Jonathan R. Orchard

Hybrid Quantum Well/Quantum Dot Structure for Broad Spectral Bandwidth Emitters

We report a hybrid quantum well (QW)/quantum dot active element for an application in broadband sources. These structures consist of an InGaAs QW and six InAs dot-in-well (DWELL) layers. The single QW is designed to emit at a wavelength coincident with the second excited state of the quantum dot. We compare two hybrid QW/quantum dot samples where the QW position is changed, and show that carrier transport effects make QW placement very important through current-voltage, capacitance-voltage, photocurrent, and temperature-dependent spontaneous emission measurements. Using the optimal structure, due to the combined effects of quantum dot ground states, first excited state, and QW emission, a positive modal gain spanning ~300 nm is achieved for the segmented contact device. The values for modal gain are further confirmed by simultaneous three-state lasing, which is studied spectroscopically. Finally, a hybrid QW/quantum dot superluminescent diode (SLD) is reported; the device exhibits a 3 dB emission spectrum of 213 nm, centered at 1230 nm with a corresponding output power of 1.1 mW. The hybrid SLD is then assessed for an application in an optical coherence tomography system; an axial resolution of ~4 μm is predicted.

In situ annealing enhancement of the optical properties and laser device performance of InAs quantum dots grown on Si substrates

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Tradeoffs in the Realization of Electrically Pumped Vertical External Cavity Surface Emitting Lasers

The design and realization of substrate emitting 980-nm electrically pumped vertical-external-cavity surface-emitting lasers (EP-VECSELs) is reported. A method to characterize the detuning of the cavity and spontaneous emission of the epitaxial material is described, and an experimental study of the effect of substrate doping on the operating characteristics of devices is presented. A reduction in optical loss and enhanced current-gain characteristics with a reduction in substrate doping from 2 × 108 to 4 × 1017 cm-3 is demonstrated. Spatial carrier distributions, evidenced by near-field profiling of devices without external feedback indicate similar current spreading behavior for the two-substrate dopings. Devices with diameter greater than 70 μm and current spreading layer thickness of 100 μm are shown to suffer from nonuniform carrier injection into the active region. Power scaling properties of the devices are investigated in both pulsed and CW operation. We realize devices with CW powers of 133 mW at 981 nm from a 150 μm device with 4 × 1017 cm-3 substrate doping at 0 °C, which is limited by nonoptimal cavity-gain peak detuning.

Design Rules and Characterisation of Electrically Pumped Vertical External Cavity Surface Emitting Lasers

We present an experimental study of the effect of substrate doping on the operating characteristics of substrate emitting electrically pumped vertical external cavity surface emitting lasers. We demonstrate a reduction in substrate doping from 2×1018 to 4×1017 cm-3 leads to reduced optical loss and enhanced current–gain characteristics. Spatial carrier distributions, evidenced by near field profiling of devices without external feedback indicates essentially identical behavior for the two substrate dopings. Devices with diameter greater than 100 µm and current spreading layer thickness of 100 µm suffer from non-uniform carrier injection into the active region, below this diameter output power scales linearly with device diameter. We show CW powers of 130 mW from a 100 µm device with 4×1017 cm-3 substrate doping at 0 °C can be obtained.

Trade-offs in the realization of electrically pumped vertical external cavity surface emitting lasers

External cavity surface emitting lasers are becoming widespread due to the possibility of power scaling and high beam qualities. However, optical pumping currently limits application to high price systems. We report on the design and realisation of electrically pumped devices to reduce cost and complexity.

Hybrid quantum well/quantum dot structures for broad spectral bandwidth devices

In this paper we report a hybrid quantum well (QW) and quantum dot (QD) structure to achieve a broad spontaneous emission and gain spectra. A single quantum well is introduced into a multi-layer stack of quantum dots, spectrally positioned to cancel the losses due to the second excited state of the dots. Attributed to the combined effect of QW and QDs, we show room temperature spontaneous emission with a 3dB bandwidth of ~250 nm and modal gain spanning over ~300 nm. We describe how this is achieved by careful design of the structure, balancing thermal emission from the QW and transport/capture processes in the QDs. We will also compare results from a QD-only epitaxial structure to describe how broadband gain/emission can be achieved in this new type of structure.

Analysing radiative and non-radiative recombination in InAs QDs on Si for integrated laser applications

Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied.

Optical characterisation of catalyst free GaAsP and GaAsP core-shell nanowires grown directly on Si substrates by MBE

We realise growth of both GaAsP and GaAs core nanowires (NWs), as well as GaAsP core-shell NWs grown on (111) Si substrates using solid source molecular beam epitaxy (MBE). By modifying the growth conditions it is possible to change the dimensions of the GaAsP NWs and optimisation of these conditions yields high crystal quality structures. Scanning electron microscopy (SEM) as well as temperature, power and time resolved photoluminescence (PL) are used to study the optical and structural properties of the NWs. The incorporation of P into the NWs is used to shift the PL emission for ~ 810 nm to ~ 730 nm at 77 K, and also results in enhanced PL and an improved carrier lifetime. The addition of a p-doped GaAsP shell to a GaAsP core NW reduces the nonradiative recombination at surface states, as evidenced by x14 reduction of PL quenching with temperature, enhanced carrier lifetime, as well as a x3.5 increase in 77 K integrated PL intensity.

Optimising the defect filter layer design for III/V QDs on Si for integrated laser applications

We introduce the concept of using strained superlattice structures as defect filters, with their purpose to reduce the upwards propagation of dislocations that result from the lattice mismatch which occurs when III-V materials are grown on silicon substrates. Three samples with defect filter layers are grown on Si with and without in situ annealing and are compared to a similar structure grown on a GaAs substrate. Transmission electron microscopy is used to verify the effectiveness of the different designs grown on Si, with the twice-annealed sample reducing the number of defects present in the active region by 99.9%. Optical studies carried out exhibit brighter room temperature emission and reduced photoluminescence quenching with temperature in samples where annealing is performed. Photoluminescence excitation measurements reveal a ~20 meV redshift in the position of the GaAs exciton for the samples grown on Si compared to that of GaAs, indicating a residual inplane tensile strain ~0.35% in the GaAs of the active region for the samples grown on Si.

Design and characterization of electrically pumped vertical external cavity surface emitting lasers

The design of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) for high power applications require a number of optimisations in design trade-offs, mainly that of doping for improved electrical performance and optical loss. Devices with diameter greater than 70μm and current spreading layer thickness of 100μm suffer from non-uniform carrier injection into the active region, below this diameter output power scales linearly with device area. We show CW powers of 133mW from a 150μm device with 4x1017cm-3 substrate doping at 0°C can be obtained.