Si-doped Barriers Research Articles

Impact of doped barriers on the recombination coefficients of c-plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). Despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

The effect of barrier layers on 2D electron effective mass in Al0.3Ga0.7N/AlN/GaN heterostructures

The Shubnikov de Haas (SdH) effect measurements have been performed to evaluate the influence of Si3N4 passivation, a spacer layer, and Si-doped barrier layer on the electronic transport parameters of two-dimensional (2D) electrons in Al0.3Ga0.7N/AlN/GaN heterostructures under temperatures from 1.8 K to 40 K and at a magnetic field up to 11 T. The 2D electron effective mass (m*), 2D carrier density (N 2D), the difference between Fermi level and subband energy levels (E F − E 1), quantum lifetime (τ q) are determined by analyzing SdH oscillations. Although investigated samples with equal 2D electron density are examined, the effective mass values of 2D electrons are deduced within the range of (0.16 ± 0.005)m 0 and (0.23 ± 0.005)m 0. Results reveal that passivation, a spacer layer, and doping affect 2D electron effective mass. Furthermore, the dominant scattering mechanisms that limited electron transport is determined as a long-range scattering for all investigated sample. The results obtained provide information for the high-performance device application of these samples.

Systematical and Numerical Investigations on InGaN-Based Green Light-Emitting Diodes: Si-Doped Quantum Barriers, Engineered p-Electron Blocking Layer and AlGaN/GaN Structured p-Type Region

Abstract Here, we numerically prove that the severe polarization-induced electric field in the active region for [0001]-oriented InGaN-based green light-emitting diodes (LEDs) is reduced when heavily Si-doped GaN quantum barriers are adopted. However, the electron injection is accordingly sacrificed for the insufficient confinement capability of the p-type electron blocking layer (p-EBL). Hence, p-EBL structures with/without gradient AlN alloys are discussed to reduce the electron leakage, and the importance of the positive sheet polarization charges at the interface between the last quantum barrier (LQB) and the p-EBL on affecting the blocking barrier height for electrons is especially highlighted. Moreover, we also suggest utilizing specially designed p-AlGaN/p-GaN structure instead of p-GaN structure to serve as the p-type hole supplier, which is able to increase the kinetic energy of holes, thus assisting holes to overcome the energy band barrier height in p-EBL and increasing the hole concentration within the quantum wells.

Reduction of efficiency droop in GaN/InGaN based multiple quantum well light emitting diode by varying Si-doping and thickness in barrier layers

In this paper, InGaN/GaN based multiple quantum well blue light emitting diodes have been optimized by varying Si doping concentrations and incorporating step-wise graded thickness in quantum barriers for achieving improved internal quantum efficiency. Optimized Si-doped barrier profile along with graded thickness in quantum barriers shows improved performance of light emitting diodes. The improvement in results may be attributed to the fact that grading made the transportation of holes more homogeneous, which inhibits the electron leakage and enhances the radiative recombination.

Polarization-Engineered High-Efficiency GaInN Light-Emitting Diodes Optimized by Genetic Algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.

Effect of Si doping in barriers of InGaN/GaN multiple quantum wells on the performance of green light-emitting diodes

Green InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) are of high In composition in well layers, a condition which results in large lattice mismatch between InGaN well layers and GaN barrier layers and hence boosts up the quantum-confined Stark effect (QCSE). Screening of the polarization electric fields within MQWs can effectively improve the internal quantum efficiency (IQE). This work investigates the effect of Si doping in barrier layers on the performance of green InGaN/GaN MQWs LEDs. It is revealed that a suitable doping density of Si (3.4 × 1016 cm−3) in barrier layers can promote the interfacial abruptness between the barrier and the well layers, reduce the In-composition fluctuation, and effectively screen the polarization electric fields. At the same time, suitable Si doping in barrier layers improves the lateral current spreading, alleviating the current crowding effect which is responsible for local heating and efficiency droop. However, overdoping of Si in barrier layers causes a series of negative effects, such as deteriorated interfacial quality and large current leakage. An effective approach for improving the performance of green LEDs is hence proposed.

Enhancement of hole injection with Mg–Si-codoped barriers in InGaN-based light-emitting diodes

The advantages of an InGaN light-emitting diode (LED) structure with Mg–Si-codoped barriers are studied numerically in this paper. Energy band diagrams, carrier concentrations in the active region, radiative recombination rates in the quantum wells (QWs), light-current performances, and internal quantum efficiency (IQE) are investigated. Simulation results suggest that by employing the Mg–Si-codoped barriers, the light-output power and IQE are more significantly improved than LEDs with step-Si-doped barriers when compared with the conventional LEDs with wholly Si-doped barriers, due to modified band diagrams, which are favorable for improvement of hole injection efficiency. Moreover, simulation also indicates that the optical performance can be further improved if the Si doping concentration of quantum barriers (QBs) is decreased gradually along the growth direction for the LED with Mg–Si-codoped barriers.

