Hemispherical GaP:N green electroluminescent diodes

Hemispherically shaped, reduced active junction area, GaP green light-emitting diodes have been fabricated by mechanical polishing and electrochemical etching from $\text{p-n}$ junctions grown by double liquid phase epitaxy on pulled crystal substrates. The hemispherical structure with the light-emitting area restricted to the central portion of the flat surface represents an idealized geometry in which to study luminescent generation and optical coupling phenomena in green-emitting material and devices. Experiments reported here show that the efficiency with which internally generated light is extracted from these GaP structures is at least twice as large as in encapsulated reduced junction area slurry cut cubical dice or mesa diodes. The junction reduction procedure is described and the properties of reduced junction area cubical diodes examined. A maximum external quantum efficiency of 0·41 per cent has been achieved with the hemispherical structure from a crystal which for other geometric devices yielded a maximum external efficiency of 0·17 per cent.     

GaP:N绿色LEDn_2层载流子浓度的优化

在已报道的ｐ－ｎ２－ｎ１结的势垒分布计算的基础上，对该结构的浓度分布进行了计算。对于正偏情形，计入了ｎ２区产生的压降。考虑到ＧａＰ：ＮＬＥＤ发光区主要在ｐ区，注入效率γ＝ｊｎ/（ｊｎ＋ｊｐ），ｊｎ和ｊｐ分别为电子电流和空穴电流。ｐ区内的少子扩散可视为向无限远处的一维扩散；ｎ２区内外加正向偏压时电场不能忽略，空穴又被ｎ２－ｎ１结势垒阻挡（设被完全阻挡），则问题归结为求解有限厚度层中空穴的扩散和复合方程，由边界条件求出空穴扩散电流。将求出的电子扩散电流和空穴扩散电流代入注入效率的表达式即可求得γ。对在合理的参数值范围内的计算结果进行了讨论。分析表明：当ｎ２值在１０１５～１０１６ｃｍ－３范围内时，注入效率较高，与实验结果基本相符。     

Analysis of room-temperature luminescence spectra of VPE-grown nitrogen-doped gallium phosphide

The effect of nitrogen doping on the 300°K quantum efficiency and luminescence spectra of VPE-grown gallium phosphide has been investigated. The range of nitrogen concentrations studied was 2 × 10¹⁸ –1.5 × 10¹⁹ cm{⁻³ on the corrected Lightowlers scale. Over this range, the external quantum efficiency of contactless dice at 40 amp/cm² in air varied from 0.04% to 0.13%, increasing monotonically with increasing nitrogen concentration. The photoluminescence spectra from as-grown epi layers and the electroluminescence spectra of contacted dice in reflecting cups were compared, and were characterized by the parameters lumens/watt, radiometric center of gravity, and photometric center of gravity. The relationship of these parameters to each other and their variation with nitrogen concentration in GaP has been determined. The problem of filtering the spectra to obtain greener devices with acceptable brightness has been investi-gated using the transmission spectra of two specific filters.     

Degradation of GaP:N LEDs

The degradation of light output with operating time has been studied for nitrogen doped GaP light emitting diodes fabricated from vapor phase epitaxial material. The degradation is found to saturate at a non-zero value of efficiency. The process is characterized in terms of a degradation rate constant and the saturation value of efficiency. The rate is found to be a strong function of current density during operation and to a lesser degree materials parameters such as dislocation density. The saturation value appears to be independent of these parameters. The degradation is specifically associated with a decrease in the minority carrier lifetime in the p-side of these diffused LEDs. A model for the generation of non-radiative recombination centers which describes the degradation process quantitatively is presented.     

用SiH_4-N_2进行PECVD生长高质量SiN研究

用ＳｉＨ４┐Ｎ２进行ＰＥＣＶＤ生长高质量ＳｉＮ研究刘英坤李明月（电子工业部第十三研究所，石家庄，０５００５１）１引言众所周知，氮化硅薄膜，尤其是低温等离子体淀积的氮化硅薄膜ＰＥＣＶＤ—ＳｉｘＮｙ，因其良好的物理化学性质和优越的制备工艺，在现代半导体器...     

