells Used for Solar Cells and Photodetectors

The resonant excitation is used to generate photo-excited carriers in quantum wells to observe the process of the carriers transportation by comparing the photoluminescence results between quantum wells with and without a p-n junction. It is observed directly in experiment that most of the photo-excited carriers in quantum wells with a p-n junction escape from quantum wells and form photoeurrent rather than relax to the ground state of the quantum wells. The photo absorption coei^cient of multiple quantum wells is also enhanced by a p-n junction. The results pave a novel way for solar cells and photodetectors making use of low-dimensional structure.

Room-temperature direct-bandgap photoluminescence from strain-compensated Ge/SiGe multiple quantum wells on silicon

Strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells were grown on an Si0.1 Ge0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition technology on an n＋-Si（001） substrate. Photoluminescence measurements were performed at room temperature, and the quantum confinement effect of the direct-bandgap transitions of a Ge quantum well was observed, which is in good agreement with the calculated results. The luminescence mechanism was discussed by recombination rate analysis and the temperature dependence of the luminescence spectrum.

Direct-bandgap electroluminescence from tensile-strained Ge/SiGe multiple quantum wells at room temperature

Tensile-strained Ge/SiGe multiple quantum wells （MQWs） were grown on a Ge-on-Si virtual substrate using ultrahigh vacuum chemical vapor deposition on an n＋-Si （001） substrate. Direct-bandgap electroluminescence from the MQWs light emitting diode was observed at room temperature. The quantum confinement effect of the direct-bandgap transitions is in good agreement with the theoretical calculated results. The redshift mechanism of emission wavelength related to the thermal effect is discussed,

Room-Temperature Annealing of 1 MeV Electron Irradiated Lattice Matched In0.53Ga0.47As/InP Multiple Quantum Wells

Long-term room-temperature annealing effects of InGaAs/InP quantum wells with different wells （namely triple wells and five wells embedded） and bulk InCaAs are investigated after high energy electron irradiation. It is observed that the photoluminescence （PL） intensity of bulk InGaAs materials is enhanced after low dose electron irradiation and the PL intensity for all the three samples is degraded dramatically when the electron dose is relatively high. With respect to the room-temperature annealing, we find that the PL intensity for both samples recovers relatively fast at the initial stage. The PL performance of multiple quantum-well samples shows better recovery after irradiation compared with the results of bulk InGaAs materials. Meanwhile, the recovery speed factors of multiple quantum-well samples are relatively faster than those of the bulk InGaAs materials as well. We infer that the recovery difference between the quantum-well materials and bulk materials originates from the fact that the radiation induced defects are confined in the quantum wells as a consequence of the free energy barrier between the In0.53Ga0.47 As wells and InP barrier layers.

Epitaxial evolution on buried cracks in a strain-controlled AlN/GaN superlattice interlayer between AlGaN/GaN multiple quantum wells and a GaN template

Epitaxial evolution of buried cracks in a strain-controlled AIN/GaN superlattice interlayer （IL） grown on GaN tem- plate, resulting in crack-free AIGaN/GaN multiple quantum wells （MQW）, was investigated. The processes of filling the buried cracks include crack formation in the IL, coalescence from both side walls of the crack, build-up of an MQW-layer hump above the cracks, lateral expansion and merging with the surrounding MQW, and two-dimensional step flow growth. It was confirmed that the filling content in the buried cracks is pure GaN, originating from the deposition of the GaN thin layer directly after the IL. Migration of Ga adatoms into the cracks plays a key role in the filling the buried cracks.

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths

Four blue-violet light emitting InGaN/GaN multiple quantum well（MQW） structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.

Raman spectrum study of δ-doped GaAs/AlAs multiple-quantum wells

Three samples of GaAs/A1As multiple-quantum wells with different quantum well widths and tS-doped with Be ac- ceptors at the well center were grown on （100） GaAs substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4-50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical （LO） phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.

Visualizing light-to-electricity conversion process in InGaN/GaN multi-quantum wells with a p-n junction

Absorption and carrier transport behavior plays an important role in the light-to-electricity conversion process, which is difficult to characterize. Here we develop a method to visualize such a conversion process in the InGaN/GaN multiquantum wells embedded in a p-n junction. Under non-resonant absorption conditions, a photocurrent was generated and the photoluminescence intensity decayed by more than 70% when the p-n junction out-circuit was switched from open to short. However, when the excitation photon energy decreased to the resonant absorption edge, the photocurrent dropped drastically and the photoluminescence under open and short circuit conditions showed similar intensity. These results indicate that the escaping of the photo-generated carriers from the quantum wells is closely related to the excitation photon energy.

