Luminescence spectra and efficiency of GaN-based quantum-well heterostructure light emitting diodes: Current and voltage dependence

Luminescence spectra and quantum yield in light emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells (MQWs) were studied in the range of currents J=10⁻⁶–10⁻¹ A. Minor spread in the quantum yield at operating currents (±15% at J≈10 mA) was observed in these LEDs, which were fabricated by Hewlett-Packard. The spread is due to differences in the current and voltage dependences of the diode emission intensity, caused by differences in the charged center distribution across the space-charge region (SCR) of the structures and in the role of the tunnel current component at low voltages. In the diodes with a thin (≲120 nm) SCR, a tunnel emission band was observed for J≲100 μA; the peak energy of this band ℏω_max=1.92–2.05 eV corresponds to the voltage applied. At low currents (J=0.05–0.5 mA), the spectral position of the main peak ℏω_max=2.35–2.36 eV is independent of the voltage and is determined by the radiative transitions between the localized states. At J>1 mA, this band shifts with the current (ℏω_max=2.36–2.52 eV). Its shape corresponds to the model for the occupation of states in the two-dimensional energy band tails, which are caused by the microscopic potential fluctuations. The four parameters in this model are related to the calculated energy band diagram of the MQW structure. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 861–868. Original Russian Text Copyright ? 2001 by Kudryashov, Mamakin, Turkin, Yunovich, Kovalev, Manyakhin. Part of this study was reported at the 3rd All-Russia Workshop on Structures and Devices Based on Gallium, Indium, and Aluminum Nitrides (Moscow State University, 1999); the 3rd International Conference on Nitride Semiconductors (Montpellier, 1999); and the 4th European Gallium Nitride Workshop (Nottingham, 2000).     

Tensile strain and threshold currents in GaAsP-AlGaAssingle-quantum-well lasers

The effects of tensile strain on threshold current in GaAsP-AlGaAs quantum well lasers are studied theoretically and experimentally. A comprehensive model for the light-current characteristics of separate-confinement strained-layer lasers, which is based on a six-band Luttinger-Kohn valence dispersion model, is first developed. Theoretical and experimental results for broad stripe single-well laser diodes with a constant well width of 115 Å are then presented. Experimentally observed variations in threshold currents and TE/TM polarization switching are accurately described by the model for phosphorus compositions in the quantum-well ranging from 0 to 0.30 and cavity lengths ranging from 300 to 1500 μm. Constant-gain contours generated from the theoretical model are shown to provide a simple and powerful guide to various regimes of operation. Our studies show that tensile-strain-related effects lower threshold currents in GaAsP-AlGaAs only in the high gain (short cavity) regime, and suggest more generally that the threshold advantages offered by tensile strain are conditional     

三终端多臂量子环中的持续电流

本文提出了三终端多臂量子环模型,并采用特殊方法来计算持续电流,研究发现：总磁通为零时,持续电流随半导体环增大做非周期和不等幅的振荡.总磁通不为零时,持续电流随AB磁通增强做周期性等幅振荡,Rashba自旋轨道耦合具有改变两种持续电流位相和位相差的效应. 平均持续电流的大小与各臂的臂长和所处的位置以及磁通分布相关.AB磁通对两种持续电流具有不同影响,两种持续电流是可分离的.     

Experimental evidence of inelastic tunneling in stress-inducedleakage current

We propose a new experimental technique to study the transport properties of stress-induced leakage current (SILC). Based on the carrier separation measurement for p-channel MOSFETs, the quantum yield of impact ionization for electrons involved in the SILC process is evaluated directly from the change in the source and gate currents of p-MOSFETs before and after stressing. Since the relationship between the electron energy and the quantum yield is established for direct and FN tunneling currents, the electron energy of electrons involved in the SILC process can be determined from the quantum yield. The results reveal that the measured energy of electrons in the SILC process is lower roughly by 1.5 eV than the energy expected in the elastic tunneling process. Trap-assisted inelastic tunneling model is proposed as a conduction mechanism of SILC accompanied by energy relaxation. It is shown, through the evaluation of the substrate hole current in n-channel MOSFETs, that the contribution of trap-assisted valence electron tunneling, another possible mechanism to explain the energy relaxation, to SILC is small     