Dopant diffusion in GaP and related compounds: recent results and new considerations

The paper recalls some major progress made over the past decade in the understanding of dopant diffusion in compound semiconductors. However, existing models to describe the diffusion behaviour of acceptors such as e.g. Zn and Cd in III–V compounds reveal serious discrepancies with respect to the whole body of available experimental data. We present new experiments on GaP and an alternative theoretical approach which both may contribute to find a consistent interpretation.     

Defect centers and degradation of GaP:N LED's

A new degradation property of N-doped GaP light emitting diodes has been found. Diodes Zn-diffused through a thin layer of SiO₂ exhibit a complete saturation of the degradation process at 70 to 80 % of the initial light output. After saturation of the degradation process the emission spectrum is broadened on the low energy side. This broadening is identified as due to the formation of additional nearest neighbour N-pairs during degradation. Diodes diffused without a SiO₂ cover show neither a saturation of the degradation process nor a change of the emission spectrum. Deep level transient spectroscopy revealed different defect levels for the two types of diodes, therefore different degradation properties might be expected.     

InP quantum dots in (1 0 0) GaP: Growth and luminescence

We describe the growth and optical emission from strained InP quantum wells and quantum dots grown on GaP substrates using gas-source molecular beam epitaxy. Self-organized quantum dot formation takes place for InP coverage greater than 1.8 monolayers on the (1 0 0) GaP surface. Atomic force and scanning-electron microscopy studies indicate that unburied dots have a lateral size of 60–100 nm and are about 20 nm high, with dot densities in the range of 2–6×10⁸ cm⁻² for InP coverage between 1.9 and 5.8 MLs. Intense photoluminescence is emitted from both the quantum wells and the quantum dots at energies of about 2.2 and 2.0 eV, respectively. Time-resolved measurements indicate rather long carrier lifetimes of about 19 ns in the quantum wells and about 3 ns in the quantum dots. The data indicate that the InP/GaP quantum wells form a type-II band system, with electrons in the X valleys of the GaP recombine with holes in the InP. Furthermore, in the InP/GaP quantum dot system, the conduction band edge in the X valley of the GaP is nearly aligned with that in the Γ valley of the InP. Rapid thermal annealing of the quantum dots results in at least a six-fold enhancement of integrated emission intensity as well as some Ga-In interdiffusion. The low interdiffusion activation energy indicates that the material near the interface between the GaP matrix and the InP dots is not free of defects.     

Growth and properties of VPE GaP for green LEDs

The PH₃-HCl-Ga-H₂ technique for VPE growth of GaP is described. The influence of various growth parameters, including substrate temperature, orientation, and PH₃ flow rate on morphology and growth rate are described. For both VPE and LPE nitrogen doping is known to be a major factor in obtaining high green luminescence efficiency. The major emphasis of this paper is an examination of the effect of nitrogen concentration in the range less than 10¹⁹ cm⁻³ (using the Lightowlers correction factor) on the growth process and materials properties, such as defect structure, photoluminescence spectra (at 300 and 77K) and photoluminescence intensity and lifetime. The LED device performance (B/J and efficiency) is used as the final test of material quality. Nitrogen is found to be incorporated far in excess of the solubility limit, and the solid gas distribution coefficient for nitrogen is found to increase rapidly with decreasing temperature below 840°C . The optimum nitrogen concentration for high diode efficacy, photoluminescence intensity, and lifetime is found to be approximately 5 × 10¹⁸ cm⁻³, where diodes fabricated by Zn diffusion into the VPE GaP have efficiencies at a current density of 10 A/cm² of 0.1%, comparable to the state-of-the-art in the more widely used grown p-n junctions using LPE.     

Characterization of closed space vapor transport GaP epitaxial layers

The growth of homoepitaxial GaP layers using Te-doped GaP as source material has been obtained by the so-called closed space vapor transport technique. The photoluminescence study shows that these layers, when grown under optimized thermodynamical conditions, have both a large luminescence efficiency and the same optical quality as the ones obtained by liquid phase epitaxy. The variation of the luminescence properties with the conditions of growth has been investigated. Both electron paramagnetic resonance and deep level transient spectroscopy detect the presence of deep levels that are not observed in liquid phase epitaxy materials.     

