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.     

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.     

Influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells grown by MOCVD

We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces.     

The injected carbon impurities in intermixed GaAs/AlAs multiple quantum wells during thermal treatment

Photoluminescence (PL) measurements were performed in order to investigate the carbon impurity effects on the intermixing behavior of GaAs/AlAs multiple quantum wells (MQWs) grown by molecular beam epitaxy. The GaAs/AlAs MQWs were annealed with a carbon source in a furnace annealing system. The PL spectra show that the magnitude of the intermixing of Al and Ga induced by thermal annealing in GaAs/AlAs MQWs increases with depth. The nonuniformity of the intermixing as a function of the depth originated from the carbon impurities which were injected during thermal treatment.     

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.     

Influence of Si doping on the optical and structural properties of InGaN films

Influence of Si doping on the optical and structural properties of InGaN epilayers with different Si concentrations was investigated in detail by means of high-resolution X-ray diffraction (HRXRD), scanning electron microscope (SEM), Cathodoluminescence (CL) and photoluminescence (PL). It was found that a small amount of Si doping in InGaN could enhance luminescence intensity, improve the crystal quality of InGaN and suppress the formation of V-defects in InGaN. Further investigation by CL showed that V-defects act as nonradiative center, which lower the luminescence efficiency of InGaN. Based on above-mentioned results, one possible mechanism of influence of Si doping on the formation of V-defects in InGaN was also proposed in this paper.     

Room-temperature luminescence study on the effect of Mg activation annealing on p-GaN layers grown by MOCVD

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10¹⁷ cm⁻³ at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.     

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.     

Chemical mapping of InGaN MQWs

High power LEDs fabricated from InGaN/GaN layers have received much research interest. Hence, in this paper we identify structural and chemical defects resulting from the epitaxial growth of these layers, which directly effect the performance of the device. TEM, annular dark field imaging (ADF), energy filtered TEM (EFTEM) and X-ray mapping were used to study multiple quantum wells structures capped with a p-type GaN layer. TEM and ADF studies of the samples show a number of V-defects which are roughly 100–200 nm apart along the MQW. Each V-defect incorporates a pure edge ( ) dislocation, which runs through the apex of the V-defect up to the free surface. These V-defects contain GaN with no InGaN layers, suggesting that the capping layer has filled in the open V-defects.     

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.     

Growth of Eu-doped GaN by gas source molecular beam epitaxy and its optical properties

Eu-doped GaN with various Eu concentrations were grown by gas source molecular beam epitaxy, and their structural and optical properties were investigated. With increasing Eu concentration from 0.1 to 2.2 at%, deterioration of the structural quality was observed by reflection high-energy electron diffraction, atomic force microscopy and X-ray diffraction. Such a deterioration may be caused by an enhancement of island growth and formation of dislocations. On the other hand, room temperature photoluminescence spectra showed red emission at 622 nm due to an intra-atomic f–f transition of Eu³⁺ ion and Fourier transform infrared spectra indicated an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the increasing behavior was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with relation to the defect.     

Selective area epitaxy of InGaN quantum well triangular microrings with a single type of sidewall facets

Triangular microrings have been formed by selective area epitaxy of GaN and InGaN quantum wells (QWs) on patterned (0 0 0 1) AlN/sapphire. SiO₂ patterns consist of triangular ring openings oriented with edges parallel to two different orientations. InGaN QW microrings with each edge parallel to the 〈1 1? 0 0〉 direction have very rough sidewalls while microrings with each edge parallel to the 〈1 1 2¯ 0〉 direction exhibit well formed and smooth sidewalls as a result of the generation of a single type of {1 1? 0 1} facets on the inner and outer sidewalls. These {1 1? 0 1} facets demonstrate similar cathodoluminescence (CL) spectra that appear to be the superposition of two peaks at photon energies ∼2.5 eV (500 nm) and 2.7 eV (460 nm). Moreover, spatially matched striations are observed in the CL intensity images and surface morphologies of the {1 1? 0 1} sidewall facets. The observed striations are found to be related to subtle surface morphologies of the underlying GaN structures.     

