InGaN/GaN LEDs with a Si-doped InGaN/GaN short-period superlattice tunneling contact layer

Nitride-based light-emitting diodes (LEDs) with Si-doped n⁺-In_0.23Ga_0.77N/GaN short-period superlattice (SPS) tunneling contact top layer were fabricated. It was found that although the measured specific-contact resistance is around 1 × 10⁻² Ω-cm² for samples with an SPS tunneling contact layer, the measured specific-contact resistance is around 1.5×10⁰ Ω-cm² for samples without an SPS tunneling contact layer. Furthermore, it was found that one could lower the LED-operation voltage from 3.75 V to 3.4 V by introducing the SPS structure. It was also found that the LED-operation voltage is almost independent of the CP₂Mg flow rate when we grow the underneath p-type GaN layer. The LED-output intensity was also found to be larger for samples with the SPS structure.     

A monolithic white LED with an active region based on InGaN QWs separated by short-period InGaN/GaN superlattices

A new approach to development of effective monolithic white-light emitters is described based on using a short-period InGaN/GaN superlattice as a barrier layer in the active region of LED structures between InGaN quantum wells emitting in the blue and yellow-green spectral ranges. The optical properties of structures of this kind have been studied, and it is demonstrated that the use of such a superlattice makes it possible to obtain effective emission from the active region.     

AlGaN/GaN/InGaN/GaN DH-HEMTs with an InGaN notch for enhanced carrier confinement

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.     

The study of lateral carrier transport in structures with InGaN quantum dots in the active region

GaN-based structures with InGaN quantum dots in the active region, which emit in the blue and green spectral ranges, are studied. The structures grown by both the conventional method and with the use of special procedures of the growth of the active region were compared. The use of these special growth modes stimulates the activated phase decomposition, which leads to the formation of quantum dots with substantially larger localization depth of electrons. It is shown that the formation of such deep quantum dots, like the formation of larger inhomogeneities of the active region, substantially suppresses the lateral carrier transport. This effect improves the characteristics of light-emitting diode structures at a low injection level and increases the temperature stability of quantum efficiency.     

Lasing in the vertical direction in InGaN/GaN/AlGaN structures with InGaN quantum dots

InGaN/GaN structures with dense arrays of InGaN nanodomains were grown by metallorganic chemical vapor deposition. Lasing in vertical direction occurs at low temperatures, indicating ultrahigh gains (～ 10⁵ cm^?1) in the active region. Fabrication of an effective AlGaN/GaN distributed Bragg reflector with reflectivity exceeding 90% enables vertical lasing at room temperature in structures with a bottom distributed Bragg reflector, despite the absence of a well-reflecting upper mirror. The lasing wavelength is 401 nm, and the threshold excitation density is 400 kW/cm².     

Diffusion and tunneling currents in GaN/InGaN multiple quantum welllight-emitting diodes

We have studied the electrical characteristics and optical properties of GaN/InGaN multiple quantum well (MQW) light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition. It appears that there is an essential link between material quality and the mechanism of current transport through the wide-bandgap p-n junction. Tunneling behavior dominates throughout all injection regimes in a device with a high density of defects in the space-charge region, which act as deep-level carrier traps. However, in a high-quality LED diode, temperature-dependent diffusion-recombination current has been identified with an ideality factor of 1.6 at moderate biases. Light output has been found to follow a power law, i.e., L ∝ I^m in both devices. In the high-quality LED, nonradiative recombination centers are saturated at current densities as low as 1.4 × 10^-2 A/cm². This low saturation level indicates that the defects in GaN, especially the high density of edge dislocations, are generally optically inactive     

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.     

Mechanisms for photon-emission enhancement with silicon doping in InGaN/GaN quantum-well structures

Material and optical analyses of three InGaN/GaN quantum-well (QW) samples with different silicon-doping conditions were conducted. Quantum-dot (QD) structures were observed in samples of silicon doping either in barriers or wells. The calibrated-radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of zero-dimensional (0-D) structure upon silicon doping. In optical characterization, the barrier-doped sample showed a blueshift of the photoluminescence (PL) peak, enhancement of integrated PL intensity, reduction of Stokes shift (SS), decrease of carrier-activation energy, and shortening of PL decay time. Except the insignificant PL peak shift, the well-doped sample showed similar trends, although they are not as prominent as the barrier-doped sample. Such results mainly originated from the reduction of the quantum-confined Stark effect (QCSE) within the clusters. Contrary to the interpretation in the past, the major mechanism for QCSE reduction is due to strain relaxation, instead of carrier screening, in conjunction with the formation of QD structures. Such a conclusion is supported by the result of smaller changes of optical behavior in the well-doped sample, in which carrier screening is expected to be more significant. In this sample, besides strain relaxation, enhanced carrier localization (CL) might represent another important mechanism for photon-emission improvement.     

Hopping transport in the space-charge region of p-n structures with InGaN/GaN QWs as a source of excess 1/f noise and efficiency droop in LEDs

Semiconductors - It is shown that the emission efficiency and the 1/f noise level in light-emitting diodes with InGaN/GaN quantum wells correlate with how the differential resistance of a diode...     

