红橙光InGaN/GaN量子阱的结构与光学性质研究

采用金属有机物化学气相沉积(MOCVD)技术生长了具有高In组分InGaN阱层的InGaN/GaN多量子阱(MQW)结构,高分辨X射线衍射(HRXRD)ω-2θ扫描拟合得到阱层In含量28%。比较大的表面粗糙度表明有很大的位错密度。室温下光致荧光(PL)研究发现该量子阱发射可见的红橙光,峰位波长在610 nm附近。变温PL(15～300 K)进一步揭示量子阱在低温下有两个发光机制,对应的发射峰波长分别为538 nm和610 nm。由于In分凝和载流子的局域化导致的载流子动力改变,使得量子阱PL发光峰值随温度增加呈明显的"S"变化趋势。     

Characterization of undoped and silicon-doped InGaN/GaN single quantum wells

We investigated the influence of doping and InGaN layer thickness on the emission wavelength and full width at half maximum (FWHM) of InGaN/GaN single quantum wells (SQW) of thicknesses between 1 nm and 5 nm by temperature and intensity resolved photoluminescence (PL). The crystalline quality of the GaN claddings was assessed by low temperature PL. The emission energy of 5 nm Si doped SQW could be tuned from 3.24 eV to 2.98 eV by reducing the deposition temperature. An increase of piezoelectric (PE) field screening with increasing deposition temperature is attributed to an increase of the SiH₄ decomposition efficiency. Piezoelectric (PE) fields between 0.5 MV/cm and 1.2 MV/cm in undoped structures of varying SQW thicknesses were calculated. Two activation energies of 15 meV and 46 meV of the SQW emission could be observed in temperature resolved measurements. The higher value was assigned to the confined exciton binding energy, whereas the activation energy of 15 meV is probably due to a decrease in carrier supply from the absorption zone in the GaN cladding into the SQW.     

InGaN/GaN MQW双波长LED的MOCVD生长

利用金属有机物化学气相淀积(MOCVD)系统生长了InGaN/GaN多量子阱双波长发光二极管(LED).发现在20 mA正向注入电流下空穴很难输运过蓝光和绿光量子阱间的垒层,这是混合量子阱有源区获得双波长发光的主要障碍.通过掺入一定量的In来降低蓝光和绿光量子阱之间的垒层的势垒高度,增加注入到离p-GaN层较远的绿光有源区的空穴浓度,从而改变蓝光和绿光发光峰的强度比.研究了蓝光和绿光量子阱间垒层In组分对双波长LED的发光性质的影响.此外,研究了双波长LED发光特性随注入电流的变化.     

InGaN/GaN多量子阱LED电致发光谱中双峰起源的研究

研究了MOCVD生长的具有双发射峰结构的InGaN/GaN多量子阱发光二极管(LED)的结构和发光特性.在透射电子显微镜(TEM)下可以发现量子阱的宽度不一致,电致发光谱(EL)发现了位于2.45eV的绿光发光峰和2.81eV处的蓝光发光峰.随着电流密度增加,双峰的峰位没有移动,直到注入电流密度达到2×104 mA/cm2时,绿光发光峰发生蓝移,而蓝光发光峰没有变化.单色的阴极荧光谱(CL)发现绿光发射对应的发光区包括絮状区域和发光点,而蓝光发射对应的发光区仅包含絮状区域.通过以上的结果,我们认为蓝光发射基本上源于InGaN量子阱发光,而绿光发射则起源于量子阱和量子点的发光.     

A monolithic white LED with an active region based on InGaN QWs separated by short-period InGaN/GaN superlattices

A new approach to development of effective monolithic white-light emitters is described based on using a short-period InGaN/GaN superlattice as a barrier layer in the active region of LED structures between InGaN quantum wells emitting in the blue and yellow-green spectral ranges. The optical properties of structures of this kind have been studied, and it is demonstrated that the use of such a superlattice makes it possible to obtain effective emission from the active region.     

Characterization of quaternary AlInGaN epilayers and polarization-reduced InGaN/AlInGaN MQW grown by MOCVD

We have demonstrated the growth of quaternary AIlnGaN compounds at different growth temperatures and pressures with metalorganic chemical vapor deposition (MOCVD). The optical properties of the samples have been investigated by photoluminescence (PL) at different temperatures. The results show that the sample grown at higher temperature (850℃) exhibits the best optical quality for its sharp band edge luminescence and weak yellow luminescence. The AlInGaN exhibited three-dimensional (3D) growth mode at higher pressure. The band edge emission almost disappeared. With the optimization of AlInGaN growth parameters, we replaced the traditional barrier in InGaN/GaN multiple quantum wells (MQWs) with AlInGaN barriers. The peak wavelength for the InGaN/AlInGaN-MQW based light emitting diodes (LEDs) was very stable at various injection current levels because of the polarization-matched InGaN/AlInGaN MQWs.     

