Short-period Superlattices Layers Research Articles

High Efficiency InGaN Blue Light-Emitting Diode With ${>}{\rm 4}\hbox{-}{\rm W}$ Output Power at 3 A

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.

Optimization of InGaN/GaN superlattice structures for high-efficiency vertical blue light-emitting diodes

We investigate carrier distribution and efficiency characteristics of GaN-based vertical blue light-emitting diodes (LEDs) having InGaN/GaN short-period superlattice (SL) structures by using numerical simulations. The SL structures exist between multiple-quantum-well active layers and an n-GaN layer. The In composition and doping concentration of SL layers are found to have strong influence on the electron concentration distribution in the plane of QWs, internal quantum efficiency, and forward voltage of vertical LEDs. The electron distribution becomes homogenous as the In composition increases or doping concentration decreases, which results in the reduction of efficiency droop and the increase of forward voltage. Based on the simulation results, optimum SL structures for obtaining the maximum wall-plug efficiency are found.

Substrate effects on the strain relaxation in GaN/AlN short-period superlattices

We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

AlN/GaN Distributed Bragg Reflectors Grown via Metal Organic Vapor Phase Epitaxy using GaN Insertion Layers

ABSTRACTDistributed Bragg Reflectors (DBRs) are an important component of various optoelectronic devices for ultra violet and visible wavelengths. In the III-Nitride material system, Aluminum Nitride (AlN) and Gallium Nitride (GaN) offer a large contrast in refractive index and are therefore well suited for fabricating DBRs with high reflectivity and wide bandwidths using relatively few periods. However, the large lattice and thermal mismatch leads to cracking in these heterostructures. In this work short period superlattice layers have been used to fabricate high reflectivity (> 94%) nitride based DBRs via Metal Organic Vapor Phase Epitaxy. Short period AlN/GaN superlattices containing three to four monolayers of GaN have been employed as the low refractive index layer in DBRs to minimize cracking. Using this technique, crack-free DBRs reflecting from 440-475 nm with up to 25 periods have been fabricated. The technique has been proven to be versatile and resulted in large area yield DBRs grown on a variety of different sapphire substrates.

Thermal escape process of photogenerated carriers from GaAs single-quantum-well contained in GaAs/AlAs superlattices

Thermal escape process of carriers has been investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy in GaAs single-quantum-well (SQW) contained in GaAs/AlAs short-period superlattices (SPSs) over a wide temperature (T) range between 15 and 300 K. A typical PL spectrum indicates two emission bands corresponding to SQW and SPS layers. When the sample temperature is increased from 15 K, the PL intensity of SQW increases and reaches the maximum value at 50 K, while the PL intensity of SPS drastically decreases below 60 K. With further increase in temperature up to 120 K, the PL intensity of SPS increases. These results indicate the following processes: in the region of 15 < T < 50 K, the photogenerated carriers in the SPS layer can quickly relax into the SQW before radiative recombination within the SPS layer, as long as the vertical carriers transport assisted by phonon scattering is sufficient. In the region of 60 < T < 120 K, the carriers in the SQW can easily escape toward the SPS layer due to thermal excitation. These processes are supported by the time-resolved PL results where the escape time from the SQW to the SPS layers gets short with increase in temperature.

Carrier leakage suppression utilising short‐period superlattices in 980 nm InGaAs/GaAs quantum well lasers

AbstractShort‐period Super Lattices (SSLs) have been proposed as a method of reducing carrier leakage in semiconductor lasers. SSLs increase the carrier confinement by increasing the effective barrier height due to the carriers being “reflected” from the SSL. Thermionically driven leakage of electrons from the quantum wells has been suggested as an important non‐radiative current path in (Al)GaAs based devices which is exacerbated at high temperatures. Due to the fact that the application of high pressure causes a reduction in the zone centre Γ–X minima splitting, pressure can be used to investigate the extent to which such leakage increase the threshold current. In this study, we compare devices with and without SSL layers and find direct evidence for leakage suppression in devices containing SSLs up to a temperature of 60 °C. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Lateral composition modulation in AlAs/InAs and GaAs/InAs short period superlattices structures: The role of surface segregation

