GaN-based light emitting diodes on nano-hole patterned sapphire substrate prepared by three-beam laser interference lithography

Nano-hole patterned sapphire substrates (NHPSSs) were successfully prepared using a low-cost and high-efficiency approach, which is the laser interference lithography (LIL) combined with reactive ion etching (RIE) and inductively coupled plasma (ICP) techniques. Gallium nitride (GaN)-based light emitting diode (LED) structure was grown on NHPSS by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) measurement was conducted to compare the luminescence efficiency of the GaN-based LED structure grown on NHPSS (NHPSS-LED) and that on unpatterned sapphire substrates (UPSS-LED). Electroluminescence (EL) measurement shows that the output power of NHPSS-LED is 2.3 times as high as that of UPSS-LED with an injection current of 150 mA. Both PL and EL results imply that NHPSS has an advantage in improving the crystalline quality of GaN epilayer and light extraction efficiency of LEDs at the same time.     

基于蓝宝石纳米图形衬底上氮化镓生长的研究

利用两步生长法在蓝宝石纳米图形衬底（NPSS）上生长得到高质量的氮化镓薄膜。通过XRD和SEM对薄膜质量的表征和研究发现,为得到高质量的氮化镓（GaN）薄膜,在NPSS上生长时得到的最优缓冲层厚度为15nm,而在微米级尺寸的图形衬底（MPSS）上得到的最优缓冲层厚度远大于15nm。同时,在NPSS上生长氮化镓薄膜的过程中观察到一个有趣的现象,即GaN在NPSS上生长的初始阶段,氮化镓晶粒主要在图形之间的平面区域生长,极少量的GaN在衬底图形的侧面上聚集生长。这一有趣的现象明显不同于GaN在MPSS上的生长过程。接着,又在NPSS上生长了GaN基LED结构,并对其光电性能进行了研究。     

High Brightness GaN-Based Light-Emitting Diodes

This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces     

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.     

图形化蓝宝石衬底GaN基LED的研究进展

蓝宝石衬底作为发光二极管最常用的衬底,经过不断发展,在克服其与GaN间晶格失配和热膨胀失配问题上,研究人员不断提出解决方案。近期发展起来的图形化衬底技术,除了能减少生长在蓝宝石衬底上GaN之间的差排缺陷,提高磊晶质量以解决失配问题,更能提高LED的出光效率。从衬底图形的形状、尺寸、制备工艺出发,回顾了图形化蓝宝石衬底GaN基LED的研究进展,详细介绍了近年来关于图形化衬底技术与其他技术在提高LED性能方面的结合,总结了图形化蓝宝石衬底应用于大尺寸芯片的优势,并对其未来在大功率照明市场的应用进行了展望。     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

Study of GaN-Based Light-Emitting Diodes Grown on Chemical Wet-Etching-Patterned Sapphire Substrate With V-Shaped Pits Roughening Surfaces

We investigate the mechanism responding for performance enhancement of gallium nitride (GaN)-based light-emitting diode (LED) grown on chemical wet-etching-patterned sapphire substrate (CWE-PSS) with V-Shaped pit features on the top surface. According to temperature-dependent photoluminescence (PL) measurement and the measured external quantum efficiency, the structure can simultaneously enhance both internal quantum efficiency and light extraction efficiency. Comparing to devices grown on planar sapphire substrate, the threading dislocation defects of LED grown on CWE-PSS are reduced from 1.28 times 10⁹/cm² to 3.62 times 10⁸/cm², leading to a 12.5% enhancement in internal quantum efficiency. In terms of the theoretical computing of radiation patterns, the V-Shaped pits roughening surface can be thought of as a strong diffuser with paraboloidal autocorrelation function, increasing the escape probability of trapped photons and achieving a 20% enhancement in light extraction efficiency. Moreover, according to the measurement of optical diffraction power, CWE-PSS demonstrated superior guided light extraction efficiency than that of planar sapphire substrate, thus an extra 7.8% enhancement in light extraction efficiency was obtained. Therefore, comparing to the conventional LED, an overall 45% enhancement in integrated output power was achieved.     

Improvement of light extraction efficiency of flip-chip light-emitting diode by texturing the bottom side surface of sapphire substrate

A high light-extraction efficiency was demonstrated in the flip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction efficiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The light-output power of the FCLED was increased by 40.2% for a 0.4-/spl mu/m deep FCLED with a periodic distance of 13-/spl mu/m mesh-type texture on the bottom side of the sapphire substrate.     

