Multiquantum-well Active Region Research Articles

Photoluminescence Properties of InGaN/InGaN MQWs with Different Electron Injection Layers

The structural and optical properties of InxGa1-xN/InyGa1-yN multi quantum well (MQW) light emitting devices with/without electron injection layers were studied. The samples with electron injection layer consist of step-graded (GIE) and two step staircase (SEI) electron injection layer between n-type GaN and MQWs active region. Edge and screw type of dislocation densities were deduced from High Resolution X-Ray Diffraction (HR-XRD) curves and no significant difference were realized. The zeroth and higher order satellite peaks were more clearly observed in the sample without electron injection layer. Optical characterization was carried out by temperature dependent photoluminescence (PL) technique. It was found that the PL densities of samples with step-graded and two step-staircase electron injection layers had almost two times lower temperature dependence compared to the reference sample without electron injection layer. On the other hand, the line width of the photoluminescence peak associated with MQWs is much narrower at low temperature for sample without electron injection layer than the other two samples.

Low-threshold strain-compensated InGaAs/(In,Al)GaAs multi-quantum well nanowire lasers emitting near 1.3 μm at room temperature

Realizing telecom-band lasing in GaAs-based nanowires (NW) with low bandgap gain media has proven to be notoriously difficult due to the high compressive strain built up in the active regions. Here, we demonstrate an advanced coaxial GaAs-InGaAs multi-quantum well (MQW) nanowire laser that solves previous limitations by the introduction of a strain compensating InAlGaAs buffer layer between the GaAs core and the MQW active region. Using a buffer layer thickness comparable to the core diameter applies a significant tensile strain to the GaAs core which efficiently minimizes the compressive strain in the InGaAs MQW and enables large In-content without plastic relaxation. Experimental verification is shown for NW-lasers with an In-content of up to 40% in the MQW, evidencing a clear strain-relieved redshift of the lasing emission and a strong reduction of the lasing threshold compared to highly strained MQWs in state-of-the-art GaAs NW-lasers. This way we achieve optically pumped room temperature lasing operation with a threshold below 50 μJ cm−2 in the telecom O-band close to 1.3 μm.

Quantum Barrier Growth Temperature Affects Deep Traps Spectra of InGaN Blue Light Emitting Diodes

Electroluminescence (EL) efficiency, deep electron and hole traps spectra, microcathodoluminescence (MCL), electron beam induced current (EBIC) imaging, and MCL spectra were studied for blue GaN/InGaN multi-quantum-well (MQW) light emitting diodes differing by the temperature at which the GaN barriers of the MQW active region were grown. It was found that increasing the growth temperature from 850 to 920°C very strongly suppressed the formation of deep electron traps with level at Ec-1 eV in the GaN barriers and of the hole traps with levels at Ev+0.7 eV in InGaN QWs. The suppression of the formation of the Ec-1 eV electron trap, a known prominent nonradiative recombination center in n-GaN, improved the carrier injection efficiency into the InGaN QWs and increased the external quantum efficiency by about 9%. EBIC and MCL imaging showed that the density of threading dislocations and terminating them V-pits was relatively low and similar for both studied growth temperatures, close to 108 cm−2. The cross-sectional dimensions of the V-pits were measurably higher for increased growth temperature. However, the rather low dislocation density and rather high dimensions of the V-pits were believed to result in minor contribution of these defects to the observed EL efficiency changes.

Control of optical loss in GaN-based planar cavities

Optical losses in GaN-based resonant planar cavity are discussed and experimentally examined in respects of indium tin oxide (ITO) inner-cavity current spreading layer, the defect-rich GaN layer, and multi-quantum well (MQW) active region. Devices with different structures are adopted to study the optical loss. It is demonstrated that a thin ITO layer and a coupled MQW active region are essential to achieve a low loss and high Q-value cavity. The results show that low-loss MQW design is particularly important in fabricating high-Q value GaN-based planar cavities.