Threshold currents for AlGaAs quantum well lasers

The threshold currents for AlGaAs quantum well lasers are studied theoretically. The structure dependent gain coefficient is obtained by taking into account the electron distribution inLvalleys. Theoretical threshold current densities calculated using the gain coefficient agree well with reported experimental results for separate-confinement heterostructure lasers. A design procedure for low threshold current laser is elucidated. The lowest threshold currents are 570 and 53 μA per 1 μm stripe width for modified multiple quantum well lasers with 32 percent and 90 percent reflectivity facet mirrors, respectively.     

Persistent spin currents in a triple-terminal quantum ring with three arms

A new model of a triple-terminal quantum ring with three arms is proposed.We develop an equivalent method for reducing the triple-terminal quantum ring to the double-terminal quantum ring and calculate the persistent spin currents in this model.The results indicate that the persistent spin currents show behavior of nonperiodic and unequal amplitude oscillation with increasing semiconductor ring size when the total magnetic flux is zero. However,when the total magnetic flux is non-zero,the persistent spin currents make periodic equal amplitude oscillations with increasing AB magnetic flux intensity.At the same time,the two kinds of spin state persistent spin currents have the same frequency and amplitude but the inverse phase.In addition,the Rashba spin-orbit interaction affects the phase and the phase difference of the persistent spin currents.The average persistent spin currents relate to the arm length and the terminal position as well as the distribution of the magnetic flux in each arm.Furthermore, our results indicate that the AB magnetic flux has different influences on the two kinds of spin state electrons.     

Current-injection efficiency in semiconductor lasers with a waveguide based on quantum wells

A dynamic distributed diffusion-drift model of laser heterostructures, which takes into account carrier capture by quantum wells, is developed. The leakage currents in the lasing mode are calculated for different laser structures without wide-gap emitters: InGaAs/GaAs (lasing wavelength λ = 0.98 μm), InGaAsP/InP (λ = 1.3 μm), and InGaAs/InP (λ = 1.55 μm). It is shown that consideration of the finite carrier-capture time is of major importance for calculating structures with deep quantum wells. The ratio of the leakage currents to the total current in the structures with deep quantum wells (InGaAsP/InP and InGaAs/InP) increases with an increase in the injection current and may reach a few percent when the lasing threshold is multiply exceeded.     

Scalable etched-pillar, AlAs-oxide defined vertical cavity lasers

Etched-pillar, bottom-emitting vertical cavity lasers have been fabricated using lateral oxidation of AlAs. The devices have threshold currents as low as 315 μA for a 4 μm×4 μm, three quantum well active region. Using the data and a numerical model the authors extract excess optical and carrier losses and an effective surface recombination velocity for the devices. The data show that size dependent optical scattering persists as the lasers are scaled to smaller sizes, but lateral carrier leakage is suppressed, allowing for scaling of lasers to small sizes to achieve lower threshold currents     

A self-consistent pseudo-two-dimensional model for hot-electron current in MOST' s

A theoretical formulation for the hot-electron currents (substrate and gate currents) in MOST's with nonuniform impurity profile has been built. By applying a gradual channel approximation for the source section and a pseudo-two-dimensional approximation for the drain section, saturation voltage is obtained by considering the voltage and channel current continuity at the boundary of the two sections. Three fitting parameters in the model are determined by comparing the theoretical calculation results with the observed substrate current in samples with various device parameters. The present model was successfully applied to describe the two experimental results: the gate oxide thickness dependence of the gate current injection efficiency and the kink in the maximum channel electric field strength versus gate voltage (= drain voltage) relation. The nonuniform channel impurity profile is approximated by the modified Gaussian distribution, which is found to agree well with the estimation by the substrate bias effect of MOST's. The calculated gate currents for the device can well explain the implantation energy dependence of the measured gate currents.     