Emission spectra of InGaN/AlGaN/GaN quantum well heterostructures: Model of the two-dimensional joint density of states

The luminescence spectra of light emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are analyzed in the context of a model of the two-dimensional density of states in the active region. The model accounts for the potential fluctuations, the statistics of occupation of the wells with charge carriers, and specific features of the extraction of radiation from the structure. The model describes the position of the maxima of the spectra and the exponential decline of the emission intensity in the short-and long-wavelength regions as well as the modification of the spectra under variations in the current. The problems of limitations of the model and the physical meaning of the parameters are discussed. The examples of approximation of the spectra of blue light emitting diodes based on InGaN/AlGaN/GaN heterostructures show the necessity of determining the temperature in the active region independently and taking into account the interference in the planar structure. The differences of the shape of the spectra from that obtained in the simple model depend not only on the properties of the quantum wells but also on the nonuniformities in the distribution of In in InGaN.     

Lasing in Cd(Zn)Se/ZnMgSSe heterostructures pumped by nitrogen and InGaN/GaN lasers

Photoluminescence and lasing at a wavelength of λ=510–530 nm (green spectral region) in Cd(Zn)Se/ZnMgSSe structures with a different design of the active region are studied in a wide range of temperatures and nitrogen laser pump intensities. A minimal lasing threshold of 10 kW/cm², a maximal external quantum efficiency of 12%, and a maximal output power of 20 W were obtained for the structure with the active region composed of three ZnSe quantum wells with fractional-monolayer CdSe inserts. The lasers exhibited a high temperature stability of the lasing threshold (characteristic temperature T ₀=330 K up to 100°C). For the first time, an integrated converter composed of a green Cd(Zn)Se/ZnMgSSe laser optically pumped by a blue InGaN/GaN laser that is grown on a Si (111) substrate and incorporates multiple quantum wells is suggested and studied.     

Blue lasing at room temperature in an optically pumped lattice-matched AlInN=GaN VCSEL structure

Laser action with low threshold average pump power density (~50 W - cm^-2 ) at room temperature is reported for a crack-free planar vertical cavity surface emitting laser (VCSEL) structure based on a bottom lattice-matched AllnN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. In addition to low threshold, a spontaneous emission coupling factor beta ~ 2 x 10^-10 is derived for this ready-to-be-processed laser structure.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

InGaN/GaN multi-quantum well distributed Bragg reflector laser diode with second-order gratings

Device fabrication and measurement results of an electrically injected distributed Bragg reflector InGaN/GaN laser operated at room temperature are presented. Using second-order gratings, the emission at 407.6 nm was achieved in the vertical direction, the device acting like a surface emitting laser. A voltage drop at threshold of 11.2 V and temperature stable emission with a wavelength shift of 0.0119 nm/K was obtained for the device.     

High efficiency integral III-N/II-VI blue-green laser converter

An integral optically-pumped laser converter comprising an ultra-low-threshold green (~540 nm) CdSe quantum dot laser chip pumped by a blue InGaN/GaN quantum well heterostructure laser grown on Si (111) substrates has been fabricated and studied. The maximum achieved quantum efficiency and pulse output power in green are as high as 14% and 3 W, respectively     

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Visible InGaN/GaN Quantum-Dot Materials and Devices

General properties of III-V nitride-based quantum dots (QDs) are presented, with a special emphasis on InGaN/GaN QDs for visible optoelectronic devices. Stranski-Krastanov GaN/AlN dots are first discussed as a prototypical system. It is shown that the optical transition energies are governed by a giant quantum-confined Stark effect, which is the consequence of the presence of a large built-in internal electric field of several MV/cm. Then we move to InGaN/GaN QDs, reviewing the different fabrication approaches and their main optical properties. In particular, we focus on InGaN dots that are formed spontaneously by In composition fluctuations in InGaN quantum wells. Finally, some advantages and limitations of nitride laser diodes with active regions based on InGaN QDs are discussed, pointing out the requirements on dot uniformity and density in order to be able to exploit the expected quantum confinement effects in future devices.     

Analysis of the temperature dependence of the capacitance-voltage characteristics of InGaN/GaN multiple quantum well light-emitting structures

The capacitance-voltage characteristics and frequency dependences of the capacitance and conductance of InGaN/GaN multiple quantum well light-emitting structures are studied in the frequency range of 60 Hz-5 MHz and the temperature range of 77–300 K. It is shown that carrier relaxation in quantum wells can be described by two emission processes, i.e., the thermally activated one and with the power-law temperature dependence of the emission rate. It is also shown that one or several quantum wells in typical InGaN/GaN-based light-emitting structures can remain filled with electrons even at comparatively high reverse biases. This makes it possible to explain the depth shift of the apparent carrier concentration profiles, obtained from the capacitance-voltage characteristics, with decreasing temperature.     

