| This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc. |
The Effect of Grain Boundaries on Electrical Conductivity in Thin GaN Layers[edit | edit source]
The Effect of Grain Boundaries on Electrical Conductivity in Thin GaN Layers
Abstract: The effect of grain boundaries on electrical properties of thin GaN layers is studied by photoconductivity and its functional dependence on surface photovoltage, and by resistance changes produced by ion implantation damage. These two independent experiments provide strong evidence that the conductivity in GaN can be described by the Grain Boundary Controlled Transport model. According to this model, charged interface states at the grain boundaries form potential barriers for inter-grain conduction.
Photoconductivity in nanocrystalline GaN and amorphous GaON.[edit | edit source]
Abstract: In this work we present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a-GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a-GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, we build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, we are able to characterize the density of states in the gap for the two materials.
Nanocolumns/Nanowires[edit | edit source]
InGaN nanorod arrays grown by molecular beam epitaxy: Growth mechanism structural and optical properties[edit | edit source]
Abstract: Vertically c-axis-aligned InGaN nanorod arrays were synthesized on c-plane sapphire substrates by radio-frequency molecular beam epitaxy. In situ reflection high-energy electron diffraction was used to monitor the growth process. X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscope, and photoluminescence were used to investigate the structural and optical properties of the nanorods. The growth mechanism was studied and a growth model was proposed based on the experimental data. A red shift of photoluminescence spectrum of InGaN nanorods with increasing growth time was found and attributed to the partial release of stress in the InGaN nanorods.
Gallium nitride nanorod arrays as low-refractive-indextransparent media in the entire visiblespectral region[edit | edit source]
Abstract: A photovoltaic device comprised of an array of 20 nm wide, 32 nm pitch array of silicon nanowires is modeled as an optical material. The nanowire array (NWA) has characteristic device features that are deep in the subwavelength regime for light, which permits a number of simplifying approximations. Using photocurrent measurements as a probe of the absorptance, we show that the NWA optical properties can be accurately modeled with rigorous coupled-wave analysis. The densely structured NWAs behave as homogeneous birefringent materials into the ultraviolet with effective optical properties that are accurately modeled using the dielectric functions of bulk Si and SiO2, coupled with a physical model for the NWA derived from ellipsometry and transmission electron microscopy.
Formation of InGaN nanorods with indium mole fractions by hydride vapor phase epitaxy[edit | edit source]
Abstract: This work demonstrates the formation of InGaN nanorod arrays with indium mole fractions by hydride vapor phase epitaxy. The nanorods grown on (0001) sapphire substrates are preferentially oriented in the c-axis direction. We found that the In mole fractions in the nanorods were linearly increased at x < 0.1. However, In mole fractions were slightly increased at x ≥ 0.1 and then were gradually saturated at x = 0.2. CL spectra show strong emissions from 380 nm (x = 0.04, 3.26 eV) to 470 nm (x = 0.2, 2.64 eV) at room temperature.
InGaN-Based Nanorod Array Light Emitting Diodes[edit | edit source]
Abstract: We demonstrate the realization of the high-brightness and high-efficiency light emitting diodes (LEDs) using dislocation-free indium gallium nitride (InGaN)/gallium nitride (GaN) multi-quantum-well (MQW) nanorod (NR) arrays by metal organic-hydride vapor phase epitaxy (MO-HVPE). MQW NR arrays (NRAs) on sapphire substrate are buried in silicon dioxide (SiO2) to isolating individual NRs and to bring p-type NRs in contact with p-type electrodes. The MQW NRA LEDs have similar electrical characteristics to conventional broad area (BA) LEDs. However, due to the lack of dislocations and the large surface areas provided by the sidewalls of NRs, both internal and extraction efficiencies are significantly enhanced. At 20 mA dc current, the MQW NRA LEDs emit about 4.3 times more light than the conventional BA LEDs, even though overall active volume of the MQW NRA LEDs is much smaller than conventional LEDs.
Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In[edit | edit source]
- Grew InN nanocolumns on InGaN substrate
- Good info on InN and nanocolumns
o InN – smallest band gap of III-nitrides at 0.65eV, large lattice mismatch with common substrates, impurity-prone surface, tight growth range of 460C to 490C o Nanocolumns – quantum confinement effect may yield novel functions and improve performance
Selective area metalorganic molecular-beam epitaxy of GaN and the growth of luminescent microcolumns on Si/SiO[sub 2][edit | edit source]
Abstract: We demonstrate the selective area growth of gallium nitride on patterned Si~111!/GaN/SiO2 wafers by metalorganic molecular beam epitaxy using triethyl gallium as a Ga source. We show that such selective area deposition may be used to grow isolated microcolumns of GaN with lateral dimensions of tens of nanometers on Si/SiO2 wafers. Via high resolution cathodoluminescence imaging we show that such microcolumn structures are highly luminescent inspite of a large surface to volume ratio, indicating that nonradiative recombination at free surfaces is not a significant issue in this system.
Periodic Si Nanopillar Arrays Fabricated by Colloidal Lithography and Catalytic Etching for Broadband and Omnidirectional Elimination of Fresnel Reflection[edit | edit source]
Abstract: Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70% at wavelengths of 200-1900 nm. The reflectance is reduced greatly for the incident angles up to 0.70% for both s- and p-polarized light. These excellent antireflection performances are attributed to light trapping effect and very low effective refractive indices, which can be modified by the fill factor of Si in the NPA layers.
Watching GaN Nanowires Grow[edit | edit source]
Abstract: We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor-liquid-solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.
Direct Observation of Vapor−Liquid−Solid Nanowire Growth[edit | edit source]
Conclusion: The direct observation of nanowire growth unambiguously confirms the validity of vapor-liquid-solid crystal growth mechanism at the nanometer scale and should allow us to rationally control the nanowire growth which is critical for their potential implementation into the nanoscale electronic and optoelectronic devices.
Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates[edit | edit source]
Abstract: Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed1, 2, 3. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies4, 5, 6. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.
Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy[edit | edit source]
Abstract: Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.
Crystallographic alignment of high-density gallium nitride nanowire arrays[edit | edit source]
Abstract: Single-crystalline,one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics1. Elucidation of the vapour–liquid–solid growth mechanism2 has already enabled precise control over nanowire position and size1,3,4–8, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization,and bandgap may be used to tune the physical properties of nanowires made from a given material.Here we demonstrate the use of metal–organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) γ-LiAlO2 and (111) MgO singlecrystal substrates resulted in the selective growth of nanowires in the orthogonal [110] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.
Single nanowire photovoltaics[edit | edit source]
Abstract: This tutorial review focuses on recent work addressing the properties and potential of semiconductor nanowires as building blocks for photovoltaic devices based on investigations at the single nanowire level. Two central nanowire motifs involving p-i-n dopant modulation in axial and coaxial geometries serve as platforms for fundamental studies. Research illustrating the synthesis of these structural motifs will be reviewed first, followed by an examination of recent studies of single axial and coaxial p-i-n silicon nanowire solar cells. Finally, challenges and opportunities for improving efficiency enabled by controlled synthesis of more complex nanowire structures will be discussed, as will their potential applications as power sources for emerging nanoelectronic devices.
Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells[edit | edit source]
Abstract: We examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, we design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.
