Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells

The effects of the growth temperature and ambient of GaN quantum barriers on the characteristics of InGaN/GaN multi-quantum wells (MQWs) grown by a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated. The improvement of optical, structural properties and surface morphology in the MQWs with increasing the growth temperature of quantum barriers was found. Without a GaN capping layer, there were many pits and the thickness of quantum pair reduced by the thermal etching during the temperature-ramping process. Photoluminescence (PL) peaks showed a blue-shift and double peaks, but relative PL intensity abruptly increased due to the suppression of deep level related defects and smooth surface morphology caused by the increased surface mobility of adatom in the high temperature region. By using a GaN capping layer on the InGaN well layer, the thermal decomposition of the InGaN well layer was suppressed and pits on the surface abruptly reduced. A hydrogen carrier gas for the GaN barrier growth also improved the optical and structural properties of MQWs.     

X-ray and cathodoluminescence study on the effect of intentional long time annealing of the InGaN/GaN multiple quantum wells grown by MOCVD

GaN-based InGaN/GaN multiple quantum wells (MQWs) structure having a high-quality epilayer and coherent periodicity was grown by metalorganic chemical vapor deposition. After thermal annealing of InGaN/GaN MQWs, the increase in temperature and annealing time caused the intermixing between the barrier and the wells, which in turn caused a decrease in periodicity on the high-resolution X-ray diffraction patterns. Thereby, we confirmed that the structural performance of InGaN MQWs is successively degrading with increasing thermal annealing temperature. Especially, InGaN MQWs of the sample annealed at 950 °C were profoundly damaged. The cathodoluminescence (CL) measurement indicated that MQWs emission intensity decreases with increasing thermal annealing temperature. Thus, the integrated CL intensity ratio of InGaN MQWs to GaN dramatically decreased while thermal annealing temperatures increased. This result caused the intermixing in MQWs to deteriorate the active layer performance. Furthermore, the peak position of MQWs showed a tendency of the red shift after high thermal annealing. It is suggested that the annealing-induced red shift in MQWs is attributed to the reduction of the inhomogeneity of the In content in the MQWs leading to the reduction of the quantized energies. Consequently, it indicates that the high temperature and the long-time thermal annealing would be inevitably followed by the structural destruction of InGaN MQWs.     

Intersubband transition at 1.55 μm in AlN/GaN multiple quantum wells by metal organic vapor phase epitaxy using the pulse injection method at 770 °C

Intersubband transition (ISBT) at 1.55 μm in AlN/GaN multi quantum wells (MQWs) was realized by metal organic vapor phase epitaxy (MOVPE) using the pulse injection (PI) method to grow GaN well layers at 770 °C. It was shown that a main factor for shifting ISBT wavelength to shorter region to cover 1.55 μm and improving ISBT properties of MQWs is the growth temperature of MQWs. Best structural and ISBT properties are observed at low growth temperature of 770 °C in this study. Carbon incorporation level in GaN layer grown by the PI method (PI-GaN) showed one order smaller value compared with that by the conventional continuous method. Moreover, further decrease in growth temperature to 770 °C did not show significant increase in carbon incorporation in PI-GaN layer. It clearly indicates that the PI method is very effective in reducing carbon concentration in GaN layer, especially at low temperature region. The low carbon concentration of 4×10¹⁸ cm^?3 released by the PI method was indispensable for realizing enough carrier concentration of 1.6×10¹⁹ cm^?3 to achieve strong ISBT at 1.55 μm.     

Cathodoluminescence study of the excitons localization in AlGaN/GaN and InGaN/GaN quantum wells grown on sapphire

Al_0.1Ga_0.9N(5 nm)/GaN(2 nm) and In_0.2Ga_0.8N/GaN quantum wells (QWs) grown on GaN/sapphire have been studied by cathodoluminescence (CL) spectroscopy and imaged using an experimental setup especially developed for scanning near-field CL microscopy, which combines a scanning force microscope and a scanning electron microscope. The CL spectra show the characteristic band edge emission peak of GaN at λ= 364 nm and the emission peaks related to the presence of QWs, at λ= 353 and 430 nm for the AlGaN/GaN and the InGaN/GaN samples, respectively. Monochromatic CL images reveal that the emission of the AlGaN/GaN and InGaN/GaN QWs is localized at the level of the grains observed by SFM. A cross sectional analysis of the InGaN/GaN sample gives insight into its growth and an estimation of the exciton diffusion length of about L=180 nm.     

TMIn流量对GaN基蓝光LED外延薄膜的影响

以蓝宝石(Al2O3)为衬底,采用有机金属化学气相沉积(MOCVD)技术生长InGaN/GaN多量子阱结构.本文通过调整外延生长过程中三甲基铟(TMIn)流量,研究了TMIn流量对InGaN/GaN多量子阱结构的合金组分、晶体质量和光学性质的影响.本文采用高分辨X射线衍射(HRXRD)、原子力显微镜(AFM)和光致发光(PL)测试表征其结构和光学性质.HRXRD测试结果表明,随TMIn流量增加,"0"级峰与GaN峰之间角偏离增大,更多的In并入薄膜中.HRXRD与AFM表征结果表明:增大TMIn流量会导致外延薄膜中的位错密度增大,V形坑数量增加,晶体质量严重恶化;PL测试结果表明,随着TMIn流量增加,发光强度逐渐降低,半高宽增大,这是由于晶体质量恶化所导致.因此严格控制铟源流量对于改善量子阱薄膜的晶体质量与光学性质有着至关重要的作用.     

