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.     

Improvement of GaAs/AlGaAs quantum well laser diodes by rapid thermal annealing

Post-growth annealing is shown to improve the laser diode quality of GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures grown at a nonoptimal substrate temperature lower than 680°C by molecular beam epitaxy. Reduction by a factor of up to three in the threshold current was accompanied by a reduction in the interface trap density. The reduced threshold current is still higher than that of laser diodes grown at the optimal temperatures which are between 680 and 695°C. The improvement in laser diode performance is ascribed to the reduction of interface nonradiative recombination centers.     

InGaN/GaN multiple quantum well green light-emitting diodes prepared by temperature ramping

High-quality InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) structures were prepared by a temperature-ramping method during metal-organic chemical-vapor deposition (MOCVD) growth. Two photoluminescence (PL) peaks, one originating from well-sensitive emission and one originating from an InGaN quasi-wetting layer on the GaN-barrier surface, were observed at room temperature (RT). The observation of high-order double-crystal x-ray diffraction (DCXRD) satellite peaks indicates that the interfaces between InGaN-well layers and GaN-barrier layers were not degraded as we increased the growth temperature of the GaN-barrier layers. With a 20-mA and 160-mA current injection, it was found that the output power could reach 2.2 mW and 8.9 mW, respectively. Furthermore, it was found that the reliability of the fabricated green LEDs prepared by temperature ramping was also reasonably good.     

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.     

Observation of the quantum confined stark effect in ZnSe/ZnCdSe single quantum well systems

We report the observation of the quantum-confined Stark effect (QCSE) in ZnSe/ ZnCdSe single quantum wells grown by molecular beam epitaxy, using photoluminescence. In our experiments the electric field was applied via a reverse-biased Schottky barrier contact. To our knowledge, this is the first observation of the QCSE in any wide gap II-VI semiconductor heterostructure. Significant red shifts, typically 10–15 meV, are detected before quenching. An associated reduction in the transition intensity, consistent with the QCSE. is clearly observed. The dependence of these results will be discussed as a function of quantum well depth and thickness. Complete quenching of the luminescence is observed with applied voltages as low as 5 V. In addition, at lowest voltages, we also detect small blue shifts (up to 4 meV), which we attribute to the interaction between the externally applied electric field and the built-in field of the structure.     

Contactless electroreflectance study of a GaAIAs/lnGaAs/ GaAs/GaAIAs step quantum well structure

Using contactless electroreflectance at 300 and 77K, we have studied the inter-subband transitions from a GaAlAs/InGaAs/GaAs/GaALAs step quantum well structure (small well inside a large well) consisting of two layers A (In_xGa_1−xAs) and B (GaAs) with widths L_A and L_B, respectively, bounded by two thick barrier regions of Ga_x Al_yAs. By comparison of the observed spectral features with an envelope function calculation, including the effects of strain, we have been able to characterize the potential profile of the structure, i.e., L_A, L_B, x, and y. There is very good agreement between experiment and the intended materials param-eters. Such configurations are of considerable importance since (a) they form the basis for pseudomorphic high electron mobility transistors, and (b) also have applications in optoelectronics due to their large Stark shifts.     

GaN基蓝光LED的多量子阱结构优化

基于量子阱结构中载流子遂穿势垒原理,使用APSYS软件模拟不同条件下具有不同垒高、垒宽及阱宽的发光二极管（LED）的I-V特性、光强和内量子效率（IQE）的变化,发现自发发射光谱存在红移现象。通过与传统LED的多量子阱（MQW）参数比较发现,当阱宽为2 nm、垒宽为4 nm、垒中In含量为0.08和驱动电流为20mA时...     

纤锌矿GaN/AlxGa1-xN量子阱中激子能量

考虑了纤锌矿GaN/Al<,x>Ga<,1-x>N量子阱(QW)材料中空穴带质量和光学声子模的各向异性以及声子频率随波矢变化的效应,采用改进的LLP变分法计算了纤锌矿氮化物QW中激子的基态能量和结合能.给出了激子的基态能量和结合能随着QW宽度和Al组分变化的函数关系,并对闪锌矿和纤锌矿GaN/Al<0.3>Ga<,0....     

Magnetic resonance studies of GaN based light emitting diodes

We report the application of electrical detection of magnetic resonance (EDMR) and electroluminescence detection of magnetic resonance (ELDMR) to study the recombination processes in InGaN/AlGaN double heterostructure p-n junctions. These techniques are especially well suited to the problems of defects in device structures in that they are much more sensitive than conventional paramagnetic resonance and are responsive to only those defects involved in the electrooptical properties of the structure. One resonance is observed at g≈2.00 and is identified as a Zn-related acceptor trap in the InGaN layer. A second resonance with g≈1.99 is identified as a deep donor.     

Cu掺杂对ZnO量子点光致发光的影响

通过溶液法合成了Cu掺杂ZnO量子点。X射线衍射(XRD)和高分辨电子透射电镜(HRTEM)图像显示Cu掺杂ZnO量子点具有六角纤锌矿结构,晶粒大小为4～5nm。Cu掺杂抑制了ZnO量子点颗粒长大。室温光致发光(PL)谱观察到紫外带边和可见区两个发射峰。随着Cu掺杂浓度的增大,紫外荧光峰位发生缓慢红移,由366nm移到370nm;可见区发射峰位发生蓝移,由525nm移到495nm;同时,两个发射峰强度降低。光谱结果表明:Cu的掺入,一方面抑制表面与O空位有关的缺陷,在495nm出现了与Cu1+有关的发射峰;另一方面,Cu离子掺入ZnO量子点引入一些非辐射中心,降低了自由激子发射。     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J_sc of 29.24 mA/cm², open circuit voltage V_oc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with J_sc of 25.27 mA/cm², V_oc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with J_sc of 27.63 mA/cm², V_oc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.     