Growth of InN nanocolumns by RF-MBE[edit | edit source]
Abstract: InN nanocolumns were grown on (0 0 0 1) sapphire substrates by radio-frequency plasma-assisted molecular-beam epitaxy, simply with substrate nitridation, InN nucleation, and high V/III growth conditions. Here, the InN nucleation was conducted by forming In droplets on the nitridated substrates and subsequently nitriding these In droplets. We investigated the growth evolution of InN nanocolumns and the growth-temperature dependence of the morphology. X-ray rocking curves (XRC) for the (0 0 0 2) reflection of the samples basically consisted of two components: a broad peak and a very sharp one. We propose that the broad peak came from the lower part of relatively short nanocolumns and the near-interface region of relatively long nanocolumns. In contrast, the sharp peak came from the (strain-free) upper part of relatively short nanocolumns and the high-crystal-quality region apart from the interface in relatively long InN nanocolumns. The relative intensity of the latter to the former increased with growth time. The shape of the nanocolumns varied with growth temperature: nanocolumns grown at 380 and 420 1C had a taper-like appearance, but the top broadened with increasing growth temperature, becoming broader at the top than the base at 470 and 500 1C. When we grew InN nanocolumns under the same conditions, but without surface nitridation, the subsequent columns were not all along a c-axis. If In droplets were not used, then columns did not form. Thus, both the surface nitridation and In droplets were needed to form c-axis aligned nanocolumns.
Ultradense, Deep Subwavelength Nanowire Array Photovoltaics As Engineered Optical Thin Films[edit | edit source]
Abstract: Vertically aligned gallium nitride (GaN) nanorod arrays grown by the catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy are shown to behave as subwavelength optical media with low effective refractive indices. In the reflection spectra measured in the entire visible spectral region, strong reflectivity modulations are observed for all nanorod arrays, which are attributed to the effects of Fabry-Pérot microcavities formed within the nanorod arrays by the optically flat air/nanorods and nanorods/substrate interfaces. By analyzing the reflectivity interference fringes, we can quantitatively determine the refractive indices of GaN nanorod arrays as functions of light wavelength. We also propose a model for understanding the optical properties of GaN nanorod arrays in the transparent region. Using this model, good numerical fitting can be achieved for the reflectivity spectra.
Plasmon effects on infrared spectra of GaN nanocolumns.[1][edit | edit source]
Abstract: Infrared (IR) transmission spectra of GaN nanocolumns were analyzed. In addition to the bulk GaN optical phonon signal, a broad absorption peak was observed from undoped and Mg-doped nanocolumns. The central position and width of the broad peak changed with the growth condition and Mg concentration. Based on the Lorentz–Drude model composed of phonon and plasmon modes associated with depolarization fields in GaN nanocolumns, IR transmission spectra were fitted by adjusting the free-electron concentration and scattering rate. Dependence of these values on the column size and impurity concentration is discussed.
Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy[2][edit | edit source]
Abstract: Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(111) and c-sapphire substrates. The nanocolumns density and diameter (600–1500 Å) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472–3.478 eV. In addition, the spectra reveal a doublet at 3.452–3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to GaI defects, given the column growth mode (Ga balling).
Stimulated emission from GaN nanocolumns[3][edit | edit source]
Abstract: Researchers from Sophia University in Tokyo, Japan compared the optical properties of a GaN film grown by an RF-plasma assisted molecular beam epitaxy process resulting in nanocolumns with an MOCVD-grown amorphous GaN film. The results of stimulated emission from the two films were compared with the following notable results:
- The wavelength at which peak emission was observed shift higher for increasing intensity of incident light. This increase was nonlinear.
- Higher MBE growth temperatures resulted in narrowing of the nanocolumns at their base, while lower temperatures caused the nanocolumns to coalesce at their base. The transition temperature for this transformation was approximately 850 C.
- The effects of residual strains from differences in thermal expansion between substrate and active layers were observed.
- The increased optical performance of the nanocolumns was explained by a reduction in dislocation density and non-radiative recombination, compared to that of the amorphous film.
Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy.[4][edit | edit source]
Abstract: We have investigated the behavior of excitons in GaN nanocolumns using time-integrated and time-resolved micro-photoluminescence measurements. In the weak confinement limit, the model of fractional-dimensional space gives an intermediate dimensionality of 2.14 for GaN nanocolumns, with an average diameter of 80 nm. Enhanced exciton and donor binding energies are deduced from a fractional-dimensional model and a phenomenological description. Time-integrated photoluminescence spectra as a function of temperature show a curved emission shift. Recombination dynamics are deduced from the temperature dependence of the PL efficiency and decay times.