The effect of In surface segregation on the microstructure of short period superlattices (SPSs) in two different material systems with nominally equivalent lattice misfit, AlAs/InAs and GaAs/InAs, has been investigated and compared. It was found that the quality of the SPSs and the appearance of lateral composition modulation are remarkably different in these two systems. For AlAs/InAs SPSs grown at temperatures of T=500 °C, uniform structures devoid of lateral composition modulation were obtained. Samples grown at T&amp;gt;500 °C exhibit lateral composition modulation. Uniform and homogeneous SPS structures were not obtained in the GaAs/InAs structures over the entire temperature range examined in this study (475 °C⩽T⩽510 °C). Instead, lateral composition modulation with varying degrees of regularity was always observed. It was found that In segregation and roughening determine the microstructure. A kinetic exchange model predicts that at an optimum temperature the SPS layers are more intermixed for the AlAs/InAs SPSs. Thus, the lattice mismatch is lower and the driving force for roughening is reduced, resulting in uniform SPS structures. Growing the structure away from the optimum temperature for smooth growth may induce roughening-driven composition modulation. The GaAs/InAs structures are less intermixed over the temperatures studied, which results in higher mismatch between the individual layers and a higher driving force for roughening and lateral composition modulation.

Lateral composition modulation in GaP/InP short-period superlattices grown by solid source molecular beam epitaxy

Transmission electron microscopy (TEM) is employed to investigate the structural properties of (GaP)/(InP) short-period superlattices (SPS) grown at temperatures in the range of 425–490 °C by solid source molecular beam epitaxy. TEM results show that lateral composition modulation (LCM) is formed in the SPS layers grown at temperatures above 460 °C. Transmission electron diffraction results show that CuPt ordering occurs in all samples. It is shown that the degree of order increases, reaches a maximum at 460 °C, and then decreases, as the growth temperature increases. Photoluminescence examination (at 9 K) shows that the samples experience a reduction in band gap, which is in the range of 55–221 meV, as the growth temperature increases. The reduction is attributed to the combined effects of the LCM and CuPt-type ordering.

Oxidation Kinetics and Microstructure of Wet-Oxidized MBE-Grown Short-Period AlGaAs Superlattices

AbstractThe kinetics of the wet oxidation process of MBE-grown high-Al-content AlAs/Al0.6Ga0.4As short-period superlattices (SPSLs) was investigated and compared to AlGaAs alloys and pure AlAs. We found that alloys and superlattices (SLs) have different oxidation characteristics. These differences were attributed to traces of the superlattice structure in the oxidized material. The microstructure and chemistry of SPSLs with an equivalent composition of Al0.98Ga0.02As was studied, using transmission electron microscopy, energy-dispersive x-ray spectroscopy, Rutherford backscattering, and nuclear reaction analysis for hydrogen-profiling. We also report on the mechanical stability of oxidized SPSL layers in optoelectronic device structures.

Improvement of Optical Properties of Multilayer Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices on GaAs(311)A

Multilayer quantum dots (MQDs) structures are fabricated on a GaAs(311)A substrate by sandwiching the quantum dots (QDs) self-formed in (GaP)1.5(InP)1.88 short-period superlattices (SLs) with InGaP/InAlP SL layers instead of InGaP layers, as barrier and cladding layers. Narrower photoluminescence (PL) and electroluminescence (EL) linewidths and weaker temperature variations are observed for the modified MQDs compared with the previously reported best values for MQDs with InGaP barrier and cladding layers. PL and EL peak energies for the modified MQDs are higher than those for the previous MQDs. These results suggest the enhancement of carrier confinement by the use of InGaP/InAlP SL layers as barrier and cladding layers. The temperature dependence of EL intensity is also improved.