Performance Enhancement of Ultraviolet Light-Emitting Diodes by Incorporating a Thin Al(In)GaN Interlayer in Multiquantum-Well Region

In this paper, the performance improvement of 380-nm InGaN/AlGaN ultraviolet light-emitting diodes (UV-LEDs) is investigated by incorporating an undoped Al(In)GaN thin interlayer between the InGaN well and AlGaN barrier in multiquantum-well (MQW) active region. By inserting the graded-composition AlGaN and AlInGaN thin interlayers, the light output powers of UV-LEDs are significantly increased by 70% and 105% at 20 mA, respectively, as compared with the LED without the interlayer. Remarkable efficiency enhancement in the UV-LEDs with graded-composition AlGaN and AlInGaN interlayers is mainly attributed to the further improvement of the electron confinement and hole injection with more uniform distribution in the MQW active region. Besides, photoluminescence and atomic force microscope analyses indicate that the MQW quality can be enhanced by incorporating a graded-composition AlInGaN thin interlayer in the MQW active region of UV-LEDs.

Effect of Sapphire Substrate Thickness on the Characteristics of 450 nm InGaN/GaN Multi-Quantum Well Light-Emitting Diodes.

450 nm InGaN/GaN multi-quantum well (MQW) ligth-emitting diodes (LEDs) prepared on sapphire substrate with different thicknesses were fabricated and characterized. By thinning the sapphire substrate to 50 µm, it was found that the LED exhibited the highest light output power of ~48 mW under high injection current of 50 mA, improved by about 35% compared to that with 200 µm-thick sapphire without increasing the operating voltage. The electroluminescence intensity was increased and the spectral peak wavelength was blue-shifted, because the wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW active region of the LED. The internal quantum efficiency was also improved by about 10% at an injection current of 50 mA. Moreover, the external quantum efficiency and light extraction efficiency were optimized because of enhanced light output intensity. The results confirmed that sapphire substrate thinning effectively alters the piezoelectric field in the InGaN/GaN active region, and hence increases both of the effective band gap and the probability of radiative recombination.

GaN-based Dual-Color Light-Emitting Diodes With a Hybrid Tunnel Junction Structure

The authors propose the use of tunnel junction (TJ) structure to cascade a green multiquantum well (MQW) active region and a blue MQW active region, and the fabrication of GaN-based dual-color light-emitting diodes (LEDs). It was found that output power observed from the TJ cascaded LED was only slightly smaller than the summation of those observed from the green LED and the blue LED. This suggests that most of the injected carriers could tunnel through the TJ and could be repeatedly used for photon generation. Furthermore, it was found that the proposed dual-color LEDs was highly reliable.

Reduced efficiency droop in blue InGaN light-emitting diodes by thin AlGaN barriers

The phenomenon of efficiency droop in blue InGaN light-emitting diodes (LEDs) is studied numerically. Simulation results indicate that the severe Auger recombination is one critical mechanism corresponding to the degraded efficiency under high current injection. To solve this issue, LED structure with thin AlGaN barriers and without the use of an AlGaN EBL is proposed. The purpose of the strain-compensation AlGaN barriers is to mitigate the strain accumulation in a multiquantum well (MQW) active region in this thin-barrier structure. With the proposed LED structure, the hole injection and transportation of the MQW active region are largely improved. The carriers can thus distribute/disperse much more uniformly in QWs, and the Auger recombination is suppressed accordingly. The internal quantum efficiency and the efficiency droop are therefore efficiently improved.

Uni-axial external stress effect on green InGaN/GaN multi-quantum-well light-emitting diodes

The influence of uni-axial external stress on green InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) is evaluated. LEDs with external tensile stress show an improvement in light output power up to 35% at an injection current of 20 mA. The maximum output power can be obtained by considering the residual mechanical stress induced by the wafer bowing. In contrast, when the LEDs were exposed to an external compressive stress, the light output power was reduced by ∼7% at an injection current of 20 mA. Moreover, when the compressive strain developed in InGaN/GaN MQW active region is relaxed, the peak wavelength of electroluminescence was blue-shifted. The results confirmed that applying external tensile stress effectively compensates for the compressive strain and alters the piezoelectric field in the InGaN/GaN active region, and hence increases the probability of radiative recombination.

GaSb-based VCSELs emitting in the mid-infrared wavelength range (2–3 μm) grown by MBE

We report the growth conditions and operations of electrically pumped monolithic Sb-based type-I quantum-well vertical cavity surface emitting lasers (VCSELs) emitting above 2.2 μm. The structures were grown on (0 0 1)-GaSb substrates by molecular beam epitaxy (MBE) and are made of two N-type GaSb/AlAsSb Bragg reflectors, a GaInAsSb/AlGaAsSb multiquantum-well active region and an InAsSb/GaSb tunnel junction. Growth conditions have been optimized for each target wavelength. Laser emission in CW up to 293 K at 2.3 μm and in pulsed regime at 2.52 μm at room temperature (RT) is demonstrated. These are the longest wavelength achieved with electrically pumped VCSELs to date.