Low driving voltage in an organic light-emitting diode using MoO_3/NPB multiple quantum well structure in a hole transport layer

The driving voltage of an organic light-emitting diode（OLED） is lowered by employing molybdenum trioxide（MoO3）/N,N＇-bis（naphthalene-1-yl）-N,N＇-bis（phe-nyl）-benzidine（NPB） multiple quantum well（MQW） structure in the hole transport layer.For the device with double quantum well（DQW） structure of ITO/[MoO3（2.5 nm）/NPB（20 nm）]2/Alq3（50 nm）/LiF（0.8 nm）/Al（120 nm）],the turn-on voltage is reduced to 2.8 V,which is lowered by 0.4 V compared with that of the control device（without MQW structures）,and the driving voltage is 5.6 V,which is reduced by 1 V compared with that of the control device at the 1000 cd/m2.In this work,the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure,which is attributed not only to the reduced energy barrier between ITO and NPB,but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.

The enhancement of light-emitting efficiency using GaN-based multiple quantum well light-emitting diodes with nanopillar arrays

The quest for higher modulation speed and lower energy consumption has inevitably promoted the rapid development of semiconductor-based solid lighting devices in recent years. GaN-based light-emitting diodes （LEDs） have emerged as promising candidates for achieving high efficiency and high intensity, and have received increasing attention among many researchers in this field. In this paper, we use a self-assembled array-patterned mask to fabricate InGaN/GaN multi- quantum well （MQW） LEDs with the intention of enhancing the light-emitting efficiency. By utilizing inductively coupled plasma etching with a self-assembled Ni cluster as the mask, nanopillar arrays are formed on the surface of the InGaN/GaN MQWs. We then observe the structure of the nanopillars and find that the V-defects on the surface of the conventional structure and the negative effects of threading dislocation are effectively reduced. Simultaneously, we make a comparison of the photoluminescence （PL） spectrum between the conventional structure and the nanopillar arrays, achieved under an experimental set-up with an excitation wavelength of 325 mm. The analysis demonstrates that MQW-LEDs with nanopillar arrays achieve a PL intensity 2.7 times that of conventional LEDs. In response to the PL spectrum, some reasons are proposed for the enhancement in the light-emitting efficiency as follows： 1） the improvement in crystal quality, namely the reduction in V-defects; 2） the roughened surface effect on the expansion of the critical angle and the attenuated total reflection; and 3） the enhancement of the light-extraction efficiency due to forward scattering by surface plasmon polariton modes in Ni particles deposited above the p-type GaN layer at the top of the nanopillars.

InGaN/GaN multiple quantum well solar cells with an enhanced open-circuit voltage

In this paper, InGaN/GaN multiple quantum well solar cells （MQWSCs） with an In content of 0.15 are fabricated and studied. The short-circuit density, fill factor and open-circuit voltage （Voc） of the device are 0.7 mA/cm2, 0.40 and 2.22 V, respectively. The results exhibit a significant enhancement of Voc compared with those of InGaN-based hetero and homojunction cells. This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing Voc of an In-rich InxGal-~N solar cell. The device exhibits an external quantum efficiency （EQE） of 36% （7%） at 388 nm （430 nm）. The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well （IMQW） structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes （LEDs） are optimized in order to obtain near white light emissions. The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain, well-coupling, valence band-mixing and polarization effect through employing a newly developed theoretical model from the k. p theory. Several structure parameters such as well material component, well width, layout of the wells and the thickness of harrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure. Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two, the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness. The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of Ga N/In Ga N asymmetric lower waveguide(LWG)layers on photoelectrical properties of In Ga N multiple quantum well laser diodes(LDs)with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of In Ga N insertion layer(In Ga N-IL)between the Ga N lower waveguide layer and the quantum wells,which is achieved with the Crosslight Device Simulation Software(PIC3D,Crosslight Software Inc.).The optimal thickness and the indium content of the In Ga N-IL in lower waveguide layers are found to be 300 nm and 4%,respectively.The thickness of In Ga N-IL predominantly affects the output power and the optical field distribution in comparison with the indium content,and the highest output power is achieved to be 1.25 times that of the reference structure(symmetric Ga N waveguide),which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells.Furthermore,when the thickness and indium content of In Ga N-IL both reach a higher level,the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor(OCF)related to the concentrated optical field in the lower waveguide.

Flexible,stretchable,and transparent InGaN/GaN multiple quantum wells/polyacrylamide hydrogel-based light emitting diodes

Visualization is a direct,efficient,and simple interface method to realize the interaction between human and machine,whereas the flexible display unit,as the major bottleneck,still deeply hinders the advances of wearable and virtual reality devices.To obtain flexible optoelectronic devices,one of the effective methods is to transfer a high-efficient and long-lifetime inorganic optoelectronic film from its rigid epitaxial substrate to a foreign flexible/soft substrate.Additionally,piezo-phototronic effect is a fundamental theory for guiding the design of flexible optoelectronic devices.Herein,we demonstrate a flexible,stretchable,and transparent InGaN/GaN multiple quantum wells(MQWs)/polyacrylamide(PAAM)hydrogel-based light emitting diode coupling with the piezo-phototronic effect.The quantum well energy band and integrated luminous intensity(increased by more than 31.3%)are significantly modulated by external mechanical stimuli in the device.Benefiting from the small Young's modulus of hydrogel and weak Van der Waals force,the composite film can endure an extreme tensile condition of about 21.1%stretching with negligible tensile strains transmitted to the InGaN/GaN MQWs.And the stable photoluminescence characteristics can be observed.Moreover,the hydrogen-bond adsorption and excellent transparency of the hydrogel substrate greatly facilitate the packaging and luminescence of the optoelectronic device.And thus,such a novel integration scheme of inorganic semiconductor materials and organic hydrogel materials would help to guide the robust stretchable optoelectronic devices,and show great potential in emerging wearable devices and virtual reality applications.