InGaN/GaN和InGaN/AlGaN多量子阱中应变对光致发光特性的影响

对蓝宝石衬底上的InGaN/GaN和InGaN/AlGaN多量子阱结构和经激光剥离去除衬底的InGaN/GaN和InGaN/AlGaN多量子阱结构薄膜样品,进行了光致发光谱、高分辨XRD和喇曼光谱测量.PL测量结果表明,相对于带有蓝宝石衬底的样品,InGaN/GaN多量子阱薄膜样品的PL谱峰值波长发生较小的蓝移,而InGaN/AlGaN多量子阱薄膜样品的PL谱峰值波长发生明显的红移;喇曼光谱的结果表明,激光剥离前后E2模的峰值从569.1减少到567.5cm-1.这说明激光剥离去除衬底使得外延层整体的压应力得到部分释放,但InGaN/GaN与InGaN/AlGaN多量子阱结构中阱层InGaN的应力发生了不同的变化.XRD的结果证实了这一结论.     

Effect of shroud flow on high quality InxGa1−xN deposition in a production scale multi-wafer-rotating-disc reactor

High quality InGaN thin films and InGaN/GaN double heterojunction (DH) structures have been epitaxially grown on c-sapphire substrates by MOCVD in a production scale multi-wafer-rotating-disc reactor between 770 to 840°C. We observed that shroud flow (majority carrier gas in the reaction chamber) is the key to obtaining high quality InGaN thin films. High purity H₂ as the shroud flow results in poor crystal quality and surface morphology but strong photolumines-cence (PL) at room temperature. However, pure N₂ as the shroud flow results in high crystal quality InGaN with an x-ray full width at half maximum (FWHM)_InGaN(0002) of 7.5 min and a strong room temperature PL peaking at 400 nm. In addition, InGaN/GaN single heterojunction (SH) and DH structures both have excellent surface morphology and sharp interfaces. The full width at half maximum of PL at 300K from an InGaN/GaN DH structure is about 100 meV which is the best reported to date. A high indium mole fraction in InGaN of 60% and high quality zinc doped InGaN depositions were also achieved.     

预应变InGaN/GaN多量子阱发光特性调控研究

为了减弱InGaN/GaN量子阱内的压电极化场,在蓝紫光InGaN/GaN多量子阱激光器结构中采用了预应变InGaN插入层,通过变温电致发光和高分辨X射线衍射测量研究了预应变插入层对量子阱晶体质量和发光特性的影响。实验结果显示,常温下有预应变层的量子阱电致发光谱积分强度显著提高。模拟计算进一步表明,预应变层对量子阱内压电极化场有调制效果,有利于量子阱中的应力弛豫,可以有效减弱量子限制斯塔克效应,有助于提高量子阱的发光效率。     

InGaN/GaN quantum well growth on pyramids of epitaxial lateral overgrown GaN

InGaN/GaN quantum wells (QW) were grown by metalorganic chemical vapor deposition (MOCVD) on pyramids of epitaxial lateral overgrown (ELO) GaN samples. The ELO GaN samples were grown by MOCVD on sapphire (0001) substrates that were patterned with a SiN_x mask. Scanning electron microscopy and cathodoluminescence (CL) imaging experiments were performed to examine lateral variations in structure and QW luminescence energy. CL wavelength imaging (CLWI) measurements show that the QW peaks on the top of the grooves are red-shifted in comparison with the QW emission from the side walls. The results show that In atoms have migrated to the top of the pyramids during the QW growth. The effects of V/III ratio, growth temperature as well as ELO GaN stripe orientation on the QW properties are also studied.     

Influence of Mg Doping on the Morphological, Optical, and Structural Properties of InGaN/GaN Multiple Quantum Wells

In this report, the influence of magnesium doping on the characteristics of InGaN/GaN multiple quantum wells (MQWs) was investigated by means of atomic force microscopy (AFM), photoluminescence (PL), and X-ray diffraction (XRD). Five-period InGaN/GaN MQWs with different magnesium doping levels were grown by metalorganic chemical vapor deposition. The AFM measurements indicated that magnesium doping led to a smoother surface morphology. The V-defect density was observed to decrease with increasing magnesium doping concentration from ∼10⁹ cm⁻² (no doping) to ∼10⁶ cm⁻² (Cp₂Mg: 0.04 sccm) and further to 0 (Cp₂ Mg: 0.2 sccm). The PL measurements showed that magnesium doping resulted in stronger emission, which can be attributed to the screening of the polarization-induced band bending. XRD revealed that magnesium doping had no measurable effect on the indium composition and growth rate of the MQWs. These results suggest that magnesium doping in MQWs might improve the optical properties of GaN photonic devices.