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Characteristics of InGaN-AlGaN multiple-quantum-well laser diodes
Abstract and Figures
We demonstrate room-temperature pulsed current-injected operation
of InGaAlN heterostructure laser diodes with mirrors fabricated by
chemically assisted ion beam etching. The multiple-quantum-well devices
were grown by organometallic vapor phase epitaxy on c-face sapphire
substrates. The emission wavelengths of the gain-guided laser diodes
were in the range from 419 to 432 nm. The lowest threshold current
density obtained was 20 kA/cm<sup>2</sup> with maximum output powers of
50 mW. Longitudinal Fabry-Perot modes are clearly resolved in the
high-resolution optical spectrum of the lasers, with a spacing
consistent with the cavity length. Cavity length studies on a set of
samples indicate that the distributed losses in the structure are on the
order of 30-40 cm<sup>-1</sup>
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... Prior to this work, the only semiconductor lasers emitting at blue wavelengths were based on II-VI materials and suffered from severe reliability issues, so that their potential for commercialization was limited. On the other hand, in a short time span after their initial demonstration, InGaN MQW lasers emitting in the 400-nm wavelength range have reached sufficient levels of performance and reliability to become attractive for practical applications [40,[213][214][215][216][217][218][219]. In fact, an operating lifetime of over 10 000 h was reported as early as in 1997 [214]. ...
This paper reviews the device physics and technology of optoelectronic devices based on semiconductors of the GaN family, operating in the spectral regions from deep UV to Terahertz. Such devices include LEDs, lasers, detectors, electroabsorption modulators and devices based on intersubband transitions in AlGaN quantum wells (QWs). After a brief history of the development of the field, we describe how the unique crystal structure, chemical bonding, and resulting spontaneous and piezoelectric polarizations in heterostructures affect the design, fabrication and performance of devices based on these materials. The heteroepitaxial growth and the formation and role of extended defects are addressed. The role of the chemical bonding in the formation of metallic contacts to this class of materials is also addressed. A detailed discussion is then presented on potential origins of the high performance of blue LEDs and poorer performance of green LEDs (green gap), as well as of the efficiency reduction of both blue and green LEDs at high injection current (efficiency droop). The relatively poor performance of deep-UV LEDs based on AlGaN alloys and methods to address the materials issues responsible are similarly addressed. Other devices whose state-of-the-art performance and materials-related issues are reviewed include violet-blue lasers, "visible blind" and "solar blind" detectors based on photoconductive and photovoltaic designs, and electroabsorption modulators based on bulk GaN or GaN/AlGaN QWs. Finally, we describe the basic physics of intersubband transitions in AlGaN QWs, and their applications to near-infrared and terahertz devices.
Nitride-based semiconductor products emerged onto the scene in the last decade, and have become a major player in the opto-electronics market with their wide wavelength tunability from the ultraviolet (UV) to the visible spectral region illustrated in Fig. 1. Revenues from the use of blue and green light emitting diodes (LEDs) in home and commercial lightening business alone are predicted to reach almost three billion dollars by 2009. Furthermore, the rapid development of high-brightness LEDs has opened applications in traffic signals consuming less than a tenths of the power of the standard filtered incandescent solution. The energy saving results in a short (< 1 year) payback time on the initial investment of the LED solution. Automotive applications of LEDs include center-high-mount-stop-lamps (CHMLs), rear combination lamps (RCLs) and turn lights. The main driving force for LED application in automotive at present is styling, where the small size of LEDs allows flexible layout. Other applications of LEDs include backlights for liquid crystal displays (LCD) displays and full color displays.
The rigorous optical model of diode lasers has been used to investigate an impact of various construction details of multi-quantum-well nitride lasers, as the number of quantum wells placed in active region, as well as designs of their waveguides and buffer layers located between the substrate and the laser structure, on room-temperature laser operation. The model is used to discuss some possible structure modifications to reduce losing thresholds. Recommended design parameters have been found for each structure.
