The principle of the operation of a Gunn laser is based on the band to band recombination of impact
ionized non-equilibrium electron-hole pairs in propagating high field space-charge domains in a
Gunn diode, which is biased above the negative differential resistance threshold and placed in a
Fabry-Perot or a vertical micro cavity (VCSEL).
In conventional VCSEL structures, unless specific measures such as the addition of oxide apertures
and use of small windows are employed, the lack of uniformity in the density of current injected
into the active region can reduce the efficiency and delay the lasing threshold. In a vertical-cavity
structured Gunn device, however, the current is uniformly injected into the active region
independently of the distributed Bragg reflector (DBR) layers. Therefore, lasing occurs from the
entire surface of the device.
The light emission from Gunn domains is an electric field induced effect. Therefore, the operation
of Gunn-VCSEL or F-P laser is independent of the polarity of the applied voltage. Red-NIR
VCSELs emitting in the range of 630-850 nm are also possible when Ga<sub>1-x</sub>Al<sub>x</sub>As (<i>x</i> < 0.45) is used
the active layer, making them candidates for light sources in plastic optical fibre (POF) based short-distance
data communications. Furthermore the device may find applications as an optical clock and
cross link between microwave and NIR communications.
The operation of a both Gunn-Fabry-Perot laser and Gunn-VCSEL has been demonstrated by us
recently. In the current work we present the potential results of experimental and theoretical studies
concerning the applications together with the gain and emission characteristics of Gunn-Lasers.
The operation of the Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure -- Vertical Cavity Surface Emitting Laser (HELLISH-VCSEL) devices is based on hot carrier transport parallel to the layers of Ga<sub>1-x</sub>Al<sub>x</sub>As p-n junction. It is therefore a field - effect device and the light emission from the device is independent of the polarity of the applied voltage. In this study, we present the temperature dependence of the operational characteristics of the device. Experimental studies comprising of the measurements of the I-V characteristics, electroluminescence, reflectivity, and temperature dependent light-applied electric field (L-F) characteristics are conducted to find the optimum operating temperature of the device.
Theoretical and experimental results concerning the study of a novel wavelength converter amplifier, which can be tuned, with the amplification of an external voltage are presented. The device consists of a Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As graded quantum well, placed on the n-side of the depletion region of a Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As p-n junction. As a result of the competition between the built-in field and the grading, in the absence of an external bias, the quantum well acts as an isolated well. Forward biasing of the junction reduces the built-in field; thus the field associated with the grading becomes effective. The tuning of the operation wavelength is based on the anti-Quantum Confined Stark Effect and achieved during the forward biasing. In this study we present the numerical results based on a 2D modeling of the device where exciton binding energy, absorption co-efficient and transition energy are obtained as a function of applied field. Experimental results show a tuning range of around 40nm.
We report on experimental characterization of GaInNAs and GaInAs semiconductor materials grown on GaAs by Chemical Beam Epitaxy. The optical characterization of the samples has been carried out by orthodox photoluminescence and ellipsometry measurements. We show that even a small amount of nitrogen added to GaInAs has a considerable effect on the optical properties, causing a red shift in the emission wavelength, a reduction in PL efficiency and an increase in the refractive index.
Hot Electron Light Emitting and Lasing Semiconductor Heterojunction device is a novel emitter that utilizes hot carrier transport parallel to the layers of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As p-n junction containing GaAs quantum well(s) in the depletion region. Electrons and holes drifting in their respective channels are heated up temperatures well above the lattice temperature and consequently transferred in real space over the built-in potential barrier into the GaAs quantum well via phonon assisted tunneling and or thermionic emission. The recombination occurs in the quantum well. The Top Hat structure HELLISH presented in this work provides a new functionality of the device where the n and p-layers are contacted separately but are biased with the same voltage longitudinally. In this configuration hot carrier injection into the active region is further enhanced in the vicinity of the cathode due to the effective forward biasing of the junction. Therefore, the emission intensity is increased compared with the conventional HELLISH device. In the vicinity of the anode, however, there is an effective reverse biasing and in this region the top hat device acts as an absorber. As a result of these two features the device can be operate as a wavelength converter and amplifier. The intensity of the emitted light is independent of the polarity of the applied voltage. However, positions of the absorber and emitter depend on the polarity. The device may offer a wide range of light logic functions. The speed and the efficiency of the device depend on both the longitudinal and the transverse fields. A simple 2D model is developed to explain the device dynamics.
Hot electron light emitting and lasing semiconductor heterostructure (HELLISH) is a novel longitudinal transport, surface emitting device. The operation of the device as a light emitter and vertical cavity surface emitting laser (VCSEL) has been previously demonstrated by us for the GaAs/GaAlAs material system. A basic GaInAsP/InP HELLISH structure and an advanced GaInAsP/InP HELLISH VCSEL structure are described in this work and designed for 1.5 micrometers emission. The basic HELLISH structure consists of a GaInAsP quantum well placed on the n-side of the InP p-n junction. The advanced structure has a similar active region to the basic HELLISH structure, but with a resonant cavity defined by the addition of DBRs, where the current is injected directly into the active region without having to pass through the DBRs. The modeling and design of these structures are described, including self-consisting numerical 1D solutions of the Poisson and Schrodinger equations.