Improved quality of InGaN/GaN multiple quantum wells by a strain relief layer

Different InGaN/GaN multi quantum wells (MQWs) structures were grown by metalorganic chemical vapor deposition (MOCVD). Samples were investigated by photoluminescence (PL), atom force microscopy (AFM) and double crystal X-ray diffractometry (DCXRD) to character their optical, morphological and crystal properties. By inserting the strain relief layer, the PL intensity was increased more than two times. The surface morphology was improved and the density of V-pits was reduced from 16–18×10⁸ to 6–7×10⁸/cm². Further, the interface abruptness was also improved. We attributed the improvements of the quality of InGaN/GaN MQWs to the relief of strain in the InGaN/GaN MQWs.     

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3 s at 610 °C.     

Role of vapor-phase diffusion in selective-area MOVPE of InGaN/GaN MQWs

Selective-area growth (SAG) of InGaN/GaN multiple quantum wells (MQWs) was performed by metalorganic vapor phase epitaxy (MOVPE). The layers of a blue light-emitting diode (LED), that includes five InGaN quantum wells, were grown on a patterned GaN template on a sapphire substrate. In order to elucidate the contribution of vapor-phase diffusion of group-III precursors to the in-plane modulation of luminescence wavelength, the width of a stripe selective growth area was 60 μm that is sufficiently larger than the typical surface diffusion length, with the mask width varied stepwise between 30 and 240 μm. The distribution of the luminescence wavelength from the MQWs was measured with cathode luminescence (CL) across the stripe growth area. The peak wavelength ranged between 420 and 500 nm. The peak shifted to longer wavelengths and became broader as the measured point approached to the mask edge. Such a shift in the peak wavelength exhibited parabolic profile in the growth area and the wider mask shifted the entire peak positions to longer wavelengths. These trends clearly indicate that the vapor-phase diffusion play a dominant role in the in-plane modulation of the luminescence wavelength in the SA-MOVPE of InGaN MQWs, when the size of a growth area and/or the mask width exceeds approximately 10 μm.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

High quality InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

High quality InAlN/GaN heterostructures are successfully grown on the (0 0 0 1) sapphire substrate by pulsed metal organic chemical vapor deposition. The InAlN barrier layer with an indium composition of 17% is observed to be nearly lattice matched to GaN layer, and a smooth surface morphology can be obtained with root mean square roughness of 0.3 nm and without indium droplets and phase separation. The 50 mm InAlN/GaN heterostructure wafer exhibits a mobility of 1402 cm²/V s with a sheet carrier density of 2.01×10¹³  cm^?2, and a low average sheet resistance of 234 Ω/cm² with a sheet resistance nonuniformity of 1.22%. Compared with the conventional continual growth method, PMOCVD reduces the growth temperature of the InAlN layer and the Al related prereaction in the gas phase, and consequently enhances the surface migration, and improves the crystallization quality. Furthermore, indium concentration of InAlN layer can be controlled by adjusting the pulse time ratio of TMIn to TMAl in a unit cycle, the growth temperature and pressure, as well as the InAlN layer thickness by the number of unit cycle repeats.     

Metalorganic vapor phase epitaxy and characterizations of nearly-lattice-matched AlInN alloys on GaN/sapphire templates and free-standing GaN substrates

The epitaxy optimization studies of high-quality n-type AlInN alloys with different indium contents grown on two types of substrates by metalorganic vapor phase epitaxy (MOVPE) were carried out. The effect of growth pressure and V/III molar ratio on growth rate, indium content, and surface morphology of these MOVPE-grown AlInN thin films were examined. The surface morphologies of the samples were characterized by scanning electron microscopy and atomic force microscopy. By varying the growth temperatures from 860 °C to 750 °C, the indium contents in AlInN alloys were increased from 0.37% up to 21.4% as determined by X-ray diffraction (XRD) measurements. The optimization studies on the growth conditions for achieving nearly-lattice-matched AlInN on GaN templates residing on sapphire and free-standing GaN substrates were performed, and the results were analyzed in a comparative way. Several applications of AlInN alloy for thermoelectric and light-emitting diodes are also discussed.     

Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition

Various techniques for morphological evolution of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been evaluated. Atomic force microscopy, photoluminescence (PL) and X-ray diffraction measurements have been used for characterization. It is shown that inclusions, that are generated into the V-defects in the InGaN quantum wells (QW), can be removed by introducing a small amount of hydrogen during the growth of GaN barriers. This hydrogen treatment results in partial loss of indium from the QWs, but smooth surface morphology of the MQW structure and improved optical quality of InGaN wells are obtained. The density of the V-defects could be reduced by reducing the dislocation density of the underlying GaN buffer.     

