Z. Y. Zhang

Self-assembled quantum-dot superluminescent light-emitting diodes

The development of low-cost, compact, high-power and broadband superluminescent n light-emitting diodes is an important research subject for a wide range of n applications. We describe how self-assembled quantum-dot structures can provide an n efficient means of realizing such devices utilizing a number of their unique physical n properties. Such quantum dot superluminescent diodes are leading to a revolution in n the development of broadband emitters for widespread medical, biological and n telecommunications applications.

Realization of extremely broadband quantum-dot superluminescent light-emitting diodes by rapid thermal-annealing process.

The first demonstration, to our knowledge, of the creation of ultrabroadband superluminescent light-emitting diodes using multiple quantum-dot layer structure by rapid thermal-annealing process is reported. The device exhibits a 3 dB emission bandwidth of 146 nm centered at 984 nm with cw output power as high as 15 mW at room temperature corresponding to an extremely small coherence length of 6.6 microm.

High-Power Quantum-Dot Superluminescent LED With Broadband Drive Current Insensitive Emission Spectra Using a Tapered Active Region

High-power and broadband quantum-dot (QD) superluminescent light-emitting diodes are realized by using a combination of self-assembled QDs with a high density, large inhomogeneous broadening, a tapered angled pump region, and etched V groove structure. This broad-area device exhibits greater than 70-nm 3-dB bandwidth and drive current insensitive emission spectra with 100-mW output power under continuous-wave operation. For pulsed operation, greater than 200-mW output power is obtained.

A p-type-doped quantum dot superluminescent LED with broadband and flat-topped emission spectra obtained by post-growth intermixing under a GaAs proximity cap

Broadband superluminescent light emitting diodes are realized by a post-growth annealing process, on modulation p-doped multiple InAs/InGaAs/GaAs quantum dot layer structures, under a GaAs proximity cap. The device exhibits a large and flat emission with spectral width up to 132 nm at 2 mW. This is mainly attributed to the reduction of the energy separation between the ground state and the excited state, in addition to the optical quality of the intermixed modulation p-doped quantum dot materials being comparable to that of the as-grown sample.

Effects of intermixing on modulation p-doped quantum dot superluminescent light emitting diodes

Different capping of quantum dot (QD) materials is known to produce different degrees of intermixing during a post-growth thermal annealing process. We report a study of the effect of different degrees of intermixing on modulation beryllium doped quantum dot superluminescent light emitting diodes (QD-SLEDs). The intermixed QD-SLEDs show high device performance whilst achieving a large central emission wavelength shift of approximately 100nm compared to the as-grown device. The evolution of the emission spectra and power with drive current suggest a transition from QD-like to QW-like behavior with increasing degree of intermixing. A selective area intermixed QD-SLED is demonstrated, and with optimized differential intermixing, such structures should allow ultra-broadband sources to be realized.

Effect of facet angle on effective facet reflectivity and operating characteristics of quantum dot edge emitting lasers and superluminescent light-emitting diodes

The authors report the creation of low reflectivity angled facets by focused-ion-beam postfabrication etching. A method to directly measure the effective facet reflectivity of such facets, utilizing gain saturation effects in the quantum dots is described. The reflectivities of the angled facets are shown to decrease by increasing the facet angle from 0° to 15°. With a reflectivity of <1×10−6 obtained for a facet with a 15° angle, allowing quantum dot superluminescent light-emitting diodes to be fabricated. The use of different angled facets to control the emission wavelength of both quantum dot lasers and superluminescent light-emitting diodes is outlined.

Tunable interband and intersubband transitions in modulation C-doped InGaAs∕GaAs quantum dot lasers by postgrowth annealing process

A bandgap and intersublevel spacing tuned laser has been realized by using a modulation C-doped InGaAs∕GaAs quantum dot structure, which utilizes a postgrowth annealing process. The intermixed laser exhibits comparable light-current characteristics, indicating little detrimental change to the quantum dot laser material, and show a ground state bandgap blueshift of ∼13nm and intersublevel energy spacing reduction of ∼30nm compared to the unannealed device. The differences in the samples during annealing are attributed to the suppression of Ga vacancy propagation for samples with modulation C doping.

Study of annealed InAs/GaAs quantum dot structures

Two similar samples of InAs/GaAs quantum dots and their response to annealing at 700°C have been studied using scanning transmission electron microscopy. The only difference in the growth of both samples is the inclusion of p-doping by carbon in the GaAs barriers between the quantum dots in one of the wafers. The size and line density of and the change of indium concentration within the quantum dots have been investigated to understand the effect of carbon doping on the diffusion of indium atoms within the structures during the anneal.

Structure and properties of InAs/AlAs quantum dots for broadband emission

The InAs quantum dots (QDs) on an AlAs layer are grown on GaAs substrates by molecular beam epitaxy technique. The properties of materials and optics of such QD structures have been investigated by cross sectional transmission electron microscopy and photoluminescence (PL) techniques. It is discovered that the inhomogeneous strain filed mainly exists below InAs QDs layers in the case of no wetting layer. The full width at half maximums (FWHMs) and intensities of PL emission peaks of InAs QDs are found to be closely related to the thickness of the thin AlAs layers. The InAs QDs on an eight monolayer AlAs layer, with wide FWHMs and large integral intensity of PL emission peaks, are favorable for producing broadband QD superluminescent diodes, external-cavity QD laser with large tuning range.

Analysis of 1.2μm InGaAs∕GaAs quantum dot laser for high power applications

The effect of modulation p-doping on the characteristics of 1.2μm quantum dot lasers is reported. Compared to undoped devices, p-doped are shown to exhibit higher saturated gain, higher internal efficiency, improved T0, lower excited state lasing current densities, and higher internal loss. Both types of the device are analyzed with regard to high power applications in the extreme cases of complete and nonexistent gain clamping. Results from a laser optimized to have minimal threshold current are discussed.