Study of deep centers in microplasma channels of silicon epitaxial avalanche diodes

Semiconductors - A statistical delay of microplasma breakdown in silicon epitaxial p-n junctions was studied. It was shown that, when the charge state of deep centers is changed by decreasing the...     

Operation-induced degradation of GaP light-emitting diodes

After GaP red light-emitting diodes (LED's) had been degraded by the forward-biased operation, changes of the light output, the minority carrier lifetime, the photocurrent, and the photocapacitance were measured. There was a linear decreasing relation between the light output and the carrier lifetime. A center which was related to the degradation of the light output was observed by means of photocurrent and photocapacitance measurements. This center's energy level is about 0.8 eV from either the valence or conduction band. This center was observed to increase in concentration with degradation. However, the contribution of this center to the minority carrier lifetime is not established since the capture cross sections of the center are unknown and is not known whether the center occurs on the n-side, or the p-side, or on both sides. This center does not appear to be an oxygen donor.     

Deep-level recombination spectroscopy in GaP light-emitting diodes

Deep-level parameters determined from an analysis of the differential coefficients of the forward-bias current-voltage curves are compared in the example of commercial GaP LEDs. It is shown that these parameters are suitable for deep-center diagnostics. The proposed measurements can be performed on semiconductor wafers in the industrial environment without sealing or dividing into individual crystals. Fiz. Tekh. Poluprovodn. 33, 723–726 (June 1999)     

A new method of growing GaP crystals for light-emitting diodes

A method named synthesis, solute diffusion (SSD) has been developed for growing compound semiconductor crystals, GaP in particular, for light-emitting diode (LED) use. The grown crystal is cylindrically shaped and is composed of fairly large-size grains. Growth rate is limited by the diffusion process of phosphorus in the gallium melt. The diffusion coefficient was obtained from the growth rate and found to be 8×10^-5cm²s^-1at 1100°C with an activation energy of 0.65 eV. Donor impurities, tellurium or sulfur, can be reproducibly incorporated from 3×10¹⁷to 4×10¹⁸cm^-3, with segregation coefficients at 1150°C, 0.038 and 1.0, respectively. The quality of the grown crystals was observed to be exceptionally good, and the saucer-type pits were hardly observable in the crystal on modified AB etching. Highly efficient red-light-emitting junctions were reproducibly grown by only one single-layer-single-liquid-epitaxy process, in which zinc was doped from the vapor phase. A double-layer-single-epitaxy process, which we call "liquid epitaxial grown-in junction" process, was also developed and it produced highly efficient green LED's. The LED's grown on the SSD wafers have efficiencies up to 7.4 percent for red and 0.15 percent for green.     

Properties of GaP light-emitting diodes grown on spinel substrates

Red and green GaP electroluminescent diodes have been successfully fabricated from GaP grown heteroepitaxially on spinel substrates by a vapor phase/liquid phase two-stage process. Current-voltage and light emission characteristics of the diodes are compared with those grown on bulk substrates. Quantum efficiencies up to 0·1 per cent in the red and 0·01 per cent in the green have been obtained.     

Electrical and luminescent properties and the spectra of deep centers in GaMnN/InGaN light-emitting diodes

Electrical and electroluminescent properties were studied for GaN/InGaN light-emitting diodes (LEDs) with the n-GaN layer up and with the top portion of the n layer made of undoped GaMnN to allow polarization modulation by applying an external magnetic field (so-called “spin-LEDs”). The contact annealing temperature was kept to 750°C, which is the thermal stability limit for retaining room-temperature magnetic ordering in the GaMnN layer. Measurable electroluminescence (EL) was obtained in these structures at threshold voltages of ∼15 V, with a lower EL signal compared to control LEDs without Mn. This is related to the existence of two parasitic junctions between the metal and the lower contact p-type layer and between the GaMnN and the n-GaN in the top contact layer.     

Solution-processed cathode-interlayer-free deep blue organic light-emitting diodes

Efficient non-doped deep blue organic light-emitting diodes (OLEDs) were fabricated by solution-processing method by using a series of small molecules consisting of various contents of triphenylamine and phosphonate-featured fluorene units as the emitting layer. Without any electron-injection layer, one of the optimal devices with a simple double-layer device configuration exhibits a maximal current efficiency of 2.59 cd A⁻¹ at 6.8 V (1.72 mA cm⁻²) with a CIE coordinates of (0.163, 0.097). These double-layer devices are demonstrated with excellent color-stability under a wide range of operating current density. The current work indicates that electron-rich triphenylamine moiety incorporated with phosphonate-featured fluorene units could be utilized as building blocks to construct a multi-functional platform combining good electron-injection property, carrier-transport property, and efficient electroluminescence. It also provides an approach to achieve a structure-simplified color-stable efficient blue OLED.     