Luminescence studies of defects and piezoelectric fields in InGaN/GaN single quantum wells

Transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL) and photoluminescence (PL) studies were performed on a 30 nm GaN/2 nm In_0.28Ga _0.72N/2 μm GaN/(0 0 0 1) sapphire single quantum well (SQW) sample. SEM-CL was performed at low temperatures ≈8 K, and at an optimum accelerating voltage, around 4–6 kV to maximise the quantum well (QW) luminescence. The CL in the vicinity of characteristic “V-shaped” pits was investigated. The near band edge (BE) luminescence maps from the GaN showed bright rings inside the boundaries of the pits while the QW luminescence maps showed pits to be regions of low intensity. These observations are consistent with TEM observations showing the absence of QW material in the pits. Variations in both the BE and QW maps in the regions between the pits are ascribed to threading edge dislocations. The CL and PL QW luminescence was observed to blue-shift and broaden with increasing excitation intensity. This was accompanied by decreasing spatial resolution in the CL QW maps implying an increasing carrier diffusion length in the InGaN layer. The reasons for this behaviour are discussed. It is argued that screening of the piezoelectric field in the material may account for these observations.     

Influence of various activation temperatures on the optical degradation of Mg doped InGaN/GaN MQW blue LEDs

GaN-based InGaN/GaN multi-quantum-well light emitting diode (MQW LED) structures were grown by metal organic chemical vapor deposition method. The optical properties of the LED structure have been investigated by using the photoluminescence and electroluminescence measurement. Both photoluminescence and electroluminescence results indicate that near pure InN clusters exist within the InGaN layers, which are responsible for the light emission in the LED. With increasing the Mg activation temperature of p-GaN layer, the optical properties of the LED structure tended to significantly degrade. This degradation was found to be deeply related to the variation of InN clusters in the active region. By the current–voltage measurement, a large forward voltage variation was observed. The voltage variation is caused to the conductivity variation of the p-GaN layer due to the different activation temperature. The turn-on voltage obtained from the best LED was 2.56 V and the forward voltage measured at 20 mA was 3.5 V. On the basis of these results, activation of the Mg-doped p-GaN layer must be carried out at the lowest possible value so as to obtain the better performance of LEDs.     

激发功率密度和阱层厚度对极化InGaN/GaN多量子阱光致发光性能的影响

用MOCVD技术在c面蓝宝石衬底上生长了具有不同阱层厚度的InGaN/GaN多量子阱结构,研究了阱层厚度和激发功率密度对多量子阱光致发光(PL)性能的影响.结果表明,随着激发功率密度的增加,PL的峰值波长会出现不同程度的蓝移,且阱层越厚,蓝移的越明显.PL的峰值波长随着激发功率密度和阱厚的变化关系可以用光生载流子对极化场的屏蔽效应和带隙填充效应来解释.阱最薄的样品(1.8 nm)由于其极化效应最弱,电致发光谱具有最高的发光强度,但其发光波长较短仅有430 nm.     

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.     

Influence of AlGaN/GaN/InGaN superlattice on the characteristics of LEDs grown by metalorganic chemical vapor deposition

This study examined the influence of strain-compensated triple AlGaN/GaN/InGaN superlattice structures (SLs) in n-GaN on the structural, electrical and optical characteristics of LEDs by analyzing the etch pits density (EPD), stress measurement, high-resolution X-ray diffraction (HRXRD), sheet resistance, photoluminescence (PL) and light–current–voltage (L–I–V). EPD, stress measurement and HRXRD studies showed that the insertion of AlGaN/GaN/InGaN SLs during the growth of n-GaN effectively distributed and compensated for the strong compressive stress, and decreased the dislocation density in n-GaN. The operating voltage at 20 mA for the LEDs grown with SLs decreased to 3.18 V from 3.4 V for the LEDs grown without SLs. In addition, a decrease in the spectral blue shift compared to the LEDs grown without SLs was observed in the LEDs grown with the SLs.     

Growth of thick GaInN on grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC

We succeeded in growing high-crystalline-quality thick (1 0 1¯ 1¯) Ga_0.92In_0.08N films on a grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580 nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure.     

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.     

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.