图形蓝宝石衬底GaN基发光二极管的研制

采用抗刻蚀性光刻胶作为掩膜,并利用光刻技术制作周期性结构,进行ICP干法刻蚀C面(0001)蓝宝石制作图形蓝宝石衬底(PSS);然后,在PSS上进行MOCVD制作GaN基发光二极管(LED)外延片;最终,进行芯片制造和测试。PSS的基本结构为圆孔,直径为3μm,间隔为2μm,深度为864 nm,呈六角形分布。与同批生长的普通蓝宝石衬底(CSS)GaN基LED芯片相比,PSS芯片的光强和光通量比CSS分别提高57.32%和28.33%(20 mA),并可减小芯片的反向漏电流,且未影响芯片的波长分布和电压特性。     

Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

InGaN-AlInGaN multiquantum-well LEDs

InGaN-GaN and InGaN-AlInGaN multiquantum-well (MQW) light-emitting diodes (LEDs) were both fabricated and their optical properties were evaluated by photoluminescence (PL) as well as electroluminescence (EL). We found that the PL peak position of the InGaN-AlInGaN MQW occurs at a much lower wavelength than that of the InGaN-GaN MQW. The PL intensity of the InGaN-AlInGaN MQW was also found to be larger. The EL intensity of the InGaN-AlInGaN MQW LED was also found to be larger than that of the InGaN-GaN MQW LED under the same amount of injection current. Furthermore, it was found that EL spectrum of the InGaN-AlInGaN MQW LED is less sensitive to the injection current. These observations all suggest that we can improve the properties of nitride-based LEDs by using AlInGaN as the barrier layer     

Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates

Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.     

Nano-Processing Techniques Applied in GaN-Based Light-Emitting Devices With Self-Assembly Ni Nano-Masks

We have developed a simple method to fabricate nanoscale masks by using self-assembly Ni clusters formed through a rapid thermal annealing (RTA) process. The density and dimensions of the Ni nano-masks could be precisely controlled. The nano-masks were successfully applied to GaN-based light-emitting diodes (LEDs) with nano-roughened surface, GaN nanorods, and GaN-based nanorod LEDs to enhance light output power or change structure properties. The GaN-based LED with nano-roughened surface by Ni nano-masks and excimer laser etching has increased 55% light output at 20 mA when compared to that without the nano-roughened process. The GaN nanorods fabricated by the Ni nano-masks and ICP-RIE dry etching showed 3.5 times over the as-grown sample in photoluminescence (PL) intensity. The GaN-based nanorod LEDs assisted by photo-enhanced chemical (PEC) wet oxidation process were also demonstrated. The electroluminescence (EL) intensity of the GaN-based nanorod LED with PEC was about 1.76 times that of the as-grown LED. The fabrication, structure properties, physical features, and the optical and electrical properties of the fabricated devices will be discussed.     

Characteristics of a-Plane Green Light-Emitting Diode Grown on r-Plane Sapphire

In this work, we have successfully grown a-plane green light-emitting diodes (LEDs) on r-plane sapphire and investigated the device characteristics of a-plane green LEDs. The apparent emission polarization anisotropy was observed and the polarization degree was as high as 67.4%. In addition, the electroluminescence (EL) spectra first revealed a wavelength blue-shift with increasing drive current to 20 mA, which could be attributed to the band-filling effect, and then the EL peak become constant. The current–voltage curve showed the forward voltage of a-plane LED grown on r-plane sapphire substrate was 3.43 V and the differential series resistance was measured to be about 24 $Omega $ as 20-mA injected current. Furthermore, the output power was 240 $mu hbox{W}$ at 100-mA drive current.      

Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates

GaN-based light-emitting diodes (LEDs) with emitting wavelength of 450 nm were grown on patterned sapphire substrates (PSSs) fabricated by chemical wet etching. The crystallography-etched facet was {1-102} R-plane with a 57/spl deg/ against {0001} C-axis and had superior capability for enhancing light extraction efficiency. The light output power of the PSS LED was 1.15 times higher than that of the conventional LED at an injection current of 20 mA. The output power and external quantum efficiency were estimated to be 9 mW and 16.4%, respectively. The improvement was attributed not only to geometrical shapes of {1-102} crystallography-etched facets that efficiently scatter the guided light to find escape cones, but also to dislocation density reduction by adopting the PSS growth scheme.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes through higher pressure grown underlying GaN layers

The 410-nm near-ultraviolet (near-UV) InGaN-GaN multiple quantum-wells light-emitting diodes (LEDs) with low-pressure-grown (200 mbar) and high-pressure-grown (400 mbar) Si-doped GaN underlying layers were grown on c-face sapphire substrates by metal-organic vapor phase epitaxy. Increasing the growth pressure during the initial growth of the underlying n-type GaN epilayers of the near-UV InGaN-GaN LEDs was found to reduce the amount of threading dislocations that originated from the GaN-sapphire interfaces. The electroluminescence intensity of LEDs with underlying GaN layers grown at a higher pressure was nearly five times larger than that of LED with layers grown at lower pressure. Additionally, two-order reduction of leakage current was also induced for the LEDs grown at a higher pressure.     

