The radiative recombination coefficient and the internal quantum yield of electroluminescence in silicon

The results of the analysis of variations in the radiative recombination coefficient with varying doping level and concentration of excess electron-hole pairs are reported. It is shown that, along with the effect of narrowing of the band gap calculated in the many-electron approximation, the effect of screening of the Coulomb interaction responsible for the decrease in the excition binding energy should be taken into account. Both effects produce similar trends and decrease the radiative recombination coefficient with increasing levels of doping or injection. The contributions of excitonic radiative recombination and band-to-band radiative recombination to the total radiative recombination coefficient are separated from each other. It is shown that, in the region of room temperature, both contributions are comparable, while at liquid-nitrogen temperature, the excitonic component dominates over the band-to-band component. The results obtained by refined calculations of the limiting value of the internal quantum yield of electroluminescence for the silicon diodes and p-i-n structures are presented. It is shown that the internal quantum yield of electroluminescence can be as high as 14%. However, this values sharply decreases with increasing surface recombination rate and decreasing lifetime of excess charge carriers in the bulk.     

On the ultimate quantum efficiency of band-edge electroluminescence in silicon barrier structures

The ultimate quantum efficiency of electroluminescence in silicon diodes and p-i-n structures at room temperature is calculated. It is shown that the internal quantum yield of electroluminescence is about 10% and is implemented at optimal doping levels for the n-and p-type regions of silicon diodes, ～10¹⁵ and 5×10¹⁶ cm^?3, respectively. With a decrease in the Shockley-Read-Hall lifetimes of electrons and holes, the internal quantum yield of electroluminescence in silicon barrier structures drops. The physical processes related to the effect of excitons in silicon has much in common with those in electroluminescence, photoluminescence, and photoconversion. It is shown that only electroluminescent p-i-n structures are promising for use in silicon integrated circuits.     

Edge electroluminescence in small-area silicon p +-n diodes heavily doped with boron: Analysis of model representations

The experimental results and model representations of the edge electroluminescence of two published studies for small-area silicon p ⁺-n diodes heavily doped with boron are analyzed. In one of these studies it was assumed that edge electroluminescence appears in the p ⁺ region of the diode, and in the other, in the n region of the diode. In the latter case, it was demonstrated that electroluminescence indeed arose in the n region and was caused predominantly by the radiative recombination of free excitons. It is shown that similar model concepts are also applicable to the other study. Based on several independent experimental studies (of edge photoluminescence, electroluminescence, and radiation absorption by free carriers), it is demonstrated that the linear or close-to-linear dependences of the edge-luminescence intensity on the excitation intensity, observed in single-crystal silicon at high injection levels, are caused by the close-to-linear dependences of the exciton concentration on the free-carrier concentration. The results of this study extend the capability of luminescence methods for determining the carrier lifetimes to the region of high injection levels.     

Low Roll‐Off Perovskite Quantum Dot Light‐Emitting Diodes Achieved by Augmenting Hole Mobility

The external quantum efficiencies (EQEs) of perovskite quantum dot light‐emitting diodes (QD‐LEDs) are close to the out‐coupling efficiency limitation. However, these high‐performance QD‐LEDs still suffer from a serious issue of efficiency roll‐off at high current density. More injected carriers produce photons less efficiently, strongly suggesting the variation of ratio between radiative and non‐radiative recombination. An approach is proposed to balance the carrier distribution and achieve high EQE at high current density. The average interdot distance between QDs is reduced and this facilitates carrier transport in QD films and thus electrons and holes have a balanced distribution in QD layers. Such encouraging results augment the proportion of radiative recombination, make devices with peak EQE of 12.7%, and present a great device performance at high current density with an EQE roll‐off of 11% at 500 mA cm^?2 (the lowest roll‐off known so far) where the EQE is still over 11%.     

The Effect of Temperature on the Ionization Coefficient in Silicon†

The avalanche breakdown of p-n junction diodes across their space charge regions is known to be analogous to the Townsend mechanism for gases. Electric charge carriers, which gain sufficient energy from the field, are able to produce secondary electron-hole pairs. Both the holes and the electrons can themselves have ionizing collisions and thus the process leads to an avalanche. An important factor, controlling the breakdown, is the ionization coefficient a, defined as the number of electron-hole pairs produced by a carrier moving unit distance in the direction of the field.