Nonpolar Al_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N multiple quantum wells on GaN nanowire for UV emission

Nonpolar m-plane AlGaN offers the advantage of polarization-free multiple quantum wells(MQWs)for ultraviolet(UV)emission and can be achieved on the sidewalls of selective area grown GaN nanowires.We reveal that the growth of AlGaN on GaN nanowires by metal organic chemical vapor deposition(MOCVD)is driven by vapor-phase diffusion,and consequently puts a limit on the pitch of nanowire array due to shadowing effect.An insight into the difficulty of achieving metal-polar AlGaN nanowire by selective area growth(SAG)in MOCVD is also provided and can be attributed to the strong tendency to form pyramidal structure due to a very small growth rate of{1011}semipolar planes compared to(0001)c-plane.The nonpolar m-plane sidewalls of GaN nanowires obtained via SAG provides an excellent platform for growth of nonpolar AlGaN MQWs.UV emission from mplane Al_(x)Ga_(1−x)N/Al_(y)Ga_(1−y)N MQWs grown on sidewalls of dislocation-free GaN nanowire is demonstrated in the wavelength range of 318–343 nm.

Variation of efficiency droop with quantum well thickness in In GaN/GaN green light-emitting diode

In GaN/GaN multiple quantum well（MQW） green light-emitting diodes（LEDs） with varying In GaN quantum well layer thickness are fabricated and characterized. The investigation of luminescence efficiency versus injection current reveals that several physical mechanisms may jointly influence the efficiency droop, resulting in a non-monotonic variation of droop behavior with increasing quantum well（QW） thickness. When the QW is very thin, the increase of In GaN well layer thickness makes the efficiency droop more serious due to the enhancement of polarization effect. When the QW thickness increases further, however, the droop is alleviated significantly, which is mainly ascribed to the enhanced nonradiative recombination process and the weak delocalization effect.

Metamorphic growth of 1.55 μm InGaAs/InGaAsP multiple quantum wells laser structures on GaAs substrates

We fabricate a GaAs-based InGaAs/InGaAsP multiple quantum wells （MQWs） laser at 1.55 pm. Using two-step growth method and thermal cyclic annealing, a thin low-temperature InP layer and a thick InP buffer layer are grown on GaAs substrates by low-pressure metal organic chemical vapor deposition technology. Then, high- quality MQWs laser structures are grown on the InP buffer layer. Under quasi-continuous wave （QCW） condition, a threshold current of 476 mA and slope efficiency of 0.15 mW/mA are achieved for a broad area device with 50 μm wide strip and 500 μm long cavity at room-temperature. The peak wavelength of emission spectrum is 1549.5 nm at 700 mA. The device is operating for more than 2000 h at room-temperature and 600 mA.

Efficiency droop in InGaN/GaN-based LEDs with a gradually varying In composition in each InGaN well layer

Temperature-dependent and driving current-dependent electroluminescence spectra of two different InGaN/GaN multiple quantum well structures SA and SB are investigated,with the In composition in each well layer(WL)along the growth direction progressively increasing for SA and progressively decreasing for SB.The results show that SB exhibits an improved efficiency droop compared with SA.This phenomenon can be explained as follows:owing to the difference in growth pattern of the WL between these two samples,the terminal region of the WL in SB contains fewer In atoms than in SA,and therefore the former undergoes less In volatilization than the latter during the waiting period required for warming-up due to the difference in the growth temperature between well and barrier layers.This results in SB having a deeper triangular-shaped potential well in its WL than SA,which strongly confines the carriers to the initial region of the WL to prevent them from leaking to the p-GaN side,thus improving the efficiency droop.Moreover,the improvement in the efficiency droop for SB is also partly attributed to its stronger Coulomb screening effect and carrier localization effect.

Phase separations in graded-indium content InGaN/GaN multiple quantum wells and its function to high quantum efficiency

Phase separations have been studied for graded-indium content In_xGa_（1-x）N/GaN multiple quantum wells（MQWs） with different indium contents by means of photoluminescence（PL）,cathodeluminescence（CL） and time-resolved PL（TRPL） techniques.Besides the main emission peaks,all samples show another 2 peaks at the high and low energy parts of the main peaks in PL when excited at 10 K.CL images show a clear contrast for 3 samples,which indicates an increasing phase separation with increasing indium content.TRPL spectra at 15 K of the main emissions show an increasing delay of rising time with indium content,which means a carrier transferring from low indium content structures to high indium content structures.