InAs avalanche photodiodes (APDs) can be designed such that only electrons are allowed to initiate impact ionization, leading to the lowest possible excess noise factor. Optimization of wet chemical etching and surface passivation produced mesa APDs with bulk dominated dark current and responsivity that are comparable and higher, respectively, than a commercial InAs detector. Our InAs electron-APDs also show high stability with fluctuation of ~0.1% when operated at a gain of 11.2 over 60 s. These InAs APDs can detect very weak signal down to ~35 photons per pulse. Fabrication of planar InAs by Be implantation produced planar APDs with bulk dominated dark current. Annealing at 550 °C was necessary to remove implantation damage and to activate Be dopants. Due to minimal diffusion of Be, thick depletion of 8 μm was achieved. Since the avalanche gain increases exponentially with the thickness of avalanche region, our planar APD achieved high gain > 300 at 200 K. Our work suggest that both mesa and planar InAs APDs can exhibit high gain. When combined with a suitable preamplifier, single photon detection using InAs electron-APDs could be achieved.
Sensitive detection of mid-infrared light (2 to 5 μm wavelengths) is crucial to a wide range of applications. Many of the applications require high-sensitivity photodiodes, or even avalanche photodiodes (APDs), with the latter generally accepted as more desirable to provide higher sensitivity when the optical signal is very weak. Using the semiconductor InAs, whose bandgap is 0.35 eV at room temperature (corresponding to a cut-off wavelength of 3.5 μm), Sheffield has developed high-sensitivity APDs for mid-infrared detection for one such application, satellite-based greenhouse gases monitoring at 2.0 μm wavelength. With responsivity of 1.36 A/W at unity gain at 2.0 μm wavelength (84 % quantum efficiency), increasing to 13.6 A/W (avalanche gain of 10) at -10V, our InAs APDs meet most of the key requirements from the greenhouse gas monitoring application, when cooled to 180 K. In the past few years, efforts were also made to develop planar InAs APDs, which are expected to offer greater robustness and manufacturability than mesa APDs previously employed. Planar InAs photodiodes are reported with reasonable responsivity (0.45 A/W for 1550 nm wavelength) and planar InAs APDs exhibited avalanche gain as high as 330 at 200 K. These developments indicate that InAs photodiodes and APDs are maturing, gradually realising their potential indicated by early demonstrations which were first reported nearly a decade ago.
We demonstrate an AlInP detector grown on lattice-matched GaAs substrate for underwater communication applications.
This detector has a narrow inherent spectral response of 22 nm with central wavelength at ~ 480 nm and is capable of
having avalanche gain of ~ 20 which gives peak responsivity of ~ 2 A/W. A much higher multiplication of ~167 was
shown in the previous work. The full-width-half-maximum (FWHM) and responsivity of this detector is fairly
insensitive to the angle of the incident light. These properties enable it to detect an optical signal at 480 nm even in the
presence of high background illumination.
the magnitude of the change in threshold current with temperature in InP quantum dot lasers caused by the distribution of carriers among dot states is quantified and demonstrated. Samples with differing distributions of allowed states, as assessed using absorption spectra and achieved by varying the composition of the quantum well above each layer of quantum dots, are affected differently by this thermal broadening although the underlying mechanism is the same. This difference is shown to be a result of different optical loss and the different gain magnitude achieved at a similar inversion level in the different samples. Uncoated, cleaved facet Fabry-Perot lasers with 2 mm long cavities are demonstrated with a threshold current density of 138 Acm-2 at 300 K that increases to 235 Acm-2 at 350 K (77ºC).
We employ a device which exploits the properties of InP quantum dots (QD), (emitting from 650-730 nm), to produce simultaneous dual-λ lasing from a single ridge-waveguide comprising two sections. Due to the effects of state-filling in an inhomogeneously broadened QD ensemble, the wavelength is strongly dependent on magnitude of the gain (or cavity loss). Therefore, by altering the loss of each section of the device we are able to demonstrate a large range of difference-wavelengths, up to 63 nm. Here, we test the performance of the device and measure effects of temperature and difference-wavelength on the stability of the two lasing modes.
Al0.52In0.48P is the largest bandgap material in III-V non-nitride semiconductors that is lattice matched to a readily available substrate (GaAs). Having a bandgap narrower than that of GaN enables it to detect wavelengths around 480 nm. Such wavelengths have the best transmittance underwater and may be used as a carrier in underwater communication systems. We present an Al0.52In0.48P homo-junction Separate-Absorption-Multiplication-Avalanche-Photodiode (SAMAPD) as a high sensitivity detector for such an application. By increasing the neutral and space-charge region thicknesses, the peak response wavelength can be tuned to longer wavelengths with a narrower full-width-half-maximum (FWHM). The quantum efficiency of the detector reduces with FWHM and this is compensated by having an avalanche gain. At room temperature, the SAM-APD has a dark current of <20 pA for a 210 μm radius device up to 99.9% of breakdown voltage. The structure gives a narrow spectral FWHM of 22 nm with centre wavelength of 482 nm. An external quantum efficiency of 33% and 6410% at 482 nm is obtained at bias voltage of -19 V and -92.6 V respectively.