A detailed threshold analysis of room-temperature pulsed operation of GaN/AlGaN/AlN verticalcavity surface-emitting lasers (VCSELs) is carried out. The model takes advantage of the latest results concerning gain in active regions, material absorption in the cladding layers, as well as cavity diffraction and scattering losses. The simulation showed that although VCSELs with single (S) or multiple (M) quantum-well (QW) active regions exhibit lower threshold currents, they are much more sensitive to any increase in optical losses than their bulk counterparts. In particular, decreasing the active region radius of gain-guided QW VCSELs below 5 μm (which increases diffraction losses) or increasing dislocation densities (which, in turn, raises scattering losses) gives an enormous rise to their threshold currents. Therefore small-size GaN VCSELs should have an index-guided structure. In the case of MQW VCSELs, the optimal number of quantum wells strongly depends on the reflectivities of resonator mirrors. According to our study, MQW GaN lasers usually require noticeably lower threshold currents compared to SQW lasers. The optimal number of QW active layers is lower in laser structures exhibiting lower optical losses. Although the best result occurred for an active region thickness of 4 nm, threshold currents for the various sizes differ insignificantly.
Photoluminescence and time-resolved spectroscopy of gallium nitride (GaN)- and indium gallium nitride (InGaN)-based materials and devices are discussed in this chapter. The chapter discusses the results obtained in various studies of the photoluminescence properties of GaN and related materials. The experimental techniques covered in the chapter include optical absorption, photoreflection (PR), modulation spectroscopy, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, micro-PL, cathodoluminescence, pump-probe spectroscopy, femtosecond spectrally resolved and time-resolved degenerate four-wave mixing, second harmonic generation, laser-induced gratings, and quantum beats. The development of large area single-crystal growth of GaN may provide the ultimate solution to the problem of lattice mismatch. GaN and other III–V nitrides have been realized as potential candidates in photonics. Light emitting diode (LEDs) and semiconductor lasers that emit in a variety of wavelengths from blue to yellow are now commercially available. They are being used in full-color displays and traffic lights. LEDs that emit various colors are also substitutes for traditional fluorescent lamps and can be used in homes and shops. The market for LEDs is huge. Blue LEDs also find applications in laser printers and underwater optical communications for the Navy. In the near future, it is possible to develop digital video disks in which InGaN and GaN lasers are used.
GaN and related materials have received a lot of attention because of their applications in a number of semiconductor devices such as LEDs, laser diodes, field effect transistors, photodetectors etc. An introduction to optical phenomena in semiconductors, light emission in p-n junctions, evolution of LED technology, bandgaps of various semiconductors that are suitable for the development of LEDs are discussed first. The detailed discussion on photoluminescence of GaN nanostructures is made, since this is crucial to develop optical devices. Fabrication technology of many nanostructures of GaN such as nanowires, nanorods, nanodots, nanoparticles, nanofilms and their luminescence properties are given. Then the optical processes including ultrafast phenomena, radiative, non-radiative recombination, quantum efficiency, lifetimes of excitons in InGaN quantum well are described. The LED structures based on InGaN that give various important colors of red, blue, green, and their design considerations to optimize the output were highlighted. The recent efforts in GaN technology are updated. Finally the present challenges and future directions in this field are also pointed out.
The effect of Si doping on the strain and microstructure in GaN films grown on sapphire by metalorganic chemical vapor deposition was investigated. Strain was measured quantitatively by x-ray diffraction, Raman spectroscopy, and wafer curvature techniques. It was found that for a Si concentration of 2×1019 cm−3, the threshold for crack formation during film growth was 2.0 μm. Transmission electron microscopy and micro-Raman observations showed that cracking proceeds without plastic deformation (i.e., dislocation motion), and occurs catastrophically along the low energy {100} cleavage plane of GaN. First-principles calculations were used to show that the substitution of Si for Ga in the lattice causes only negligible changes in the lattice constant. The cracking is attributed to tensile stress in the film present at the growth temperature. The increase in tensile stress caused by Si doping is discussed in terms of a crystallite coalescence model.
The amount of strain was measured in GaN films using X-ray diffraction, Raman, and curvature techniques as a function of film thickness and the Si doping concentration. It was found that for a doping concentration of 2×1019, the threshold thickness for crack formation was about 2.5μm. Transmission electron microscopy observations showed that cracking proceeds without plastic deformation (i.e., no dislocation motion), and occurs catastrophically along the low-energy {11̄00} cleavage plane of GaN.
Laser-diode heterostructures of InGaAlN containing a third-order diffraction grating for distributed optical feedback have been examined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The grating was defined holographically and etched by chemically assisted ion-beam etching into the upper GaN confinement layer of the laser structure. After the etch step, it was overgrown with an Al0.08Ga0.92N upper cladding layer. Threading dislocations were present that initiated at the sapphire substrate, but no new dislocations were observed at the grating/Al0.08Ga0.92 N interface. A comparison of TEM and SEM micrographs reveals that there is a compositional gradient in the AlGaN upper cladding layer; however, calculations show that it did not reduce the optical coupling coefficient of the grating. (C) 1998 American Institute of Physics. [S0003-6951(98)04545-].