InP-based multiple quantum well structures grown with tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP): Effects of growth interruptions on structural and optical properties

In this paper, we investigate the effect of interfacial layers on GalnAs(P)/GalnAsP and GalnAs/InP multiple quantum well structures with x-ray diffraction and photoluminescence. We observe a decrease in the room temperature and low temperature photoluminescence intensity as the number of periods is increased which we attribute to the interfaces. Furthermore, different growth interruption schemes show that decomposed As species from TBA have an effect on the structural and optical quality of these structures at both the lower and upper interfaces due to As carry-over. The effect of this carry-over is shown in structural measurements and laser diode results.     

金属腔量子阱红外探测器共振波长的角度依赖性

测量了金属腔量子阱红外探测器在斜入射条件下的光电流谱,斜入射条件分为入射面垂直于器件长轴和平行于器件长轴两种情形.从实验和理论上研究了金属腔共振模对入射角度的依赖性.实验结果表明:入射面垂直于器件长轴时,腔模共振波长不随入射角度变化;入射面平行于器件长轴时,腔模共振波长随入射角度变大而向短波移动.测试结果和推导出的共振...     

Unusual temperature dependence of electroluminescence intensity in blue InGaN single quantum well diodes

Temperature dependence of electroluminescence (EL) spectral intensity of the super-bright blue InGaN single quantum well (SQW) light emitting diodes (LEDs) has been carefully investigated over a wide temperature range (T=15-300 K) and as a function of injection current level (0.1-10 mA) in comparison with high quality GaAs SQW-LEDs. When T is slightly decreased to 180 K, the EL intensity efficiently increases in both cases due to the reduced non-radiative recombination processes. However, further decreasing T below 100 K, striking differences exist in EL intensity as well as injection current dependences between the two types of diodes. That is, the EL efficiency at lower T is found to be quite low for the blue diode in strong contrast to that of red GaAs SQW-LED where significant enhancement of the EL efficiency persists down to 15 K. These results indicate that the carrier capture efficiency of the blue SQW diode is unusually worse at lower T than at T=180-300 K, reflecting the unique radiative recombination processes under the presence of high-density dislocation (10¹⁰ cm⁻²).     

Photoluminescence characteristics of GaN/lnGaN/GaN quantum wells

Photoluminescence (PL) characteristics of GaN/lnGaN/GaN single quantum wells (QWs) and an InGaN/GaN single heterojunction were studied using continuous wave (CW) and pulsed photoluminescence in both edge and surface emitting configurations. Samples were grown on c-plane sapphire substrates by atmospheric pressure metalorganic chemical vapor deposition (MOCVD). Room temperature and 77K PL measurements were performed using a CW Ar-ion laser (305 nm) and a frequency tripled (280 nm), pulsed, mode-locked Ti: sapphire laser. CW PL emission spectra from the quantum wells (24, 30, 80Å) were all blue shifted with respect to the reference sample. The difference (i. e., the blue shift) between the measured value of peak emission energy from the QW and the band-edge emission from the reference sample was attributed to quantum size effects, and to strain arising due to a significant lattice mismatch between InGaN and GaN. In addition, stimulated emission was observed from an InGaN/GaN single heterojunction in the edge and surface emitting configu-ration at 77K. The narrowing of emission spectra, the nonlinear dependence of output emission intensity on input power density, and the observation of a strongly polarized output are presented.     

Optical properties and structure of InAs quantum dots in near-infrared band

The InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are studied as a function of growth temperature at a specific InAs coverage of 2.7 ML. The QDs density is significantly reduced from 8.0 × 1010 to 5.0 × 109 cm-2 as the growth temperature increases from 480℃ to 520℃, while the average QDs diameter and height becomes larger. The effects of the growth temperature on the evolution of bimodal QDs are investigated by combining atomic force microscopy (AFM) and photoluminescence (PL). Results show that the formation of the bimodal QDs depends on the growth temperature: at a growth temperature of 480℃,large QDs result from the small QDs coalition; at a growth temperature of 535℃, the indium desorption and InAs segregation result in the formation of small QDs.     

Optical and electrical characterization of (Ga,Mn)N/InGaN multiquantum well light-emitting diodes

(Ga,Mn)/N/InGaN multiquantum well (MQW) diodes were grown by molecular beam epitaxy (MBE). The current-voltage characteristics of the diodes show the presence of a parasitic junction between the (Ga,Mn)N and the n-GaN in the top contact layer due to the low conductivity of the former layer. Both the (Ga,Mn)N/InGaN diodes and control samples without Mn doping show no or very low (up to 10% at the lowest temperatures) optical (spin) polarization at zero field or 5 T, respectively. The observed polarization is shown to correspond to the intrinsic optical polarization of the InGaN MQW, due to population distribution between spin sublevels at low temperature, as separately studied by resonant optical excitation with a photon energy lower than the bandgap of both the GaN and (Ga,Mn)N. This indicates efficient losses in the studied structures of any spin polarization generated by optical spin orientation or electrical spin injection. The observed vanishing spin injection efficiency of the spin light-emitting diode (LED) is tentatively attributed to spin losses during the energy relaxation process to the ground state of the excitons giving rise to the light emission.