Direct Growth of Nanopatterned Graphene on Sapphire and Its Application in Light Emitting Diodes

Direct growth of graphene films on functional substrates is immensely beneficial for the large‐scale applications of graphene by avoiding the transfer‐induced issues. Notably, the selective growth of patterned graphene will further boost the development of graphene‐based devices. Here, the direct growth of patterned graphene on the c‐plane of nanopatterned sapphire substrate (NPSS) is realized and the superiority of the patterned graphene for high‐performance ultraviolet light‐emitting diodes (UV‐LED) is demonstrated. As confirmed by density functional theory calculations and analog simulations, compared to the concave r‐plane the flat c‐plane of NPSS is characterized by a lower active barrier for methane decomposition and carbon species diffusion, as well as a greater supply of carbon precursor for graphene growth. The synthesized patterned graphene on the c‐plane of NPSS is verified to be monolayer and high quality. The patterned graphene enables the selective and well‐aligned nucleation of aluminium nitride (AlN) to promote rapid epitaxial lateral overgrowth of single‐crystal AlN films with low dislocation density. Consequently, the fabricated UV‐LED demonstrates high luminescence intensity and stability. The method is suitable for obtaining various patterned graphene by substrate design, which will allow for greater progress in the cutting‐edge applications of graphene.     

High Light-Extraction GaN-Based Vertical LEDs With Double Diffuse Surfaces

High light-extraction (external quantum efficiency ~40%) 465-nm GaN-based vertical light-emitting diodes (LEDs) employing double diffuse surfaces were fabricated. This novel LED structure includes one top transmitted diffuse surface and another diffuse omnidirectional reflector (ODR) on the bottom of a LED chip. The diffusive ODR consists of a roughened p-type GaN layer, an indium-tin-oxide (ITO) low refractive index layer, and an Al layer. The surface of the p-type GaN-layer was naturally roughened while decreasing the growth temperature to 800 degC. After flip-bonding onto a Si substrate by AuSn eutectic metal and laser lift-off processes to remove the sapphire substrate, an anisotropic etching by dilute potassium hydroxide (KOH) was employed on the N-face n-GaN layer to obtain transmitted diffuse surfaces with hexagonal-cone morphology. The double diffused surfaces LEDs show an enhancement of 56% and 236% in light output power compared to single side diffused surface and conventional LEDs, respectively. The devices also show a low leakage current in the order of magnitude of 10 ^-8 A at -5 V and a calculated external quantum efficiency of about 40%. The high scattering efficiency of double diffused surfaces could be responsible for the enhancement in the device light output power     

Influence of GaN material characteristics on device performance for blue and ultraviolet light-emitting diodes

An analysis of blue and near-ultraviolet (UV) light-emitting diodes (LEDs) and material structures explores the dependence of device performance on material properties as measured by various analytical techniques. The method used for reducing dislocations in the epitaxial III-N films that is explored here is homoepitaxial growth on commercial hybride vapor-phase epitaxy (HVPE) GaN substrates. Blue and UV LED devices are demonstrated to offer superior performance when grown on GaN substrates as compared to the more conventional sapphire substrate. In particular, the optical analysis of the near-UV LEDs on GaN versus ones on sapphire show substantially higher light output over the entire current-injection regime and twice the internal quantum efficiency at low forward current. As the wavelength is further decreased to the deep-UV, the performance improvement of the homoepitaxially grown structure as compared to that grown on sapphire is enhanced.     

InGaN/GaN light-emitting diodes with a reflector at the backside of sapphire substrates

Nitride-based light-emitting diodes (LEDs) with a reflector at the backside of the sapphire substrates have been demonstrated. It was found that an SiO₂/TiO₂ distributed-Bragg reflector (DBR) structure could reflect more downward-emitting photons than an Al-mirror layer. It was also found that the 20-mA output power was 2.76 mW, 2.65 mW, and 2.45 mW for the DBR LED, Al-reflector LED, and conventional LED, respectively. With the same 50-mA current injection, the integrated-electroluminescence (EL) intensity of a DBR LED and an Al-reflector LED was 19% and 15% larger than that observed from a conventional LED.