Improved carrier injection and efficiency droop in InGaN/GaN light-emitting diodes with step-stage multiple-quantum-well structure and hole-blocking barriers

Hole transport control and carrier injection improvement have been demonstrated in the InGaN/GaN light-emitting diodes (LEDs) with step-stage multiple-quantum-well (MQW) structure and Si-doped hole-blocking barriers. Single-wavelength emission was obtained under electrical pumping in these LEDs by utilizing hole-blocking effect. The light emission around 450 nm showed a substantial increase compared with the reference sample with single or step-stage indium-content MQWs. The droop behavior and wavelength stability were also improved significantly. These improvements were attributed to the enhanced carrier injection to the active region due to the alleviation of the quantum-confined Stark effect and the effective hole-blocking effect of the Si-doped barriers.

Suppression of Nonradiation Recombination by Selected Si Doping in AlGaN Barriers for Ultraviolet Light-Emitting Diodes

The effect of selective Si doping on the emission efficiency in ultraviolet (UV) light-emitting diodes (LEDs) is investigated both experimentally and theoretically. The results show that the light output power increases with the number of Si-doped barriers (QBs). Experimental results indicate that compared with an all-undoped-QB LED, a factor of 3.17 can be achieved for the output power of an all-doped-QB LED at 350 mA. Detailed analysis on this phenomenon shows that the Si-doped QB is beneficial to suppress the nonradiative recombination rate by excess electrons in doped barriers.

On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes

InGaN/GaN light-emitting diodes (LEDs) make an important class of optoelectronic devices, increasingly used in lighting and displays. Conventional InGaN/GaN LEDs of c-orientation exhibit strong internal polarization fields and suffer from significantly reduced radiative recombination rates. A reduced polarization within the device can improve the optical matrix element, thereby enhancing the optical output power and efficiency. Here, we have demonstrated computationally that the step-doping in the quantum barriers is effective in reducing the polarization-induced fields and lowering the energy barrier for hole transport. Also, we have proven experimentally that such InGaN/GaN LEDs with Si step-doped quantum barriers indeed outperform LEDs with wholly Si-doped barriers and those without doped barriers in terms of output power and external quantum efficiency. The consistency of our numerical simulation and experimental results indicate the effects of Si step-doping in suppressing quantum-confined stark effect and enhancing the hole injection, and is promising in improving the InGaN/GaN LED performance.

Simulation Studies on GaN/InGaN Based Multi Quantum Well Light Emitting Diode for Reducing Efficiency Droop by Imposing Improved Si-Doped Barrier Model

Simulation Studies on GaN/InGaN Based Multi Quantum Well Light Emitting Diode for Reducing Efficiency Droop by Imposing Improved Si-Doped Barrier Model

Investigation of dominant effect on efficiency droop in InGaN light emitting device

To understand a major effect on efficiency droop, radiative characteristics of InGaN laser diodes (LDs) of emission wavelength of ∼445 nm are studied at subthreshold levels for different active structures; (1) InGaN single quantum well (SQW), (2) double quantum wells (DQWs) with Si-doped barrier, and (3) DQWs with undoped barrier. For InGaN LDs with DQWs, the absolute radiative efficiency is most dominant for the case of undoped barrier, and least for the case of Si-doped barrier. The efficiency droop in InGaN LDs with DQWs, regardless of the doping condition of barriers, is typical of reported InGaN light emitting devices, whereas for the InGaN LD with SQW, the efficiency droop is significantly improved for all current density levels due to the least polarization field of InGaN QW.

InGaN ∕ GaN Multiple-Quantum-Well LEDs with Si-Doped Barriers

multiple-quantum-well (MQW) light-emitting diodes (LEDs) with undoped and Si-doped barriers were fabricated. It was found that we could enhance crystal quality of the MQW structure by introducing Si doping into the barrier layers. It was also found that carriers can be injected into the MQW active region more easily for the LED with Si-doped barriers. With a injection current, it was found that forward voltages were 3.29 and for the LEDs with undoped barriers and Si-doped barriers, respectively. The output power and external quantum efficiency of the LED with Si-doped barriers were also found to be larger.

Electron magnetotransport in GaAs/AlGaAs superlattices with weak and strong inter-well coupling

We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.

Illumination-induced changes of the Fermi surface topology in three-dimensional superlattices

The magnetoresistance of the molecular beam epitaxy grown $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state and after brief illumination by a red-light diode at low temperature, $T\ensuremath{\approx}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also that the coupling among two-dimensional electron layers in neighboring quantum wells has been reduced.