This paper presents the results of nn investigation into the effect of lattice temperature on the ionization coefficient. This has been achieved by observing the breakdown voltage of a range of silicon diodes with either step or linear graded junctions, and applying simple and well-known relationships between ionization coefficient and breakdown voltage.

Measurements have been made over the temperature range 77 to 400°K, and for field strengths from 4 to 9 × 10⁵ volts/cm. Results show the ionization to become more efficient with decrease in temperature over this range of field strength. Temperature is found to have a greater effect at the lower field strength. This is shown to be consistent with modern theory.     

Electroluminescence at 1.54 μm in Si:Er/Si structures grown by sublimation molecular-beam epitaxy

In Si:Er/Si diode structures grown by sublimation molecular-beam epitaxy in a vacuum with a pressure of ～10^?7 mbar at temperatures 520–580°C, the intensity of room-temperature electroluminescence at 1.54 μm is studied as a function of the concentration and distribution of erbium and donor impurities in the space-charge region (SCR) and the SCR width. Methods for obtaining electroluminescence in diodes with a wide (0.1–1 μm) SCR are developed. The mean free path of electrons with respect to their interaction with Er centers and the threshold energy a free electron needs in order to excite an Er-shell electron are determined. The values of electric-field strength corresponding to breakdown in silicon p-i-n diodes with and without Er doping are obtained experimentally. A model describing the interaction of hot electrons with Er centers is suggested.     

Shifting of the peak generation rate in double-drift silicon IMPATT diodes

In double-drift (DD) silicon IMPATT diodes, it is observed that the peak generation rates of both carriers (electrons and holes) lie within the n side. The shifting is due to the unequal ionization rates for electrons and holes in silicon. By neglecting the reverse saturation current, a simple analytical expression for the location where the peak generation occurs is derived. This simple result may be useful for the design of double-drift as well as complementary single-drift IMPATT diodes.     

Edge electroluminescence of the effective silicon point-junction light-emitting diode in the temperature range 80–300 K

The edge electroluminescence spectra of silicon point-junction light-emitting diodes with a p-n junction area of 0.008 mm² are studied at temperatures ranging from 80 to 300 K. Unprecedentedly high stability of the position of the spectral peak is observed at temperatures in the range between 130 and 300 K. The spectral characteristics of the light emitting diodes are studied at 80 K at different current densities up to 25 kA/cm². In contrast to the earlier reported data obtained at 300 K, the data obtained at 80 K do not show any noticeable Augerrecombination-related decrease in the quantum efficiency. From an analysis of the electroluminescence spectra at 80 K in a wide range of currents, it follows that radiative annihilation of free excitons is not a governing mechanism of electroluminescence in the entire emitting region in the base of the point-junction light-emitting diode at all currents used in the experiment.     

Mechanism of radiative recombination in the region of interband transitions in Si-Ge solid solutions

The electroluminescence of Si-Ge diodes (with a Ge content of 5.2% in the corresponding solid solutions) in the region of interband transitions has been studied at the temperatures T = 82 K and 300 K. The emission spectra, the linear dependence of the electroluminescence intensity on current, and the exponential decay of the intensity suggest an exciton mechanism of radiative recombination with and without the involvement of phonons during radiative transitions.     

Analysis of hot carrier transport in AlGaAs/InGaAs pseudomorphicHEMTs by means of electroluminescence