Composite InGaN/GaN/InAlN heterostructures emitting in the yellow-red spectral region

The results of studies of the properties of composite InGaN/GaN/InAlN heterostructures are reported. It is shown that, in the InAlN layer, there is substantial phase separation that brings about the formation of three-dimensional islands consisting of AlN-InAlN-AlN regions. The dimensions of these islands depend on the thickness of the InAlN layer and the conditions of epitaxial growth. Interruptions in the growth of InAlN provide a means for influencing the structural properties of the InAlN islands. The use of composite InGaN/GaN/InAlN heterostructures, in which the InGaN layer with a high In content serves as the active region in light-emitting diode structures, makes it possible to achieve emission in the yellow-red wavelength range 560?C620 nm.     

Base transit time in abrupt GaN/InGaN/GaN HBT's

Base transit time in an abrupt GaN/InGaN/GaN HBT is reported. Temperature and doping concentration dependence of low field mobility is obtained from an ensemble Monte Carlo simulation. Base transit time, τ_b, decreases with increasing temperature. The low temperature τ_b is dominated by the diffusion constant or, in other words, transport within the neutral base region. However, at elevated temperatures base transit time is dependent more upon the base-collector junction velocity or, in other words, by the transport across the heterointerface. τ_b increases with In-mole fraction showing a stronger dependence at lower temperatures. Unity gain current cut-off frequency, f_T, is a strong function of temperature and base doping concentration. An f_T of 20 GHz is obtained for a 0.05 μm HBT     

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.     

p型InGaN/GaN超晶格N面GaN基LED的光电特性

随着氮(N)面GaN材料生长技术的发展,基于N面GaN衬底的高亮度发光二极管(LED)的研究具有重要的科学意义.研究了具有高发光功率的N面GaN基蓝光LED的新型结构设计,通过在N面LED的电子阻挡层和多量子阱有源层之间插入p型InGaN/GaN超晶格来提高有源层中的载流子注入效率.为了对比N面GaN基LED优异的器件性能,同时设计了具有相同结构的Ga面LED.通过对两种LED结构的电致发光特性、有源层中能带图、电场和载流子浓度分布进行比较可以发现,N面LED在输出功率和载流子注入效率上比Ga面LED有明显的提升,从而表明N面GaN基LED具有潜在的应用前景.     

Effect of pressure in the growth reactor on the properties of the active region in the InGaN/GaN light-emitting diodes

Effect of pressure in the reactor in the case of growth of active regions in the InGaN/GaN light-emitting diodes by the method of vapor-phase epitaxy from metalorganic compounds on their electroluminescent and structural properties has been studied. It is shown that, as pressure is increased, the InGaN layers become transformed from being continuous in the lateral direction to the layers of separate InGaN islands. This transformation affects both the emission efficiency and the dependence of efficiency on current.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

自组装InGaN/GaN量子点的研究进展

GaN基量子点的生长及其特性已成为目前Ⅲ-Ⅴ族半导体研究的热点.由于其较强的量子效应,量子点结构的光电器件有望获得比量子阱器件更优异的性能.总结了InGaN/GaN量子点的最新研究进展,从生长模式、发光特性和制造工艺三个方面对InGaN/GaN量子点做了较详细的介绍,并对其在半导体器件的具体应用前景上提出了合理的设想.     

Analysis of Interface Scattering in AlGaN/GaN/InGaN/GaN Double-Heterojunction High-Electron-Mobility Transistors

This paper presents detailed investigations on the direct-current (DC) characteristics of an AlGaN/GaN/InGaN/GaN double-heterojunction high-electron-mobility transistor (DH-HEMT) using two-dimensional numerical analysis. In this work, the hot-electron effect is taken into account and implemented in the hydrodynamic model. The results indicate that carrier transport in this kind of device exhibits properties significantly different from that in a conventional AlGaN/GaN HEMT. Due to imperfections at the GaN/InGaN interface, scattering caused by the interface roughness, phonons, etc. inhibit the negative differential conductance in high electric field. In addition, the velocity increment of electrons around the gate edge is dominated by the overshoot effect rather than the phonon effect. The energy exchange between phonons and electrons, as presented in this paper, illustrates that the dissipated power is just a small portion of the exchanged energy. For further performance improvement, more lattice-matched material with strong polarization for the barrier layer is proposed.     

Direct observation of piezoelectric fields in GaN/ InGaN/GaN strained quantum wells

Off-axis electron holography is used to examine a single thin InGaN quantum well in GaN viewed in cross-section. The results show a phase offset across the well, which, under weakly diffracting conditions, is an approximately linear function of specimen thickness. This phase offset is ascribed to a change AV0 in the specimen mean inner potential V0 caused by a piezoelectric field induced by misfit strains in the InGaN layer. This paper examines the dependence of the phase offset on the diffracting conditions and on thin foil relaxation effects. It is shown that relaxation is negligible for the film thicknesses involved. Using a range of weakly diffracting conditions, the phase offset is measured as deltaV0/V0 = 0.042+/-0.012. Zone axis convergent beam electron diffraction patterns were taken and compared to simulations to determine the crystal polarity, showing the magnitude of the inner potential increased in the [0001] direction. By using dark-field displacement fringes to measure the InGaN layer thickness, and recent estimates of V0, the magnitude of the piezoelectric field is determined. This paper assesses the accuracy and limitations of electron holography for the studies of electric fields in other GaN structures.     

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al_0.3Ga_0.7N/GaN/In_0.1Ga_0.9 N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In_0.1Ga_0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In_0.1Ga_0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF₄-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs     

InGaN/GaN light emitting diodes with a p-down structure

Nitride-based p-down blue light emitting diodes (LEDs) were successfully fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn-on voltage could be reduced from 15 V to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 mA output power was 1 mW for the p-down LED with an Mg+Si codoped interlayer and a rough p-tunnel layer.