Effect of Si doping on the photoluminescence properties of AlGaInP/GaInP multiple quantum wells

The influence of Si doping on the photoluminescence (PL) properties of (Al0.3Ga0.7)0.5In0.5P/Ga0.5In0.5P multiple-quantum-wells (MQWs) was studied. For the samples without p-type layers, the PL peak wavelength from (Al0.3Ga0.7)0.5In0.5P/Ga0.5In0.5P MQWs did not vary when Si was doped in MQWs, the PL peak intensity did not change obviously and the PL FWHM broadened. We consider that Si doping results in worse interface quality of (Al0.3Ga0.7)0.5In0.5P/Ga0.5In0.5P MQWs. However, for the full light-emitting diode (LED) structure samples, the PL intensity of MQWs obviously increased when Si was doped in MQWs. The PL intensity from MQWs with Si-doped barriers was about 13 times stronger than that of undoped MQWs. The PL intensity from MQWs with Si-doped barriers and wells was strong as 28 times as that of undoped MQWs. The reasons are discussed.

Magnetoresistance oscillations in GaAs/AlGaAs superlattices subject to in-plane magnetic fields

The MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been used to study a 3D → 2D transition under the influence of the in-plane component of an applied magnetic field. The longitudinal magnetoresistance data measured in tilted magnetic fields have been interpreted in terms of a simple tight-binding model. The data provide values of basic parameters of the model and make it possible to reconstruct the superlattice Fermi surface and to calculate the density of states for the lowest Landau subbands. Positions of van Hove singularities in the DOS agree excellently with magnetoresistance oscillations, confirming that the model describes adequately the magnetoresistance of strongly coupled semiconductor superlattices.

Growth and fabrication of AlGaN-based ultraviolet light emitting diodes on 6H-SiC(0 0 0 1) substrates and the effect of carrier-blocking layers on their emission characteristics

Growth, fabrication, and the electrical and optical characterization of ultraviolet light emitting diodes and their components, including Al x Ga 1− x N films, quantum wells (QWs), and ohmic contacts, and the problems encountered in the process integration of these components have been investigated. Ni/Au ohmic contacts with specific contact resistivities of 2.2 × 10 −4 and 2.0 × 10 −2 Ω cm 2 were achieved on annealed, Mg-doped ([Mg] ∼ 5 × 10 19 cm −3), p-type GaN layers that had been cleaned in HCl at 85 °C and on the backside of the SiC substrates after annealing in nitrogen, respectively. The emission intensity of the diodes increased with an increase in the number of Al 0.06Ga 0.94N/Al 0.10Ga 0.90N QWs and with the use of Si-doped n-type barrier layers. The highest intensities of the principle emission at 353 nm were measured at all values of the injection current in the device with a p-type carrier-blocking layer at the top of the QWs; this device also exhibited the highest values of light output power. Growth of an n-type carrier-blocking layer at the bottom of the QWs had an adverse effect on their characteristics. A broad peak centered at ∼540 nm exhibited yellow luminescence and was present in the spectra acquired from all the devices. This peak is attributed to absorption of the ultraviolet emission by and re-emission from the p-GaN and/or to the luminescence from the AlGaN within QWs by current injection.

Effects of Si-doped GaAs layer on optical properties of InAs quantum dots

We have investigated the optical properties of InAs self-assembled quantum dots (SAQDs) with the Si-doped GaAs barrier layer. Two types of samples are fabricated according to the position of the Si-doped GaAs layer. For type A samples the Si-doped GaAs layer is grown below the QDs, whereas for type B samples the Si-doped GaAs layer is grown above the QDs. For both types of samples the excited-state emissions caused by state filling are observed in photoluminescence (PL) spectra at high excitation power densities. The bandgap renormalization of QDs can be found from the shift of the PL peak energy. Particularly, for type A samples the Si atoms act as nucleation centers during the growth of InAs QDs on the Si-doped GaAs layer and affect the density and the size of the QDs. The Si-doped GaAs layer in type A samples has more effects on the properties of QDs, such as state filling and bandgap renormalization than those of type B samples.

The influence of Si-doping to the characteristics of AlGaInP/GaInP multiple quantum wells

AlGaInP/GaInP multiple quantum well structures were grown by low-pressure MOCVD on n-GaAs substrates. The influence of Si-doping to the characteristics of AlGaInP/GaInP multiple quantum wells was studied. The PL intensity from MQW with Si-doped barriers is about 13 times stronger than that of undoped MQW. The PL intensity from MQW with Si-doped barriers and wells is strong as 28 times as that of undoped MQW.