Growth Of Multiple Quantum Wells Research Articles

Fabrication of crack-free strained SiGe/Ge multiple quantum wells on Ge-on-Si(111) by the patterning method

We fabricate strained SiGe/Ge multiple quantum well (MQW) structures on Ge-on-Si(111). It is clearly found that high-density crystal cracks are generated in the MQW due to the accumulated tensile strain when the total thickness of the MQW increases beyond the critical thickness. In this study, we determine the critical thickness of the strained SiGe/Ge (111) MQW. Moreover, we demonstrate the mesa-patterning of the Ge-on-Si prior to the MQW growth can drastically suppress the crack formation, leading to the significant increase in the critical thickness. As a result, we find that room-temperature photoluminescence from the crack-suppressed MQW is highly enhanced. This indicates that the suppression of crack formation by the patterning method is very promising toward SiGe/Ge MQW embedded light-emitting device applications.

Improved external quantum efficiency of deep UV LEDs with an ultra-thin AlGaN last quantum barrier by controlling the desorption-kinetics process

An ultra-thin AlGaN last quantum barrier (LQB) with a high Al content is effective in enhancing the hole injection in AlGaN-based deep UV LEDs (DUV-LEDs). However, it is very challenging to realize the ultra-thin LQB by the metalorganic chemical vapor deposition technique. In this work, a well-defined 1.0 nm thick AlGaN LQB with unintentionally high Al content was achieved by controlling the surface desorption kinetics for the growth of multiple quantum wells. The light output power of the DUV-LEDs with 1 nm thick LQB was increased by 44% to 7.16 mW at 40 mA, and the peak external quantum efficiency reached 4.04%.

Growth of (101¯1) Semipolar GaN‐Based Light‐Emitting Diode Structures on Silicon‐on‐Insulator

The growth and characterization of semipolar GaN buffer, InGaN multiple quantum wells (MQWs), and light‐emitting diode (LED) structure on patterned silicon‐on‐insulator (SOI) substrates, implementing the aspect ratio technique (ART), are reported. The early growth stages of GaN result in continuous and uniform stripes with small height variations that cause the formation of chevrons. Three coalescence strategies are tested to improve surface morphology and optical quality. Scanning electron microscopy identifies no crack formation but undulations of the surface. A roughness of ≈10 nm is measured by atomic force microscopy on large areas. The impact of MQW growth temperature shows similar surface morphology in terms of undulations and roughness. Room temperature photoluminescence spectra show wavelength emission redshifting when decreasing the MQW growth temperature. Room‐temperature cathodoluminescence (CL) highlights first the presence of threading dislocations (TDs) in between the coalescence boundary despite the use of the ART technique. Second, CL shows a spatially homogeneous emission wavelength of around 485 nm only perturbed by lower‐wavelength emission (455 nm) arising from the chevrons. Blue LED structures exhibit uniform emission wavelength at 450 nm, having a crack‐free surface, and roughness of ≈5 nm. These results pave the way for the fabrication of semipolar micro‐LEDs on SOI substrates.

The role of surface states and point defects on optical properties of InGaN/GaN multi-quantum wells in nanowires grown by molecular beam epitaxy

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO x deposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range.

A comparison study of InGaN/GaN multiple quantum wells grown on (111) silicon and (0001) sapphire substrates under identical conditions

Due to an increasing demand of developing III-nitride optoelectronics on silicon substrates, it is necessary to compare the growth and optical properties of III-nitride optoelectronics such as InGaN based light emitting diodes (LEDs) on silicon substrates and widely used sapphire substrates. GaN-on-silicon suffers from tensile strain, while GaN-on-sapphire exhibits compressive strain. This paper presents a comparative study of InGaN/GaN multiple quantum wells (MQWs) grown on a silicon substrate and a sapphire substrate under identical conditions. It has been found that GaN strain status has a significant influence on the growth and the optical properties of InGaN/GaN MQWs. Photoluminescence measurements indicate the InGaN/GaN MQWs grown on a silicon substrate exhibit significantly longer wavelength emission than those on a sapphire substrate. Detailed x-ray diffraction measurements including reciprocal space mapping measurements confirm that both indium content and growth rate in the InGaN MQWs on the silicon substrate are enhanced due to the tensile strain of the GaN underneath compared with those on the sapphire substrate. This work also presents an investigation on strain evolution during the InGaN MQWs growth on the two different kinds of substrates. A qualitative study based on in-situ curvature measurements indicates that a strain change on the silicon substrate is much more sensitive to a growth temperature change than that on the sapphire substrate. It is worth highlighting that the results provide useful guidance for optimising growth conditions for III-nitrides optoelectronics on silicon substrates.

Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content AlGaN Quantum Wells for Deep-Ultraviolet LED

A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode.

Phase-transition critical thickness of rocksalt MgxZn1-xO layers.

The rocksalt structure of ZnO has a very promising bandgap for optoelectronic applications. Unfortunately, this high-pressure phase is unstable under ambient conditions. This paper presents experimental results for rocksalt-type ZnO/MgO superlattices and theoretical considerations of the critical thickness of MgxZn1-xO layers. The correlations between the layer/spacer thickness ratio, elastic strain, chemical composition, and critical thickness are analyzed. The Matthews and Blakeslee model is revisited to find analytic conditions for the critical layer thickness resulting in phase transition. Our analysis shows that due to the decrease in misfit stresses below some critical limit, the growth of multiple quantum wells composed of rocksalt ZnO layers and MgO spacers is possible only for very large layer/spacer thickness ratios.

Single peak deep ultraviolet emission and high internal quantum efficiency in AlGaN quantum wells grown on large miscut sapphire substrates

Deep ultraviolet (DUV) AlxGa1-xN/AlyGa1-yN-based multiple quantum wells (MQWs) were grown on sapphire substrates with miscut angles of 0.2° and 4°. Significantly reduced edge type dislocation density and in-plane anisotropy of dislocation distributions were demonstrated for MQWs grown on high miscut sapphire substrate. Up to 10-fold enhanced MQW emission was observed for MQWs grown on 4° miscut sapphire compared to that grown on 0.2° miscut sapphire, and internal quantum efficiency (IQE) as large as 88% was achieved. Compositional inhomogeneity and thus formation of potential minimum were ambiguously demonstrated by localized cathodoluminescence spectra across the step bunched surface of the MQW sample. However, from the macroscale point of view, single peak PL luminescence was shown for MQW grown on high miscut sapphire without peak separation, suggesting a strongly carrier localization effect. A three-dimensional atom diffusion and desorption model describing the MQW growth was also proposed. This work provides a promising approach towards the realization of highly-efficient DUV emitters.

Effects of hole-injection through side-walls of large V-pits on efficiency droop in III–nitride LEDs**Project supported by the National Key Research and Development Project of China (Grant No. 2017YFB0403303) and the Key Technologies Research and Development Program of Tianjin, China (Grant Nos. 18YFZCGX00760 and 18YFZCGX00400).

Although the solid-state lighting market is growing rapidly, it is still difficult to obtain ultra-high brightness white light emitting diodes (LEDs). V-pits are inevitably introduced during the metalorganic chemical vapor deposition (MOCVD) growth of multiple quantum wells (MQWs) in III–nitride LEDs, and thus affecting the carrier dynamics of the LEDs. Specifically designed structures are fabricated to study the influence of the V-pits on the hole transportation and efficiency droop, and double quantum wells (QWs) are used to monitor the transportation and distribution of holes based on their emission intensity. It is found that when compared with the planar QWs, the injection of holes into the QWs through the side walls of the V-pits changes the distribution of holes among the MQWs. This results in a higher probability of hole injection into the middle QWs and enhanced emission therein, and, consequently, a lower efficiency droop.