The carrier transport phenomena occurring in pseudomorphic AlGaAs/InGaAs HEMTs biased in the on-state impact-ionization regime is analyzed in this paper. We confirm the presence, in the electroluminescence spectra of pseudomorphic HEMTs, of a dominant contribution due to electron-hole recombination and we identify a composite peak due to recombination of cold carriers. We analyze the recombination peak using a high-resolution monochromator, which reveals the fine structure due to transitions between electron and hole subbands in the channel quantum well, thus providing useful data concerning the properties of the InGaAs HEMT channel. We also demonstrate that recombination between nonenergetic electrons and holes occurs in the gate-source region, as already observed in InAlAs/InGaAs HEMT's on InP. This recombination emission is superimposed to a less intense contribution mostly coming from the gate drain region. This contribution has a nearly Maxwellian distribution which extends to fairly high energies (>3 eV) and has equivalent temperatures in the 1000-3000 K range. Finally we show evidence of recombination in the AlGaAs layers (observed at high electric field), which demonstrates, in these devices, real space transfer of both electrons and holes     

Mechanisms of transport and injection of carriers into a porous silicon: Electroluminescence in electrolytes

A model describing the transport and injection of carriers into a heterophase system (silicon substrate)/(silicon nanocrystals)/(electrolyte) under excitation of electroluminescence is proposed. The main fraction of current passes from the electrolyte directly into a substrate by-passing the nanocrystals. Electromechanical processes at the electrolyte-substrate interface produce the electroactive particles, which inject one or both types of carriers in macro-and nanocrystals, respectively. For the bipolar injection, the nanocrystals play the role of catalyst in the exothermal reaction of charge exchange between the electroactive particles, which possess the opposite charges. In this case, the particles transfer the energy to nanocrystals, which they accumulated in the process of their formation. A fraction of this energy is released by the radiative recombination. The resulting efficient visible electroluminescence is weakly dependent on the doping level and conduction type of the initial silicon.     

Electroluminescence in porous silicon at a reverse bias voltage applied to the Schottky barrier

A model of electroluminescence originating in a structure composed of a metal and porous silicon at a reverse bias voltage applied to the arising Schottky barrier is suggested. In this model, the avalanche multiplication of hot charge carriers and nonradiative Auger recombination in porous silicon are taken into account. It is ascertained that the difference in the systematic features of an increase in electron and hole currents due to generate nonequilibrium charge carriers, as a result of avalanche multiplication of hot electrons, brings about a superlinear increase in the radiative-recombination intensity as a current function. The radiative-recombination efficiency is lowered in the conditions of an avalanche breakdown as a result of an increase in the contribution of the Auger processes. It is shown that an increase in the concentration of nanocrystallites in porous silicon represents a way for increasing the electroluminescence efficiency in this material.     

Spectra of persistent photoconductivity in InAs/AlSb quantum-well heterostructures

Persistent photoconductivity at T = 4.2 K in AlSb/InAs/AlSb heterostructures with two-dimensional (2D) electron gas in InAs quantum wells is studied. Under illumination by IR radiation (?ω = 0.6–1.2 eV), positive persistent photoconductivity related to the photoionization of deep-level donors is observed. At shorter wavelengths, negative persistent photoconductivity is observed that originates from band-to-band generation of electron-hole pairs with subsequent separation of electrons and holes by the built-in electric field, capture of electrons by ionized donors, and recombination of holes with 2D electrons in InAs. It is found that a sharp drop in the negative photoconductivity takes place at ?ω > 3.1 eV, which can be attributed to the appearance of a new channel for photoionization of deep-level donors in AlSb via electron transitions to the next energy band above the conduction band.     

A mechanism of charge transport in electroluminescent structures consisting of porous silicon and single-crystal silicon

Electroluminescent structures that emit in the visible region of the spectrum and are based on porous silicon (por-Si) formed on the p-Si substrate electrolytically using an internal current source are fabricated. The photoluminescent and electroluminescent properties, as well as the current-and capacitance-voltage characteristics of the structures are studied. Electroluminescence is observed only if the forward bias voltage is applied to the structure; the electroluminescence mechanism is based on the injection and is related to the radiative recombination of electrons and holes in quantum-dimensional Si nanocrystals. The injection of holes is controlled by the condition of their accumulation in the space-charge region of p-Si and by a comparatively low concentration of electronic states at the por-Si/p-Si interface. The charge transport in por-Si is caused by the direct tunneling of charge carriers between the quantum-mechanical levels, which is ensured by an appreciable number of quantum-dimensional Si nanocrystals. The leakage currents are low as a result of a small variance in the sizes of Si nanocrystals and the absence of comparatively large nanocrystals.     