Injection-enhanced transformation of luminescence spectra of green GaP:N light-emitting diodes

Complex kinetics of the injection-enhanced change of the GaP:N LED emitting power and transformation of luminescence and DLTS spectra were investigated. It is shown that injection-enhanced processes in GaP:N LED’s includes: a) the interstitial defects (Zn_i) diffusion from the highly doped layer near the contact into the active layer; b) the relief of stresses due to the arising of a dislocation network and the generation of point defects (vacancies antisites); c) diffusion of the impurity atoms via the vacancies which leads to dissociation of the nitrogen atom complexes (NN _i ).     

Transition metal deep centers in GaAs,GaP and Si

A simple model predicts that the energy of a transition element relative to the valence band edge is related to elastic stress and therefore to the radius. This model has been originally applied to a few elements in GaAs, but is shown to give moderately quantitative results for a range of substitutional iron-group transition elements from Sc to Cu in GaAs and to a lesser extent between measurements and theory for GaP and Si. The radius term involved is of the form R_M²δR, where δR is the difference between the metal atom with a full 3d shell and the tetrahedral covalent radius of the atom with +2 coordination. Other transition metal properties, such as distribution coefficients and hole lifetimes also appear to be related to atomic radius, implying that impurity induced stresses may be involved.     

High efficiency deep blue phosphorescent organic light-emitting diodes

Deep blue phosphorescent organic light-emitting diodes have been developed by using tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium (FCNIr) as a blue phosphorescent dopant. The FCNIr showed a wide triplet bandgap of 2.8 eV for deep blue emission due to a strong electron withdrawing CN substituent in addition to F unit. Doping of the FCNIr in N,N’-dicarbazolyl-3,5-benzene gave a high quantum efficiency of 9.2% with a CIE color coordinate of (0.15, 0.16).     

Statistical delay of microplasma breakdown in GaP p-n junctions

The mechanism for switching on a microplasma in gallium phosphide p-n junctions is investigated. It is shown that changing the distribution function of the statistical breakdown delay with respect to distance makes it possible to determine the energy spectrum of deep levels localized in the microplasma channels. In these experimental studies, commercial gallium phosphide AL102 red-light LEDs were used. In the temperature range 100–380 K the influence of a number of energy levels was detected. In these diodes, deep levels were observed to have an unusually strong effect on the statistical breakdown delay when their charge states are changed by a fractional decrease in the voltage across the p-n junction. Fiz. Tekh. Poluprovodn. 33, 1345–1349 (November 1999)     

Spectroscopy of impurity levels by measuring the microplasma turn on probabilities in GaP/Zn,O/diodes

The modifying effects of the avalanche breakdown on the space charge density of the depletion layer of GaP/Zn,O/p-n junctions have been studied by the double square pulsed method. The thermal-emission time constant of the impurity levels may be determined from measurements of the microplasma turn-on probability vs the time interval between the two pulses. By making these measurements at various temperatures, one can find the activation energy and also the thermal-capture cross-section of the impurity levels. The analysis of the breakdown-voltage variation with time and temperature permits one to draw conclusions about the type of the individual levels (minority or majority-carrier trap) and also about the impurity impact ionization. Five impurity centers were identified in the temperature interval 77–140°K; among them two centers which might be responsible for the anomalous short minority carrier lifetime, i.e., the low quantum efficiency of the red GaP lamps.     

Silicon carbide light-emitting diodes with epitaxial junctions

Silicon carbide light-emitting diodes have been produced by vapour growth techniques and liquid-phase epitaxy. The fabrication methods for diodes with emission in the blue region of the spectrum are described. Emission spectra and efficiency data are presented for several types of diodes.     

Silicon light-emitting diodes with strong near-band-edge luminescence

Electroluminescence (EL) in the region of interband transitions from silicon light-emitting diodes (LEDs) fabricated by cutting a solar cell with an area of 21 cm² and external quantum efficiency η_ext of EL up to 0.85% has been studied at room temperature. Despite the considerable decrease in η_ext because of the cutting and Auger recombination, record-breaking values of the total power emitted by a diode (up to W = 8 mW) and emitted power per unit area (up to P ₀ = 65 mW/cm²) were achieved at pulse currents of up to 10 A and structure areas in the range S = 0.1–0.9 cm². The EL decay kinetics was measured for LEDs with different areas. The emission pattern of a Si LED with a textured surface and the emission intensity distribution along different directions in the plane of the emitting area of the LED were measured.     