I–V and Differential Conduction Characteristics of an AlGaAs/GaAs Lateral Quantum Dot Infrared Photodetector

A new infrared detector design, henceforth referred to as a lateral quantum dot infrared photodetector (LQDIP), with the potential for a tunable internal spectral response was investigated. In this design, InAs quantum dots are buried in a GaAs quantum well, which is in turn tunnel-coupled to a second GaAs quantum well. Photoexcited electrons from the quantum dots are expected to tunnel over to the second well, where they are then swept out via a lateral (perpendicular to the growth direction) bias voltage. The lateral photocurrent is in part directed to tunnel into the second quantum well by the depletion field of a narrow pinch-off gate, applied vertically (parallel to the growth direction). Under a proper biasing arrangement, this detector architecture is expected to exhibit the ability to tune to select infrared frequencies as well as operate with reduced dark currents and unity gain in the second well. The LQDIP detector architecture, operating principles and conditions, and preliminary results of I–V, photocurrent, and differential conductance measurements are all discussed.     

Calculation of the surface characteristics and pressures of InAs quantum dots in a GaAs matrix

A theoretical model for calculating the energy characteristics of surfaces of InAs quantum dots in a GaAs(100) matrix is described. The model is based on notions of nonequilibrium thermodynamics and surface physics. The results of calculating the magnitudes of the surface energy and adhesion physical quantities as well as pressures in the vicinity of the edges of InAs quantum dots in a GaAs(100) matrix are presented. The causes of bending of the profile of the lower part of the quantum dot are presented using the Young relationship. These results can be used to asses the stress-relaxation mechanisms during the course of the selforganization of InAs quantum dots in a GaAs(100) matrix.     

Realistic Detailed Balance Study of the Quantum Efficiency of Quantum Dot Solar Cells

An attractive but challenging technology for high efficiency solar energy conversion is the intermediate band solar cell (IBSC), whose theoretical efficiency limit is 63%, yet which has so far failed to yield high efficiencies in practice. The most advanced IBSC technology is that based on quantum dots (QDs): the QD‐IBSC. In this paper, k·p calculations of photon absorption in the QDs are combined with a multi‐level detailed balance model. The model has been used to reproduce the measured quantum efficiency of a real QD‐IBSC and its temperature dependence. This allows the analysis of individual sub‐bandgap transition currents, which has as yet not been possible experimentally, yielding a deeper understanding of the failure of current QD‐IBSCs. Based on the agreement with experimental data, the model is believed to be realistic enough to evaluate future QD‐IBSC proposals.     

Improved design for the monolithic integration of a laser and anoptical waveguide coupled by an evanescent field

A model is proposed to optimize the design of a laser monolithically integrated with a transparent waveguide by an evanescent field. A five-layer structure is shown to be so sensitive to the growth parameters that the threshold current and coupled into the waveguide are basically uncontrollable. The model makes it clear that separate-confinement structures, in which the active layer is attached to a thicker waveguide, are more suitable. The increase of threshold current in these configurations can be efficiently offset by depositing high-reflectivity coatings on the facets of the devices. Moreover, these coatings are shown to further reduce the sensitivity of the devices to technological fluctuations. The use of a single quantum well embedded into a single-mode guide is proposed to obtain very low threshold currents while maintaining a low sensitivity to growth fluctuations     