High Internal Quantum Efficiency of Nonpolar a-Plane AlGaN-Based Multiple Quantum Wells Grown on r-Plane Sapphire Substrate

An internal quantum efficiency (IQE) as high as 39% was achieved with the nonpolar a-plane AlGaN-based multiple quantum wells (MQWs) grown on the r-plane sapphire substrate with metal organic chemical vapor deposition technology. Evident fourth order X-ray diffraction satellite peak and intense MQW-related exciton emission peak at a wavelength of 279.2 nm were observed, implying the successful growth of high quality nonpolar a-plane AlGaN-based MQWs. It was found that the employment of the trimethyl-aluminum (TMAl) flow duty-ratio modulation method played a crucial role in the epitaxial growth of the nonpolar a-plane AlGaN MQWs with sharp heterointerfaces and high IQE. Moreover, the unambiguous absence of the blue-shift in the MQWs-related exciton emission peak was verified in spite of the increase in the excitation power during the measurement of the excitation power-modulated photoluminescence spectra, indicating a complete elimination of the quantum confined Stark effect in the nonpolar AlGaN-based MQWs.

Advantages of InGaN/GaN multiple quantum wells with two-step grown low temperature GaN cap layers

Two-step grown low temperature GaN cap layers (LT-cap) are employed to improve the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs). The first LT-cap layer is grown in nitrogen atmosphere, while a small hydrogen flow is added to the carrier gas during the growth of the second LT-cap layer. High-resolution X-ray diffraction results indicate that the two-step growth method can improve the interface quality of MQWs. Room temperature photoluminescence (PL) tests show about two-fold enhancement in integrated PL intensity, only 25 meV blue-shift in peak energy and almost unchanged line width. On the basis of temperature-dependent PL characteristics analysis, it is concluded that the first and the second LT-cap layer play a different role during the growth of MQWs. The first LT-cap layer acts as a protective layer, which protects quantum well from serious indium loss and interface roughening resulting from the hydrogen over-etching. The hydrogen gas employed in the second LT-cap layer is in favor of reducing defect density and indium segregation. Consequently, interface/surface and optical properties are improved by adopting the two-step growth method.

GaAsBi/GaAs MQWs grown by MBE using a two-substrate-temperature technique

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) on (100) GaAs substrates were grown by molecular beam epitaxy (MBE) using the two-substrate-temperatures (TST) technique. In the TST technique, the substrate temperature was set at TGaAsBi = 350 °C for GaAsBi layers and TGaAs = 550 °C for GaAs layers for the growth of MQWs. The structural and optical characterization of GaAsBi/GaAs MQWs using TST technique was carried out for growth optimization by changing As4/Ga beam equivalent pressure (BEP) ratio. Atomic force microscope observation shows high As4/Ga BEP ratio supply can prevents roughening of the GaAsBi layer surface of MQWs. High resolution x-ray diffraction analysis results reveal Bi incorporation enhancement with increasing As4/Ga BEP ratio. The reciprocal space mapping shows highly strained MQWs layers can be grown without relaxation with respect to the GaAs substrate at optimized growth conditions. At room temperature 1.22 μm photoluminescence emission was obtained from GaAs0.962Bi0.038/GaAs MQWs grown using TST technique.

Homogeneity improvement of N-polar InGaN/GaN multiple quantum wells by using c-plane sapphire substrate with off-cut-angle toward a-sapphire plane

To improve the homogeneity of the N-polar (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy (MOVPE), the growth of GaN and MQW on two c-plane sapphire substrates with an off-cut angle of 0.8° toward the a-plane (sub-A) and the m-plane (sub-M) was performed. The effects of the off-cut direction on the structural properties and surface morphologies of −c-plane GaN films were elucidated. It was found that the step bunching and meandering of −c-plane GaN were significantly suppressed on sub-A. The spatial homogeneity of the −c-plane InGaN/GaN MQWs along the off-cut direction was observed in the submicrometer scale using microbeam X-ray diffraction. By inhibiting the step bunching of the GaN template using sub-A, the thickness homogeneity of the MQWs on sub-A has been significantly improved in comparison with that on sub-M.