Room-Temperature Continuous-Wave Operation of 2.3-$\mu$m Sb-Based Electrically Pumped Monolithic Vertical-Cavity Lasers

Room-temperature continuous-wave (CW) operation of GaSb-based monolithic microcavity vertical-cavity surface-emitting lasers operating near 2.3 mum is presented. These devices were composed of two n-doped AlAsSb-GaSb Bragg mirrors, a type-I GaInAsSb-AlGaAsSb multiquantum-well active region, and an n++-InAsSb/p++-GaSb tunnel junction. CW laser operation was observed up to 294 K. A CW threshold current density as low as 1.1 kA ldr cm-2 was obtained at 284 K for 60-mum-diameter devices (20-mum-diameter emitting area).

1.3 µm gain coupled DFB laser with InGaAlAs MQW grown on absorptive InGaAsP corrugation

An InGaAlAs multiquantum well (MQW) has been successfully overgrown on the absorptive InGaAsP corrugation for fabricating the 1.3 µm gain coupled distributed feedback (DFB) lasers. The absorptive InGaAsP corrugation was efficaciously preserved during the overgrowth of the InGaAlAs MQW active region. The absorptive InGaAsP corrugation has a relatively high intensity around the PL peak wavelength in comparison with that of the InGaAlAs MQW. The fabricated DFB laser exhibited a side mode suppression ratio of 40 dB together with a high single-mode yield of 90%.

High temperature continuous wave operation of Sb-based vertical external cavity surface emitting laser near 2.3 μm

The growth conditions and the operation of a diode-pumped AlGaAsSb/GaInAsSb type-I quantum-well vertical cavity surface emitting laser emitting near 2.3 μm in an external cavity configuration are reported. The epitaxial structure was grown in two steps on a GaSb substrate by molecular beam epitaxy. It is made of a GaSb/AlAsSb Bragg reflector, a GaInAsSb/AlGaAsSb multi quantum-well active region and an AlAsSb heatspreader layer. A TEM 00 low-divergence laser continuous wave mode operation was demonstrated from 277 K up to 350 K. A characteristic temperature T 0 as high as 74 K was measured just below 300 K. Threshold incident pump power as low as 600 W/cm 2 at 277 K and a maximum output power of 8.5 mW at 288 K was observed.

Fabrication and complete characterization of polarization insensitive 1310 nm InGaAsP–InP quantum-well semiconductor optical amplifiers

Polarization insensitive 1310 nm InGaAsP–InP multi-quantum-well (MQW) semiconductor optical amplifiers (SOAs), with 7° tilted ridge waveguide and buried-window end facets, have been fabricated and fully characterized on chip and module level. SOAs chips with an optimized complex strained MQW active region exhibited less than 1 dB polarization dependence of amplified spontaneous power in the drive current range of 50–200 mA. The amplifier module, having a residual facet reflectivity of 2.8 × 10−5, achieved 25 dB fibre-to-fibre unsaturated gain, for both transverse electric and transverse magnetic polarization states, 11.2 dBm saturation output power, and 7.6 dB noise figure at 1310 nm. The polarization dependence of gain was less than 0.6 dB in the 3 dB gain bandwidth of 56 nm. Coupling efficiency played a significant role in the gain, saturation output power and noise figure of a SOA module. Spot-size-converter integrated SOAs with buried heterostructures are expected to exhibit further improved performances.

Two-dimensional transverse cross-section nanopotentiometry of actively driven buried-heterostructure multiple-quantum-well lasers

We report results of two-dimensional local potential measurement of the transverse cross-section of operating buried-heterostructure (BH) multiple-quantum-well lasers. The measured two-dimensional image of potential distribution resolved clearly the multiquantum-well active region and the p-n-p-n current-blocking structure of the BH laser, showing close correlation to the scanning spreading resistance microscopy image. Nanopotentiometry measurements were also performed on the p-n-p-n current-blocking structure of a BH laser under different forward bias voltages. The nanopotentiometry results provide direct insight into the behavior of p-n-p-n current-blocking layers intended to minimize current leakage. Our results demonstrate the application of nanopotentiometry to the delineation of complex buried structures in quantum optoelectronic devices.