We demonstrate lower temperature sensitivity at high temperature in a strained layer InP/AlGaInP self-assembled
quantum dot design grown by MOVPE. The lasers emit between 700 - 730 nm, finding application in photodynamic
therapies and bio-photonic sensing. We previously achieved a 300 K threshold current density of 150 Acm-2 in similar
structures for 2mm long lasers with as-cleaved facets, however at elevated temperatures Jth increases rapidly with
temperature. To address this issue we redesign the layers around the active regions, consisting of five layers of dots, each
grown on a lower confining layer of (Al0.30Ga0.70)InP lattice matched to GaAs, formed from 3 mono-layers of InP and
with a GaxIn(1-x)P upper confining layer. We grew two series of samples, x=0.43-0.58 with (Al0.70Ga0.30)0.51In0.49P
waveguide claddings, and x=0.52-0.58 (AlInP claddings). Dot properties are strongly influenced by the UCL. Properties
varied with Ga fraction. Measured absorption and lasing energies increase with Ga percentage, maintaining a constant
separation from upper confining layer transition energies. A Ga fraction of x=0.54 (lightly tensile strained with respect to
GaAs) gave the strongest and most well defined absorption, the lowest 300K Jth for 2mm long broad area lasers
(uncoated facets) of 180 Acm-2 and lowest rate of Jth increase with temperature.
In this work, we demonstrate semiconductor quantum dots weakly coupled to photonic crystal cavity modes operating in
the visible spectrum. We present the design, fabrication and characterization of two dimensional photonic crystal cavities
in GaInP and measure quality factors in excess of 7,500. We demonstrate control over the spontaneous emission rate of
InP quantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode
we observe a Purcell enhancement of ~8.
By optimising an InP/AlGaInP quantum dot size distribution a broad and relatively flat topped gain spectra can be
achieved. Using the segmented contact method we measure the optical gain spectra and use this to explain the range of
lasing wavelengths that can obtained by varying the grating structure of deep etched DBR lasers. We describe the
optimisation of a simple single stage ICP etch process suitable for producing anisotropic microstructures in this material
system and the resulting deep-etched DBR lasers. Measurements of emission wavelength made between 220 and 320 K
on a ridge laser, fabricated with cleaved facets, reveals a temperature dependence on of 0.14 nm/K. DBR structures have
been used to improve this behaviour, with a dependence of peak wavelength with temperature of 0.07 nm/K, over the
same temperature range. Measurements on a 4 μm wide DBR ridge laser show they can be operated up to 17 nm from
the peak emission of a ridge laser operating at the same current density.
We review the development of high performance, short wavelength (3 μm < λ < 3.8 μm) quantum cascade lasers (QCLs)
based on the deep quantum well InGaAs/AlAsSb/InP materials system. Use of this system has enabled us to demonstrate
room temperature operation at λ ~ 3.1 μm, the shortest room temperature lasing wavelength yet observed for InP-based
QCLs. We demonstrate that significant performance improvements can be made by using strain compensated material
with selective incorporation of AlAs barriers in the QCL active region. This approach provides reduction in threshold
current density and increases the maximum optical power. In such devices, room-temperature peak output powers of up
to 20 W can be achieved at λ ~ 3.6 μm, with high peak powers of around 4 W still achievable as wavelength decreases to
Quantum dots (QD) offer significant advantages over quantum wells (QW) as the active material in high power lasers.
We have determined power density values at catastrophic optical mirror damage (COMD), a key factor limiting high
power laser diode performance, for various QW and QD red and NIR emitting structures in the in the AlGaInP system.
The devices used were 50 μm oxide stripe lasers mounted p-side up on copper heatsinks operated pulsed. The COMD
power density limit decreases as pulse length increases. At short pulse lengths the limit is higher in QD (19.1±1.1
MW/cm2) than in QW devices (11.9±2.8 MW/cm2 and 14.3±0.4 MW/cm2 for two different spot sizes). We used the high
energy Boltzmann tail of the spontaneous emission from the front facet to measure temperature rise to investigate the
physical mechanisms (non-radiative recombination of injected carriers and reabsorption of laser light at the facet)
leading to COMD and distinguish between the behaviour at COMD of QW and QD devices. Over the range 1x to 2x
threshold current the temperature rise in the QW structures was higher. Scanning electron microscopy showed a
difference between the QD and QW lasers in the appearance of the damage after COMD.