Stability of polymer light-emitting diodes

In this paper, we report on the lifetime of polymer LEDs fabricated at Philips Research. For single-layer LEDS, we find that the operational lifetime in nitrogen gas is limited by the stability of the indium-tin-oxide (ITO) anode. By using a polymeric capping layer for the ITO, we obtain more stable devices. In air, the lifetime is limited by black spot formation. Small pinholes in the cathode layer are the origins of the black spots. Water or oxygen may diffuse through these pinholes and react with the cathode, causing degradation. By encapsulating the devices we can prevent black spot formation. Our present 8 cm² devices have lifetimes of many thousands of hours at daylight visibility under ambient conditions.     

Thermal resistance of light-emitting diodes

A detailed analysis of the heat flow in a light-emitting diode is carried out in the present paper. In the thermal model of a light-emitting diode, the heat flow from the active region throughout the area between it and the top contact, the nonuniform heat flux density distribution in the active region due to the current-spreading effect as well as the temperature dependence of the thermal conductivity of the semiconductor material, are taken into account. The solutions of the thermal conduction equation for a light-emitting diode are obtained for both the steady-state condition and the transient-state condition. The heat-spreading in the heat-sink is analyzed. The effect of LED construction parameters on its thermal resistance is illustrated in numerous figures.     

High efficient light-emitting diodes with antireflection subwavelength gratings

A two-dimensional subwavelength grating (SWG) has been fabricated on a GaAlAs light-emitting diode (LED). The SWG is patterned by electron beam lithography and etched by fast atom beam with Cl/sub 2/ and SF/sub 6/ gases. The fabricated grating has 200 nm period and the tapered grating shape with aspect ratio of 1.38 to prevent reflection in the spectral region including 850 nm light emission. The emission is increased by 21.6% at the normal emission angle. The total emittance is increased by 60% with the SWG in comparison with that of the flat surface.     

具有相分离InGaN有源层的白光发光二极管

韩国Kwangju理工学院光电子材料中心和材料科学工程系利用相分离InGaN有源层，无需添加荧光材料，制造出了白光发光二极管。这种二极管的白光发射归因于分离相InGaN三元合金中铟组分和类量子点富铟区域尺寸的宽分布。     

Dissociation of excitons in organic light-emitting diodes

In this paper, the dissociation of excitons in both doped and undoped organic light-emitting diodes is investigated in detail by means of electric-field-induced photoluminescence quenching. The results show that the doped devices demonstrate lower quenching than that of undoped device. The reason is that the narrower energy band gap of guest molecules compared to that of the host molecules. In doped devices the increasing concentration of the guest molecules leads to a decrease in dissociation of excitons. The reason is that the increasing of the guest molecules concentration can result in an increase in the fraction of excitons residing on the guest molecules. Besides, the decreasing energy band gap of guest molecules can also make lower quenching in the doped devices.     

Role of LiF in polymer light-emitting diodes with LiF-modified cathodes

We report on high-efficiency polymer light-emitting diodes (PLEDs) based on poly [2-methoxy-5-(3′,7′-dimethyloctyloxyl)]-1,4-phenylene vinylene (OC1C10) with LiF-modified cathodes. Devices with different cathodes are made and characterized by the electroabsorption technique to measure their built-in voltage. Devices with a LiF/Al bilayer cathode or a LiF:Al composite cathode, all show significantly improved performance as compared to those with bare Al cathodes. The improvement is correlated with enhanced electron injection due to a decrease of the electron injection barrier, which is also indicated by the electroabsorption measurements. The same effect is also observed with LiF(0.6 nm)/Mg cathodes. However, inserting the same LiF thin film between Ag and OC1C10 does not improve the device performance. Cathodes composed of ultra-thin films of LiF(0.6 nm)/Al(1 nm) or LiF:Al(2 nm) covered by Ag (100 nm) show the same performance as LiF(0.6 nm)/Al bilayer cathode or a LiF:Al composite cathode, indicating that the enhancement is specific to LiF and Al. Our experiments can be explained by assuming that Li-ions can dissociate from LiF and diffuse into the OC1C10 layer, leading to an n-type zone close to the polymer/cathode interface. This n-doped layer at the interface facilitates electron injection at the cathode/polymer interface and eventually leads to the formation of an Ohmic contact.