A study of resonant tunneling in a 3-terminal ballistic device

Preliminary results are presented for a 3-terminal device fabricated by incorporating a narrow quantum well into the collector barrier of a unipolar hot electron transistor. The structure allows unique studies of the mechanism of resonant tunneling in which the carrier injection energy and the bias across the double barrier can be independently varied. The transistor clearly shows an additional current collection threshold representing as much as 10% of the injected current attributable to tunneling via the single subband in the quantum well. This interpretation is confirmed by measurements in perpendicular and parallel magnetic fields despite the fact that theoretical estimates of the resonant tunneling current without scattering are many orders of magnitude smaller, and there are strong indications that elastic scattering in the well is responsible for the enormous discrepancy. The data tend to support proposals (Guéret, 1989a, 1989b; Wolak, 1988) that elastic scattering is responsible for the large valley currents in resonant tunneling diodes, and it appears that the tunnel process may not be completely incoherent as has frequently been assumed up to now.     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

Biomagnetic Fourier imaging [current density reconstruction

A technique for reconstructing a current density distribution from measurements of its magnetic field is described. The technique assumes that the current distribution is confined to a single plane. The data it requires are measurements of the magnetic flux on a plane. These can be provided by an integrated planar array of superconducting quantum interference device magnetometers. The approach is based on the magnetic lead field which is derived in a simple way based on energy concepts. Using the lead field and conservation of charge conditions provides two linear, spatially invariant imaging equations relating the current density and flux measurements. These equations are solved using Fourier techniques. The validity of the resulting reconstruction technique is shown both analytically and with a computer model. The effects of not satisfying the planar assumption are described for the case where the currents are parallel but not in the same plane.     

Resonant tunneling devices with multiple negative differential resistance and demonstration of a three-state memory cell for multiple-valued logic applications

A new resonant-tunneling (RT) functional device with two peaks in the current-voltage (I-V) characteristic has been demonstrated. Contrary to conventional RT devices, the peaks are obtained using a single resonance of the quantum well. The peak's separation is voltage tunable and the peak currents are nearly equal, which is important for a variety of device applications. Using a single device, a three-state memory cell has been implemented.     

Characterizing electric fields in (111)B InGaAs quantum wells using electric field modulated photoluminescence and reflectance techniques

We have performed a series of electroreflectance, photoluminescence, and electric-field-modulated photoluminescence experiments to characterize the strain-induced electric fields in (111)B InGaAs/AlGaAs quantum well p-i-n diode structures. A 180° phase change in the lineshapes of electroreflectance spectra of these samples determines when the quantum well is biased to flatband. Using this bias and a depletion model for the diode, the polarization field in the quantum well can be determined. Contrary to expectations, this polarization field increases significantly with increasing temperature. In addition, at fixed temperature, the quantum well transition energies red-shift with increasing excitation intensity when excited by photons of energy higher than the lowest quantum well transition but lower than the AlGaAs diode's bandgap. When excited with photons of energy greater than the AlGaAs bandgap, the transition energy first red shifts then blue shifts with increasing excitation intensity.     

Vertical-cavity surface-emitting lasers with lateral carrier confinement

A novel method to reduce threshold currents in vertical-cavity surface-emitting lasers (VCSELs) is proposed. By using selective quantum well intermixing, lateral heterobarriers are created that prevent carriers from diffusing away from the optical modes. Our devices show 40% reduction of threshold currents with the implementation of lateral carrier confinement     

Analytical solution of the subbands and absorption coefficients ofAlGaAs-GaAs hyperbolic quantum wells

A hyperbolic function is used to model the carrier confinement potential profile of a disordered Al_0.3Ga_0.7As-GaAs single quantum well produced by interdiffusion of group III atoms across the well-barrier interfaces. The confined states are determined analytically for the hyperbolic profile, and their wavefunctions and energies compare well with the results obtained numerically using an error function profile. The subband edge energy in the hyperbolic quantum well increases initially for a small diffusion length and then decreases for larger diffusion lengths. The absorption coefficient is calculated for different interdiffusion lengths to demonstrate the model. The first absorption peak of the TM polarization has a larger magnitude than does the corresponding peak of the TE polarization, which is in agreement with measurements performed on quantum-well waveguide structures. A similar trend is observed for the higher-order transitions. The magnitude of the second absorption peak also varies with the amount of interdiffusion in the case of TM polarization