Defect creation in InGaAs/GaAs multiple quantum wells–I. Structural properties

We present a systematic study of extended defect creation in InGaAs/GaAs multiple quantum well (MQW) structures. Three sets of samples, grown by molecular beam epitaxy, were characterized by high-resolution x-ray diffraction and transmission electron microscopy. First, in a temperature series, optimal deposition temperature of 505°C was found for the In composition of 20% as determined from dislocation loop (DL) density and inspection of diffuse scattering patterns. InGaAs decomposition and lateral layer thickness undulations were observed above this optimal temperature. Second, increase of MQW periodicity from ×5 to ×10 revealed a thickness-related cumulative deterioration, characterized by increased likelihood of defect intersection with continued MQW growth, as suggested by an increase of the secondary DL density from ~1.6×107cm−2 to ~6×108cm−2. Additional strained layers experienced an ever-degrading quality of the growth surface. Third, a set consisting of samples with three different MQW periods was investigated. Different stages, suggested by a model of defect creation, were obtained for these different MQW periods, allowing specification of particular type, density, and spatial distribution of extended defects for each stage of defect creation.

Influences of stress on the properties of GaN/InGaN multiple quantum well LEDs grown on Si (111) substrates**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 51177175), the National Basic Research Program of China (Grant Nos. 2010CB923201 and 2011CB301903), the Ph. D. Program Foundation of the Ministry of Education of China (Grant No. 20110171110021), the International

The influences of stress on the properties of InGaN/GaN multiple quantum wells (MQWs) grown on silicon substrate were investigated. The different stresses were induced by growing InGaN and AlGaN insertion layers (IL) respectively before the growth of MQWs in metal–organic chemical vapor deposition (MOCVD) system. High resolution x-ray diffraction (HRXRD) and photoluminescence (PL) measurements demonstrated that the InGaN IL introduced an additional tensile stress in n-GaN, which released the strain in MQWs. It is helpful to increase the indium incorporation in MQWs. In comparison with MQWs without the IL, the wavelength shows a red-shift. AlGaN IL introduced a compressive stress to compensate the tensile stress, which reduces the indium composition in MQWs. PL measurement shows a blue-shift of wavelength. The two kinds of ILs were adopted to InGaN/GaN MQWs LED structures. The same wavelength shifts were also observed in the electroluminescence (EL) measurements of the LEDs. Improved indium homogeneity with InGaN IL, and phase separation with AlGaN IL were observed in the light images of the LEDs.

Electrically Driven Single Pyramid InGaN/GaN Micro Light-Emitting Diode Grown on Silicon Substrate

Electrically driven single-pyramid InGaN/GaN micro light-emitting diode ( $\mu$ -LED) on silicon substrate has been fabricated and investigated. The $\mu$ -LED exhibits a typical p-n diode behavior with a turn-on voltage of 2.5 V and realizes an efficient electroluminescence (EL) under an ultra-small injection current (3 $\mu$ A). An excellent EL pattern with a high-brightness spot at apex, a hexagonal blue ring around the base of pyramid, and six bright spots at the corners of pyramid base was observed. EL wavelength centered at about 450 nm, and a slight peak shift was found as the injection current increased from 100 to 1000 $\mu$ A. The small magnitude of spectral shift is attributed to the growth of multiple quantum wells on semi-polar GaN pyramid side facets and its accompanied reduction of the quantum-confined Stark effect. The $\mu$ -LED has the promising potential for applying in micro display and electrically driven single photon emitter.

Suppressed lattice relaxation during InGaAs/GaAsP MQW growth with InGaAs and GaAs ultra-thin interlayers