InGaN films have been grown on GaN and AlGaN epitaxial layers by metalorganic vapor phase epitaxy. The “composition pulling effect” during the initial InGaN growth stages has been studied as a function of the lattice mismatch between the InGaN and the underlying epitaxial layer. The crystalline quality of the InGaN is good near the InGaN/GaN interface and the composition is close to that of GaN. However, with increasing InGaN film thickness, the crystal quality deteriorates and the indium mole fraction increases. The composition pulling effect becomes stronger with increasing lattice mismatch. It is suggested that indium atoms are excluded from the InGaN lattice during the early growth stages to reduce the deformation energy from the lattice mismatch. TEM observations of the InGaN/GaN structure reveal that the degradation of the crystalline quality of InGaN films grown on GaN is caused by pit formation which arises from edge dislocations propagating through the InGaN film from the underlying GaN.
Vertical mirrors have been fabricated with chemically assisted ion beam etching (CAIBE) on OMVPE grown InGaN/AlGaN laser diode structures. AFM measurements show that smooth vertical sidewalls are obtained which exhibit a root mean squared (rms) roughness of only 40—60 A. The inclination angle of the etched mirrors is within 2° of vertical, as SEM studies indicate. Photopumping measurements reveal that the reflectivity of the etched mirrors corresponds to 60—70% of the value for an ideal GaN/air interface. The reduced reflectivity may be due to surface roughness, a slight tilt in the facet angle, and the excitation of higher-order transverse waveguide modes in the laser structure.
First-principles calculations for defects and impurities in GaN prov i d e a w ealth of infor-mation about doping and electronic properties. We h a ve been able to show that nitrogen vacancies are not the cause of n-type conductivity in GaN. Results for silicon and oxygen indicate that unintentional incorporation of these impurities is the most plausible explana-tion for n-type conductivity. F or p-type GaN, we that Mg incorporation is limited by solubility. The role of hydrogen in acceptor-doped material is discussed.
Continuous‐wave (cw) operation of InGaN multi‐quantum‐well structure laser diodes (LDs) was demonstrated at room temperature (RT). The threshold current and voltage of the LD were 130 mA and 8 V, respectively. The threshold carrier density was 9 kA/cm2. The lifetime of the LDs under RT cw operation was 1 s due to large heat generation. Mode hopping of the emission wavelength of the LDs was observed. The average wavelength drift due to temperature increase was 0.066 nm/K between 20 and 70 °C, because of the temperature dependence of the gain profile due to band‐gap narrowing of the InGaN active layer.
Single crystal thin films with compositions from the AlN- InN-GaN system were grown via metal-organic chemical vapor deposition on single crystal 6H-SiC substrates. AlGaN containing high and low fractions of Al was grown directly on the SiC for use as a buffer layer. Subsequent epitaxial layers of GaN and AlGaN were doped with Mg and Si to achieve p-type conductivity, respectively. N-type InGaN layers with In compositions up to approximately 50 percent were also achieved. Room temperature photoluminescence on these films exhibited single peaks in the spectral range from the UV to green. Various layers were combined to form light emitting diode (LED) and laser structures. Blue LEDs with both insulating and conductive buffer layers exhibited an external quantum efficiency of 2-3 percent with a forward operating voltage of 3.4-3.7 V. Laser diode structures having a separate confinement heterostructure multiple quantum well configuration were optically and electrically pumped. Photopumping resulted in stimulated emission at 391 nm. Electrically pumped structures resulted in a peak emission at 393 nm and a bandwidth of 12 nm. No lasing was observed.
The well-number dependence of the optical pumping threshold power for stimulated emission of GaInN multiple quantum-well (MQW) laser structures was investigated. The pumping threshold power for a three GaInN MQW sample was found to be as low as 33kW/cm2 at room temperature. The room-temperature pulsed operation of a five GaInN MQW laser diode (LD), whose number of wells was determined based on the optical pumping experiment, was also demonstrated. The lowest threshold current density was 9.5kA/cm2. The lasing wavelength was 417.5nm with a full-width at half-maximum (FWHM) less than the spectrum resolution of 0.2nm.
Quantum well structures composed of GaInN well and GaN barrier were fabricated. Room-temperature stimulated emission by pulsed current injection is observed from group III nitride using the very thin active layer, for the first time.
We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the (1120) direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50kA/cm2 and a voltage for the threshold current was 20 V.