Low temperature II–VI nanowire growth using Au–Sn catalysts

Epitaxial lateral overgrowth is reported for semi-polar (Al,Ga)N(1 1 .2) layers. The mask pattern consisted of periodic stripes of SiO₂ oriented parallel to either the GaN[1 1 .0] or the GaN[1 1 .1] direction. Lateral growth occurred either along GaN[1 1 .1] or along GaN[1 1 .0]. For growth along the [1 1 .0] direction, coalescence was achieved for layer thicknesses >4 μm. However, planarization was not observed yielding extremely corrugated surfaces. For growth in [1 1 .1] direction, coalescence was delayed by a diminishing lateral growth rate. Growth of AlGaN during ELOG resulted in coalescence. Improvement in crystal quality of such buffer layers for the growth of InGaN/GaN quantum wells was confirmed by X-ray diffraction and photoluminescence spectroscopy.     

Growth and optical characterisation of multilayers of InGaN quantum dots

We report on the growth (using metal-organic vapour phase epitaxy) and optical characterisation of single and multiple layers of InGaN quantum dots (QDs), which were formed by annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The distribution of nanostructure heights of the final QD layer of three is wider and is centred around a larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K of capped samples have confirmed the QD nature of the capped nanostructures by the observation of sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer. We have also observed much more QD emission per unit area in a sample with three QD layers, than in a sample with a single QD layer, as expected.     

Suppression of phase separation in InGaN layers grown on lattice-matched ZnO substrates

Sapphire and SiC are typical substrates used for GaN growth. However, they are non-native substrates and result in highly defective materials. The use of ZnO substrates can result in perfect lattice-matched conditions for 22% indium InGaN layers, which have been found to suppress phase separation compared to the same growths on sapphire. InGaN layers were grown on standard (0 0 0 2) GaN template/sapphire and (0 0 0 1) ZnO substrates by metalorganic chemical vapor deposition. These two substrates exhibited two distinct states of strain relaxation, which have direct effects on phase separation. InGaN with 32% indium exhibited phase separation when grown on sapphire. Sapphire samples were compared with corresponding growths on ZnO, which showed no evidence of phase separation with indium content as high as 43%. Additional studies in Si-doping of InGaN films also strongly induced phase separation in the films on sapphire compared with those on ZnO. High-resolution transmission electron microscopy results showed perfectly matched crystals at the GaN buffer/ZnO interface. This implied that InGaN with high indium content may stay completely strained on a thin GaN buffer. This method of lattice matching InGaN on ZnO offers a new approach to grow efficient emitters.     

Growth of bulk InGaN films and quantum wells by atmospheric pressure metalorganic chemical vapour deposition

InGaN bulk layers and single quantum wells were grown on 1.4 to 2.4 μm thick GaN on sapphire films by atmospheric pressure metalorganic chemical vapour deposition (AP-MOCVD). The morphology of the epitaxial layers was strongly affected by the V/III ratio in the gas phase. The incorporation efficiency of indium was observed to increase with higher growth rates and decreasing temperature, but was independent of the V/III ratio in the investigated parameter range. In_0.16Ga_0.84N single quantum wells showed intense quantum well related luminescence at room temperature, with a full width at half maximum of 7.9 nm at a thickness of 50 Å. Single quantum wells embedded in InGaN of graded composition showed superior properties compared to quantum wells with In_0.04Ga_0.94N barriers of constant composition.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.     

Transmission electron microscopy investigation of AlN growth on Si(111)

AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy. Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth. X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.     

Growth of a-plane GaN on r-plane sapphire by self-patterned nanoscale epitaxial lateral overgrowth

We report on the use of a novel technique to grow the nonpolar a-plane GaN on r-plane sapphire by metal–organic chemical vapor deposition. A thin InGaN interlayer was deposited on the substrate followed by a low temperature (LT) GaN buffer layer. A stripe-like template was obtained by annealing the LT GaN/InGaN layers at 1100 °C for 2 min. This special template facilitated the nanoscale epitaxial lateral overgrowth of a-plane GaN. Scanning electron microscopy shows that the surface morphology was rather flat for a 1 μm-thick sample. The improvement in crystalline quality was also demonstrated by high-resolution x-ray diffraction, room temperature Raman spectroscopy and photoluminescence measurements. Compared with the traditional epitaxial lateral overgrowth technique, our technique greatly simplified the template preparing process and the crystalline quality of a-plane GaN was improved.     

Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy

The propagation properties of inversion domains (IDs) in InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been investigated by transmission electron microscopy (TEM). The majority of the IDs, originating from the sapphire and/or buffer layer, propagate through the MQWs with normal wurtzite structure retaining their original structural features. Some of IDs could induce V-shaped pits in the MQW structures proposing a new formation mechanism for the so-called V-shaped defects. Detailed measurements show that a few IDs are found to be stopped in abnormal MQW regions, where In droplets appear due to phase separation. We presented direct evidence of pure In-phase droplets by means of high-resolution TEM. The above results provide new information on the structural defects in InGaN/GaN-based materials.