InGaAs/GaAsP strain-compensated multiple quantum wells (MQW) with an effective bandgap of 1.2eV are promising candidate materials for solar cells applications. Since high indium content in wells is needed to achieve such a low bandgap, lattice relaxation can easily occur during crystal growth. Here, we propose graded interlayers, consisting of ultra-thin GaAs and InGaAs with lower indium content, inserted between wells and barriers to prevent relaxation during growth and to achieve better crystal quality. Lattice relaxation was detected by in-situ monitoring of surface reflectance including an anisotropic component. Our method allowed the number of MQW periods before detectable lattice relaxation to reach approximately three times that without interlayers. Over 100 periods of In0.26Ga0.74As (7.0nm)/GaAs0.60P0.40 (9.2nm) MQW, with an absorption edge at 1.2eV, were successfully piled up as a result. Lattice relaxation without interlayers was caused dominantly by dislocations due to the large lattice mismatch between a well and a barrier, and graded interlayers made the hetero-interfaces smoother so that the subsequent layers could be more easily grown. Transmission electron microscopy confirmed the existence of the interlayer at both ends of In0.26Ga0.74As layer, with graded indium content and the thickness of 1.2–1.8nm.

Properties of surface‐pit related emission in a ‐plane InGaN/GaN quantum wells grown on r ‐plane sapphire

AbstractWe have investigated the optical and structural properties of non‐polar (11‐20) In0.03Ga0.97N/GaN multiple quantum wells (MQWs) grown on r‐plane sapphire substrates. The emission spectrum is characterised by a peak at ∼370 nm and a blue‐green emission band (400–550 nm). The near‐UV peak at 370 nm is assigned to carrier recombination in the MQWs lying on the (11‐20) plane. On the basis of microscopy and cathodoluminescence imaging, the longer wavelength band is attributed to emission from sidewall MQWs formed on the various semi‐polar facets of structural defects which form during MQW growth at the lower temperatures and high NH3 flow in a nitrogen atmosphere. The spectral width of this band is proposed to originate from electronic disorder within individual sidewall QWs and variations between the structural properties of QWs formed on different semi‐polar facets (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative Study of GaN—Based LED Grown on Different Substrates

The InGaN/GaN multiple quantum wells (MQWs) light emitting diodes (LEDs) were grown by metalorganic chemical vapor deposition (MOCVD) on silicon and sapphire substrates, respectively. The Optical and crystal properties of InGaN/GaN MQWs LEDs were investigated by room temperature photoluminescence (PL), temperature dependent PL measurements, Raman spectra and high-resolution double crystal X-ray diffraction(DCXRD). These results indicate that the crystal quality of InGaN/GaN MQWs growth on sapphire substrate are more preferable than that of InGaN/GaN MQWs growth on silicon substrate, and the interface of MQWs growth on substrate or silicon substrate is level. The peak positions of InGaN/GaN MQWs are 2.78 eV (446nm) and 2.64 eV (468.8nm) growth on sapphire substrate and silicon substrate, respectively.

Post and <i>In Situ</i> Characterization of Strain Control and Crystal Quality in Quantum Well Solar Cell Structure

To improve current matching in a tandem solar cell, a strain-compensated InGaAs/GaAsP multiple quantum-wells (MQWs) structure was grown within the middle GaAs PN junction, using metal organic vapor phase epitaxy (MOVPE) on GaAs substrate. Aiming at an accurate design of adsorption edge and precise control of crystal quality in MQWs, post- and in situ characterization was applied in such a fabrication process. Here, we report some applications of some post-characterization such as: X-ray scanning and reciprocal mapping in clarifying composition and crystal quality in these structures; photoluminescence (PL) and FTIR in determining adsorption edge and even indicate some irradiative recombination features as well. By employing an in situ optical surface reflectivity measurement, we established a way of and evaluating an instant of strain relaxation in the course of MOVPE, which deteriorates crystal quality significantly. When strain balancing was incomplete, surface reflectivity dropped during the growth of MQWs, indicating lattice relaxation. The accumulated strain, which is defined as the average strain per period of QW multiply the number of stacking for MQWs , was roughly constant for all the MQWs samples in our experiment. Therefore, it may indicate that this overall strain may be used as a tentative criterion of critical value for lattice relaxation or to predict the maximum number of MQWs for a given value of the average strain per QW period. Combining both post and in situ characterizations, we can effectively adjust the overall strain to get defects free growth for MQWs, and also significant features can be observed for better understanding the heterostructure management.