GaP：（N）的背景光谱和发光尖峰

获得高分辨ＧａＰ（Ｎ）光致发光光谱,观察到等电子陷陆束缚激子发光中ＬＯ和ｌｏｃ多声子发射,其强度分布答合泊松分布。将声子伴带区分为直接光跃迁和间接光跃迁,并进行了相应讨论,还观察到局域声子效应－－光谱相似定律和相当显著的背景光谱。     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

GaP:N(Te)发光谱上的声子双伴线

根据GaP∶N(Te)的光致发光和喇曼光谱,提出了GaP∶N发光中的光学声子双伴线可能分别由LO~г和它的束缚态LO_(XN)~г对所产生的初步设想.另外,尝试对Te的声学声子翼中所含的组合声子作出判别.     

Modulation spectroscopy studies of defects

We examine some relationships between defect characteristics and the modulated optical properties of solids. In particular, the effects of an electric field ℰ on the N absorption in GaP are presented. The crystallographic-orientation dependence is used to determine symmetry properties of the impurity electron states and their interaction with the host band structure. These data also yield an impurity concentration which is in agreement (to within a factor of 2) with results obtained using other techniques. The Franz-Keldysh mechanism responsible for the electro-absorption suggests that in indirect gap semiconductors, an electric field should double the luminescence. This enhancement will be greatest for ℰ along the orbital axis of the luminescent center. Generalizations of this work to other chemical and structural defects are discussed.     

The electroluminescent investigation of double layer Eu-complex organic electronic luminescence diodes

Double layer organic electronic luminescence diodes (OLEDs) based on europium(dibenzoylmethanato)₃monophenanthroline [Eu(DBM)₃bath], ITO/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/Eu(DBM)₃bath/LiF/Al have been fabricated. With increasing the thickness of hole transporting layer, the maximum EL efficiency was increased, and the EL efficiency of 10 cd/A was achieved when the thickness of TPD layer was 80 nm; however, at high current density, the EL efficiency of all devices was decreased drastically. Besides, the evolution of EL emission spectra with increasing operating voltage was found, the mechanisms of the symmetry around the ion improved and the annihilation of excited state of Eu(DBM)₃bath were discussed in explaining this phenomenon.     

Transient luminescence in organic light-emitting diodes explained by trap-assisted recombination of stored charges

We show that after voltage turn-off the recombination of the charges which are stored in the on-state may follow a trap-assisted mechanism instead of the Langevin formula in organic light-emitting diodes. A microscopic model based on this form of recombination is introduced, which not only fits the transient electroluminescence very well in the whole range of the off-state but also provides parameters quantitatively characterizing the stored charges in the on-state. In the last part, we briefly compare the work by Weichsel et al. [1] and our model, trying to constitute a comprehensive picture of the stored charges in OLEDs. As an OLED model system we have chosen a host/guest system co-doped with red and green phosphorescent emitters.     

短周期超晶格AlP／GaP能带结构的转变及其带间的光跃迁

介绍了间接跃迁的半导体ＡｌＰ与ＧａＰ形成的超晶格，由于零折叠效应，实现了能带由间接带隙向直接带隙的转变，从而增加了带间的光跃迁几率，并推导了该几率的表达式。     

Luminescence spectra of blue and green light-emitting diodes based on multilayer InGaN/AlGaN/GaN heterostructures with quantum wells

The luminescence spectra of blue and green light-emitting diodes based on In_xGa_1−x N/Al_yGa_1−y N/GaN heterostructures with a thin (2–3 nm) In_xGa_1−x N active layer have been investigated in the temperature and current intervals 100–300 K and J=0.01–20 mA, respectively. The spectra of the blue and green light-emitting diodes have maxima in the interavals ℏω_max=2.55–2.75 eV and ℏω_max=2.38–2.50 eV, respectively, depending on the In content in the active layer. The spectral intensity of the principal band decreases exponentially in the long-wavelength region with energy constant E ₀=45–70 meV; this is described by a model that takes into account the tails of the density of states in the two-dimensional active region and the degree of filling of the tails near the band edges. At low currents radiative tunneling recombination with a voltage-dependent maximum in the spectrum is observed in the spectra of the blue diodes. A model of the energy diagram of the heterostructures is discussed. Fiz. Tekh. Poluprovodn. 31, 1055–1061 (September 1997)     

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.     

Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.