Deep Ultraviolet AlGaInN-Based Light-Emitting Diodes on Si(111) and Sapphire

Ultraviolet light-emitting diodes (LEDs) with emission wavelength as short as 280 nm, grown by gas source molecular beam epitaxy with ammonia, are described. The typical multi-quantum well (MQW) structure LED consists of an AlN buffer layer deposited on Si(111) or sapphire, followed by a (Al)GaN buffer layer and two superlattice structures, n- and p-type, with the MQW active region placed between them. Room temperature Hall measurements of n- and p-type AlN/AlGaInN superlattice structures show average hole concentrations of 1 x 10 18 cm -3 , with mobility of 3-4 cm 2 /Vs, and electron concentrations of 3 × 10 19 cm -3 , with mobility of 10-20 cm 2 /Vs. Room temperature electroluminescence spectra of mesa-etched devices show predominant emission at 280 nm.

Design of InGaN-GaN-AlGaN vertical-cavity surface-emitting lasers using electrical-thermal-optical simulation

A three-dimensional electrical-thermal-optical numerical simulator is developed and applied to model group-III-nitride-based intracavity-contacted vertical-cavity surface-emitting lasers with InGaN multiquantum-well active region. The optical model based on the effective frequency method is combined with an electrical-thermal simulator using the control volume method. Isothermal (pulsed regime imitation) and continuous-wave modes of operation are calculated over a range of voltages, covering subthreshold spontaneous emission and lasing emission. Effects of current crowding at the active region periphery are examined, and in particular, an impact on mode profiles of spatial hole burning superimposed on nonuniform gain distribution is studied. In order to reduce the current crowding and provide more uniform gain distribution within the active region, a semitransparent p-side contact design is proposed.

High-temperature operation (70°C, 50 mW) of 660-nm-band InGaAlP Zn-diffused window lasers fabricated using highly Zn-doped GaAs layers

660-nm-band InGaAlP Zn-diffused window structure lasers have been fabricated using highly Zn-doped GaAs layers grown by metal-organic chemical vapor deposition as a Zn diffusion source. A multiquantum-well active region was disordered by Zn diffusion, resulting in band-gap expansion and photoluminescence-wavelength shortening. In this Zn diffusion method, the Zn diffusion length from the active region into the n-cladding layer and the PL-wavelength shortening were easily controlled by controlling the Zn-dopant amount and the Zn-doped GaAs layer thickness. This method is very suitable for mass production. Window lasers fabricated by this method operate at up to 188 mW at 25/spl deg/C, and as high as 50 mW at 85/spl deg/C, and 30 mW at 95/spl deg/C with no COD. The lasers have been operating stably for more than 1200 h at 70/spl deg/C temperature and 50-mW output power. The mean time to failure was predicted as being more than 37000 h.

Differential Gain Analysis of InGaAs/InGaAsP/InP Multiquantum Well Lasers with 1.55 <FONT FACE=Symbol>m</font>m Emission Wavelength

We present measurements of the differential gain of multiquantum well (MQW) broad area lasers emitting at 1.55<FONT FACE="Symbol">m</font>m wavelength, taking into account the nonuniform stimulated emission caused by filamentation within the optical cavity. The differential gain was determined from measurements of the threshold current density, considering the losses due to the inhomogenous carrier density distribution, as an apparent leakage current effect in the output power-current characteristics. The lasers were grown by low pressure MOCVD and incorporate an InGaAs/InGaAsP/InP separate confinement MQW active region. The results are compared with previous data reported on similar MQW heterostructures.

Investigation of tensile-strained InGaAlP multiquantum-well active regions by photoluminescence measurements

The use of a tensile-strained multiquantum-well (MQW) active region in 630-nm-band InGaAlP MQW laser diodes is investigated through photoluminescence (PL) measurements. The critical conditions for lattice relaxation of strained MQW layers are also discussed. Emissions due to electron–heavy-hole recombination and electron–light-hole recombination are observed in the PL spectrum. A large energy difference (about 38 meV) is found between the two peaks, and radiative recombination is dominant in the MQW structure. These are thought to decrease the threshold current of tensile-strained MQW laser diodes. The MQW active region is thought to be extremely close to the critical conditions for lattice relaxation, but no adverse effects on optical properties are observed. It is concluded that such a tensile-strained MQW active region has advantages for use as the active region of 630-nm-band laser diodes.