GaAs Quantum-Wire Laser Using Fractional Layer Superlattice

In (AlAs)k/n(GaAs)l/n (k+l=n) fractional layer superlattice (FLS) growth on the (001) GaAs vicinal surface 2° off toward [1̄10] by metal-organic chemical vapor deposition, using a (AlAs)m(GaAs)n (m, n>3) short period superlattice as the buffer layer very much improve the uniformity of lateral periodicity in the FLS near the FLS/buffer interface compared with using binary (GaAs and AlAs) and ternary AlGaAs as the buffer layers. This is due to the improvement in the uniformity of the terrace width on the surface of a short period superlattice by step-ordering. As a result, a large polarization anisotropy of about 50% is observed in the photoluminescence spectrum for the quantum-wire array structure oriented toward [110] in which a 4-nm-thick (AlAs)1/4(GaAs)3/4 FLS layer is sandwiched by (AlAs)3(GaAs)3 short period superlattice layers. Separated confinement heterostructures consisting of a 12-nm-thick undoped (AlAs)1/4(GaAs)3/4 FLS quantum-wire active layer, doped (AlAs)1(GaAs)2 carrier confinement layers, and doped Al0.6Ga0.4As optical confinement layers are grown. To prevent the disordering of the FLS layer induced by the diffusion of intentionally doped impurity atoms during the growth and the processing, low-diffusive Si and C atoms are used for n- and p-type doping into carrier and optical confinement layers. Lasing operation is obtained by current injection below 195 K for the electrode with simple stripe geometry. The lasing wavelength depends on the orientation of the stripe electrode. The electrode with [110] orientation which is parallel to the quantum wire array has a lasing wavelength of 662 nm, and the [1̄10] orientation which is perpendicular to the array has a lasing wavelength of 667 nm at 10 K.

Photoluminescence Fatigue in Laterally-Ordered (GaP)2/(InP)2 Short-Period-Superlattices (SPS) Grown by Molecular Beam Epitaxy

AbstractSpontaneously laterally ordered (GaP)2/(InP)2 short period superlattices (SPS) grown by Molecular Beam Epitaxy (MBE) on nominal (100) GaAs substrates have been studied by photoluminescence (PL) spectroscopy. The samples studied included SPS comprising 110 pairs of (GaP)2/(InP)2 (total thickness σ90 nm) and multiquantum well structures in which quantum wells comprising 12 pairs of (GaP)2/(InP)2 SPS layers (thickness σ10 nm) are alternated with lattice-matched GaInP random alloy barrier layers. The 5K PL spectra include a σ1760 meV nearband edge band, and a much broader, lower energy (σ1670 meV) luminescence band that exhibits an unusual fatiguing behavior; its intensity diminishes monotonically during continuous illumination by the exciting light. This fatigued PL state is metastable at low temperatures. In the quantum well structure, although the relative intensity of the lower energy band is significantly weaker in comparison to the higher one, the fatiguing behavior still exists. However the fatiguing rate is slower in quantum well structures than that observed in the thick SPS film.

Formation of lateral quantum wells in vertical short-period superlattices by strain-induced lateral-layer ordering process

The lateral quantum wells formed in vertical (GaAs)n/(InAs)n and (GaP)n/(InP)n short-period superlattices (SPS) grown on nominally (100) InP and GaAs substrates, respectively, have been examined. The strain induced from the deviation of superlattice periodicity from na0 is the major driving force of the lateral modulation of the composition along the [110] direction, where the integer n is the number of monolayers of each binary compound within a period of SPS structure and a0 is the lattice constant of the substrate material. When the deviation of the superlattice periodicity from na0 is larger than ∼4%, both (GaAs)n/(InAs)n and (GaP)n/(InP)n ordered vertical SPS layers were found to have a lateral periodic modulation of composition with periodicities as small as ∼200 Å. This effect is enhanced when n was increased from 1 to 2.