Photoluminescent and electroluminescent properties of spontaneously forming periodic InGaAsP structures

Photoluminescence and electroluminescence techniques were used to study spontaneously forming periodic InGaAsP structures that consisted of two types of alternating solid-solution domains differing in composition and lattice constant. It was found experimentally that the volume of the narrow-gap material domains is smaller than that of the wide-gap domains. Existence of the inelastic strain caused by the large (2–3%) lattice constant mismatch between the adjacent domains was inferred. Laser diodes with the spontaneously forming periodic InGaAsP structures in the active region were fabricated, and lasing in the long-wavelength electroluminescence spectral band that originated from the radiative recombination in the narrow-gap domains was obtained. Lasing at the threshold current densities of 70 A/cm² at 77 K and 700 A/cm² at 300 K was observed in the highest-quality samples.     

Migration of excited charge carriers in arrays of phosphorus-doped silicon nanocrystals

The rate of tunnel migration of excited charge carriers (electrons and holes) in the array of silicon nanocrystals doped with phosphorus is calculated. It is shown that, starting from certain phosphorus concentrations dependent on the relation between the dimensions of the emitting and accepting nanocrystals, the rate of tunneling of electrons sharply decreases (by several orders of magnitude) and becomes lower than the rate of interband radiative recombination     

Semiconductor lasers with asymmetric barrier layers: An approach to high temperature stability

A method for enhancing the temperature stability of injection lasers that is based on introducing asymmetric barrier layers on each side of the quantum-confined active region is suggested. The asymmetric barrier layers prevent electrons from escaping from the active region into the part of the waveguide region where holes are injected and prevent holes from escaping into the part of the waveguide region where electrons are injected. Parameters of the layers that allow implementation of the asymmetric-barrier design using pseudomorphic structures grown on GaAs substrates are determined. The calculation of the threshold characteristics of these laser structures demonstrates that suppression of electron-hole recombination outside the active region attained due to the use of asymmetric barrier layers leads to a significant decrease in the threshold current and an increase in the characteristic temperature of this type of lasers.     

Mechanism of electroluminescence of porous silicon in electrolytes

A generalized model for the appearance of visible-and infrared-range electroluminescence of porous silicon in contact with an oxidizing electrolyte is proposed. According to the model, visible-range electroluminescence arises as a result of bipolar injection of electrons and holes from the electrolyte into electrically insulated quantum-well silicon microcrystallites, while infrared-range electroluminescence is due to monopolar injection of holes from the electrolyte into macrocrystals. A mechanism of electron injection from the electrolyte is proposed. It is concluded that the character of the electroluminescence should not depend on the magnitude and even the type of conductivity of the silicon substrate. Fiz. Tekh. Poluprovodn. 31, 844–847 (July 1997)     

High-temperature luminescence in an n-GaSb/n-InGaAsSb/p-AlGaAsSb light-emitting heterostructure with a high potential barrier

The electroluminescent properties of an n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with a high potential barrier in the conduction band (large conduction-band offset) at the n-GaSb/n-InGaAsSb type-II heterointerface (ΔE _c = 0.79 eV) are studied. Two bands with peaks at 0.28 and 0.64 eV at 300 K, associated with radiative recombination in n-InGaAsSb and n-GaSb, respectively, are observed in the electroluminescence (EL) spectrum. In the entire temperature range under study, T = 290–480 K, additional electron-hole pairs are formed in the n-InGaAsSb active region by impact ionization with hot electrons heated as a result of the conduction-band offset. These pairs contribute to radiative recombination, which leads to a nonlinear increase in the EL intensity and output optical power with increasing pump current. A superlinear increase in the emission power of the long-wavelength band is observed upon heating in the temperature range T = 290–345 K, and a linear increase is observed at T > 345 K. This work for the first time reports an increase in the emission power of a light-emitting diode structure with increasing temperature. It is shown that this rise is caused by a decrease in the threshold energy of the impact ionization due to narrowing of the band gap of the active region.     

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.