Structural and optical characterization of intrinsic GaN nanocolumns[5][edit | edit source]
Abstract: Spanish researchers used photoluminescence, Raman scatteringW, SEM imaging and cathodeluminescenceW (CL) to characterize the optical and microstructural properties of GaN nanocolumns grown by MBE. Outputs from CL were used to link the emissions at various peaks with their corresponding locations in the nanocolumns; the lower-energy peaks in PL observed came from defects at the nanocolumn-substrate interface, while the higher-energy peaks were emitted along the length of the nanocolumns. Another correlation was made between stoichiometry and resulting microstucture. Under nitrogen-rich conditions, the flux of Ga could be varied to produce different nanocolumn populations.
The Controlled growth of GaN nanowires.[6][edit | edit source]
Abstract: This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 ím/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.
- Reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled based on conventional metal-organic chemical vapor deposition (MOCVD) using trimethygallium (TMGa) and ammonia (NH3) and requires no additional metal catalyst.
- The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective-growth mask that is patterned by interferometric lithography.
- This method enables to fabricate high quality symmetrical hexagonal sidewall facets over the entire micrometer length of nanowires.
- This method was carried out in two phases, first phase used continous MOCVD till nanowires emerged out of mask and second phase was pulse MOCVD to fabricate nanowires emerged out of mask at the end of phase one, using pulsed MOCVD in second phase maintained the quality of nanowires and were highly alligned and ordered as desired with respect to the mask.
- The diameter of the nanowire remains constant as it emerges from the growth mask, confirming from XTEM images that the nanowire diameter is indeed controlled by the diameter of the growth mask aperture.
- The XTEM images show no threading dislocations (TDs) in the GaN nanowires, even though TDs were observed in the planar GaN film beneath the growth mask. This was found to be the case for GaN nanowire growth on all substrates, including growth on silicon (111).
- The recorded band-edge PL peak intensity for the nanowire sample was 100 times greater than that measured for a 5 micron planar GaN film and more than 200 times greater than that measured for a 0.6 micron planar GaN film. Much of this intensity increase is undoubtedly due to the geometry of a nonplanar nanowire sample, where the input coupling of the PL pump beam and the out-coupling of the resulting PL will both be increased significantly.
Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy.[7][edit | edit source]
Abstract: GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×109 cm−2 and finally decreased to 2×108 cm−2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of 8×106 cm−2.
Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations[edit | edit source]
Abstract: The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.
InGaN and Light Emitting Diodes (LED)[edit | edit source]
InGaN is a relatively new material to photovoltaic technology, but similar materials have been used in LEDWs for some time now. In operation, an LED is to a solar cell as a fan is to a turbine. In the LED, electricity comes in and light is emitted, while in a solar cell light comes in and electricity comes out. The following articles cover details on some advancements in the field of LED technologies closely related to photovoltaics.
Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns[8][edit | edit source]
Abstract: Optical characterization has been performed on an InGaN/GaN nanocolumn structure grown by nitrogen plasma assisted molecular beam epitaxy not only in macroscopic configuration but also in a microscopic one that can be assessed to a single nanocolumn. The photoluminescence (PL) decay monitored at 500 nm is fitted with a double exponential curve, which has lifetimes of 0.67 and 4.33 ns at 13 K. These values are two orders of magnitude smaller than those taken at the same wavelength in conventional InGaN/GaN quantum wells (QWs) grown toward the C orientation. PL detection of each single nanocolumn was achieved using a mechanical lift-off technique. The results indicate that the very broad, macroscopically observed PL spectrum is due to the sum of the sharp PL spectrum from each nanocolumn, the peak energy of which fluctuates. Moreover, unlike conventional QWs, the blueshift of a single nanocolumn is negligibly small under higher photoexcitation. These findings suggest that carrier localization as well as the piezoelectric polarization field is suppressed in InGaN/GaN nanocolumns.
Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off[9][edit | edit source]
Abstract: Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.
InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures[10][edit | edit source]
Abstract: Electrical operation of InGaN/GaN quantum-well heterostructure photonic crystal light-emitting diodes (PXLEDs) is demonstrated. A triangular lattice photonic crystal is formed by dry etching into the top GaN layer. Light absorption from the metal contact is minimized because the top GaN layers are engineered to provide lateral current spreading, allowing carrier recombination proximal to the photonic crystal yet displaced from the metal contact. The chosen lattice spacing for the photonic crystal causes Bragg scattering of guided modes out of the LED, increasing the extraction efficiency. The far-field radiation patterns of the PXLEDs are heavily modified and display increased radiance, up to ~1.5 times brighter compared to similar LEDs without the photonic crystal.
Spontaneous emission of localized excitons in InGaN single and multiquantum well structures.[11][edit | edit source]
Abstract: Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum-confinement Stark effect and band filling of the localized states by excitons.
Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface[12][edit | edit source]
Abstract: We report on an InGaN-based light-emitting diode (LED) with a top p-GaN surface microroughened using the metal clusters as a wet etching mask. The light-output power for a LED chip with microroughening was increased compared to that for a LED chip without one. This indicates that the scattering of photons emitted in the active layer was much enhanced at the microroughened top p-GaN surface of a LED due to the angular randomization of photons inside the LED structure, resulting in an increase in the probability of escaping from the LED structure. By employing the top surface microroughened in a LED structure, the power conversion efficiency was increased by 62%.
Growth Kinetics and Microstructure[edit | edit source]
- ↑ Plasmon effects on infrared spectra of GaN nanocolumns. T. Iwanaga, T. Suzuki, S. Yagi, T. Motooka. Applied Physics Letters, Vol 86 (2005).
- ↑ Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy. E. Callega, M.A. Sanchez-Garcia, F. J. Sanchez, F. Calle, F.B. Naranjo, E. Munoz. Physical Review, Vol 62 (2000).
- ↑ Stimulated emission from GaN nanocolumns. A. Kikuchi, K. Yamano, M. Tada, K. Kishino. Physical Status Solidi, Vol 241, Issue 12 (2004).
- ↑ Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy. J.H. Na, R.A. Taylor, J.H. Rice, J.W. Robinson, K.H. Lee, Y.S. Park, C.M. Park, T.W. Kang. Applied Physics Letters, Vol 86 (2005).
- ↑ Structural and optical characterization of intrinsic GaN nanocolumns. J. Sanchez-Paramo, J.M. Calleja, M.A. Sanchez-Garcia, E. Calleja, U. Jhan. Physica E: Low-dimensional systems and nanostructures. Vol 13, Issue 2-4 (2002).
- ↑ The controlled growth of GaN nanowires. S.D.Hersee, X. Sun, X. Wang. Nano Letters. Vol 6, No 8 (2006).
- ↑ Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. H. Sekiguchi, T. Nakazato, A. Kikuchi, K. Kishino. Journal of Crystal Growth, Vol 300 issue 1 (2006).
- ↑ Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns. Y. kawakami, S. Suzuki, M. Funato, A. Kikuchi, K Kishino. Applied Physics Letters, Vol 89 (2006).
- ↑ Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off. W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano, N.M. Johnson. Applied Physics Letters, Vol 75 (1999).
- ↑ InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures. J.J. Wierer, M.R. Krames, J.E. Epler, N.F. Gardner, M.G. Craford, J.R. Wendt, J.A. Simmons, M.M. Sigalas. Applied Physics Letters, Vol 84 (2004).
- ↑ Spontaneous emission of localized excitons in InGaN single and multiquantum well structures. S. Chichibu, T. Azuhata, T. Sota, S. Nakamura. Applied Physics Letters, Vol 69 (1996).
- ↑ Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface. C. Huh, K. Lee, E. Jang, S. Park. Journal of Applied Physics, Vol 93 (2003).