We present the results of our studies concerning the pulsed operation of a bulk GaInAsP/InP vertical cavity surface emitting laser (VCSEL). The device is tailored to emit at around 1.5 micrometers at room temperature. The structure has a 45-period n-doped GaInAsP/InP bottom Distributed Bragg Reflector (DBR), and a 4 period Si/Al<SUB>2</SUB>O<SUB>3</SUB> dielectric top reflector defining a 3-(lambda) cavity. Electroluminescence from a 16micrometers diameter top window was measured in the pulsed injection mode. Spectral measurements were recorded in the temperature range between 125K and 240K. Lasing threshold current density has a broad minimum at temperatures between 170K-190K. In order to establish the relationship between the temperature dependence of the threshold current and the gain peak, we investigated the spectral dependence of edge and surface emission from optically pumped structures without the top reflector. Experimental result are compared with existing theories concerning the temperature dependence of threshold current.
We present the results of our studies concerning temperature dependence of photoluminescence (PL) in Ga<SUB>x</SUB>In<SUB>1-x</SUB>As<SUB>1- y</SUB>N<SUB>y</SUB>/GaAs single quantum wells. Our results at low temperatures indicate the presence of a high density of compositional and/or structural disorder and hence poor PL efficiency, common to as-grown GaInAsN material. We show, however, that the optical quality of GaInAsN can be improved while achieving a red shift in the spectra. This is unlike the results obtained by rapid thermal annealing (RTA) or conventional annealing, which are widely employed as post- growth treatment techniques, where any increase in the PL intensity is almost always accompanied by an undesired blue- shift.
The GaInAsP/InP device described in this work consists of an InP p-n junction with a GaInAsP quantum well placed on the n- side within the depletion region. This device is designed for 1.5 micrometer emission. Light emission is independent of the applied voltage polarity, and the device acts as an XOR optical logic gate. One potential application for this device is as a low cost VCSEL for optical access networks, since two diffused-in point contacts are used for longitudinal biasing. Hence, the current is injected directly into the active region without having to pass through the Distributed Bragg Reflectors (DBRs). Experimental results concerning the temperature dependence of photoluminescence and electroluminescence spectra, and light field characteristics are compared with model calculations. These include self- consistent numerical one-dimensional solutions of the Poisson and Schrodinger equations. We also studied the emission wavelength as a function of position of the GaInAsP quantum well within the built-in electric field of the InP p-n junction. The calculated overlap of the normalized electron and hole wavefunctions is in good agreement with the experimental results.
The hot Electron Light Emission and Lasing in Semiconductor Heterostructures devices (HELLISH-1) is novel surface emitter consisting of a GaAs quantum well, within the depletion region, on the n side of Ga<SUB>1-x</SUB>Al<SUB>x</SUB>As p- n junction. It utilizes hot electron transport parallel to the layers and injection of hot electron hole pairs into the quantum well through a combination of mechanisms including tunnelling, thermionic emission and diffusion of `lucky' carriers. Super Radiant HELLISH-1 is an advanced structure incorporating a lower distributed Bragg reflector (DBR). Combined with the finite reflectivity of the upper semiconductor-air interface reflectivity it defines a quasi- resonant cavity enabling emission output from the top surface with a higher spectral purity. The output power has increased by two orders of magnitude and reduced the full width at half maximum (FWHM) to 20 nm. An upper DBR added to the structure defines HELLISH-VCSEL which is currently the first operational hot electron surface emitting laser and lases at room temperature with a 1.5 nm FWHM. In this work we demonstrate and compare the operation of UB-HELLISH-1 and HELLISH-VCSEL using experimental and theoretical reflectivity spectra over an extensive temperature range.
In n-type GaAs/Gai. xAlxAs single double and multiple quantum wells continuous oscillations in the current occur when electric fields in excess of a few hundred volts cm1 are applied along the layers. The frequency of the oscillations increases with the electric field and the electron concentration over a range of frequencies up to the gegahertz region. A strong electroluminescence (EL) signal is observed at and above the threshold field of the instabilities. The intensity of the EL signal increases with electric field as ''EL Fs where s is typically s 5. The EL spectra peaks at an energy close to the PL spectra. EL is shown to be associated with hot electron recombination and it is speculated that it originates in p-channels in the well adjacent to the AlGaAs cladding. The study provides a technique which can be used as a tool to determine the quality of GaAs quantum wells.
Experimental results on high-field parallel transport in GaAs/GaAlAs quantum well structures are presented. The results are compared with a theoretical model of high field transport involving non-drifting hot phonons and scattering from remote impurities and interface roughness. The latter two effects contribute to the relaxation of the electron momentum. It is also shown that non-drifting hot phonons with a fmite life-time reduce the energy relaxation and enhance the momentum relaxation. The enhancement of the momentum relaxation at high fields inhibits negative differential conductivity via real space transfer or intervalley transfer. This is observed in our samples. The reduction of the drift velocity at high fields is also detrimental to the speed of many devices which operate in the hot electron regime.