We report the first realization of short wavelength (λ ~ 3.05 - 3.6 μm) lattice matched In0.53Ga0.47As/AlAs0.56Sb0.44/InP
quantum cascade lasers (QCLs). The highest-performance device (λ ~ 3.6μm) displays pulsed laser action for
temperatures up to 300 K. The shortest wavelength QCL (λ ≈ 3.05 μm) operates in pulsed mode at temperatures only up
to 110 K. The first feasibility study of the strain compensated InGaAs/AlAsSb/InP QCLs (λ ~ 4.1 μm) proves that the
lasers with increased indium fractions in the InGaAs quantum wells of 60 and 70% display no degradation compared
with the lattice matched devices having identical design. This strain compensated system, being of particular interest for
QCLs at λ <~ 3.5μm, provides increased energy separation between the Γ and X conduction band minima in the quantum
wells, thus decreasing possible carrier leakage from the upper laser levels by intervalley scattering. We also demonstrate
that the performance of strain compensated InGaAs/AlAsSb QCLs can be improved if AlAsSb barriers in the QCL
active region are replaced by AlAs layers. The introduction of AlAs is intended to help suppress compositional
fluctuations due to inter diffusion at the quantum well/barrier interfaces.
The notion ofusing a laser CRT as monochromatic light source for passive display technologies is discussed. Optimism
for such an application is based on the high efficiency (more than O %) of "red" lasers obtained in GaInP/AlGaInP
multi-quantum well (MQW) structures. A miniature vacuum tube designed as a light source is presented. Technological
issues of achieving high efficiency in green and blue using wide band gap II-VI compound MQW structures is also
presented. To date, maximum output power of 3.2 and 1 W was achieved at wavelengths of 535 and 462 nm respectively
under longitudinal pumping by an electron beam with electron energy of 4O keV at room temperature.
We report the realisation of spectroscopic broadband transmission experiments on quantum cascade lasers (QCLs)
under continuous wave operating conditions for drive currents up to laser threshold. This technique allows, for the first
time, spectroscopic study of light transmission through the waveguide of QCLs in a very broad spectral range (λ~1.5-12
μm), limited only by the detector response and by interband absorption in the materials used in the QCL cladding
regions. Waveguide transmittance spectra have been studied for both TE and TM polarization, for InGaAs/InAlAs/InP
QCLs with different active region designs emitting at 7.4 and 10μm. The transmission measurements clearly show the
depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy
corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second
laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key
parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain
and waveguide loss coefficients.
In this paper we present single mode quantum cascade lasers (QCLs) based on the GaAs and the InP material systems. We show results for first- and second-order distributed feedback (DFB) QC lasers with surface gratings. The InP based lasers are grown by metalorganic vapor phase epitaxy (MOVPE) and show single mode continuous wave emission up to 200 K. In pulsed operation we achieved single mode surface emission peak output powers exceeding 1 Watt at room temperature. The presented GaAs/AlGaAs laser features an air/AlGaAs waveguide, combined with a second-order distributed feedback grating. That laser shows 3 Watts of single mode output power via the surface at 78 K.
We report MOVPE-grown quantum cascade lasers with operating wavelengths between λ~7.5-9.5μm with threshold current densities as low as 2.4kA/cm2 at room temperature. Seven wafers grown for operation at ~9μm show a variation of just 3% in the superlattice periods obtained from X-ray analysis, and laser emission is observed from all wafers with a ~5meV spread of emission energies. Multimode Fabry-Perot and singlemode distributed feedback lasers have been fabricated, operating at λ~7.8μm at room temperature, corresponding with absorption lines in the infrared spectra of methane. In addition, we have produced a strain compensated MOVPE-grown quantum cascade laser operating at λ~4.5μm.
We examine the mechanisms that lead to a low value of saturated modal gain in both 1μm emitting InGaAs based and ≈ 700nm emitting InP/GaInP quantum dot laser systems. We explain the observation that the value of the saturated modal gain increases as the temperature decreases using a simple model of the filling of the available dot and wetting layer states according to a Fermi-Dirac distribution. We show that it is the relatively large number of available wetting layer valence states and their proximity in energy to the dot states that limits the modal gain. We measure the population inversion factor for samples containing different numbers of layers of dots and for samples where the dots are grown in a quantum well (DWELL) and for dots grown in bulk layers of either GaAs or Al0.15Ga0.85As (non-DWELL). Comparison of this data with that calculated for a Fermi-Dirac distribution of carriers in the available states demonstrates that for most of the samples the carriers in the ground states of the quantum dots are not in thermal equilibrium with those in higher lying energy states - the excited states or wetting layer.