Peter Vines

Avalanche Multiplication and Excess Noise in InAs Electron Avalanche Photodiodes at 77 K

The findings of a study of impact ionization, avalanche multiplication and excess noise in InAs avalanche photodiodes at 77 K are reported. It is shown that hole impact ionization is negligible in practical devices which continue to operate as electron avalanche photodiodes, as they do at room temperature. A new electron ionization coefficient capable of modeling multiplication at 77 K is presented and it is shown that significant multiplication can be achieved in practical devices without excessive tunneling currents. The characteristic changes observed between room temperature and 77 K are discussed. This paper helps to demonstrate the potential for practical InAs electron avalanche photodiodes, operating cooled.

Versatile Spectral Imaging With an Algorithm-Based Spectrometer Using Highly Tuneable Quantum Dot Infrared Photodetectors

We report on the implementation of an algorithm-based spectrometer capable of reconstructing the spectral shape of materials in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) wavelengths using only experimental photocurrent measurements from quantum dot infrared photodetectors (QDIPs). The theory and implementation of the algorithm will be described, followed by an investigation into this algorithmic spectrometers performance. Compared to the QDIPs utilized in an earlier implementation, the ones used here have highly varying spectral shapes and four spectral peaks across the MWIR and LWIR wavelengths. It has been found that the spectrometer is capable of reconstructing broad spectral features of a range of bandpass infrared filters between wavelengths of 4 and 12 as well as identifying absorption features as narrow as 0.3 in the IR spectrum of a polyethylene sheet.

Multiple stack quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIP) have established themselves as promising devices for detecting infrared (IR) radiation for wavelengths <20μm due to their sensitivity to normal incidence radiation and long excited carrier lifetimes. A limiting factor of QDIPs at present is their relatively small absorption volume, leading to a lower quantum efficiency and detectivity than in quantum well infrared photodetectors and mercury cadmium telluride based detectors. One means of increasing the absorption volume is to incorporate a greater number of quantum dot (QD) stacks, thereby increasing the probability of photon capture. Growth of InAs/InGaAs dot-in-a-well (DWELL) QDIPs with greater than 10 stacks is challenging due to the increased strain between layers, leading to high dark current. It is known that strain can be reduced in QDIPs by reducing the width of the InGaAs well and incorporating a second well consisting of GaAs and barriers consisting of AlGaAs. A number of InAs/InGaAs/GaAs DWELL QDIPs with 30-80 stacks have been grown, fabricated and characterised. Dark current in these layers appears to be constant at given electric field, suggesting strain does not increase significantly if the number of QD stacks is increased. IR spectral measurements show well defined peaks at 5.5μm, 6.5μm and 8.4μm. In this work a comparison between dark current, noise, gain, responsivity and detectivity in these layers is presented and compared to existing data from conventional DWELL QDIPs.

Modeling and analysis of intraband absorption in quantum-dot-in-well mid-infrared photodetectors

Intraband absorption in quantum-dot-in-a-well (DWELL) mid-infrared photodetectors is investigated using photocurrent spectroscopy and computationally cost-effective modeling linked to experimental data. The DWELL systems are challenging for modeling the electronic structure, which involves both discrete levels and the continuum energy spectrum. We show that the latter can be successfully approximated by a quasi-continuum in a large three-dimensional (3D) “quantum box” in which the electronic structure is calculated in the effective mass approximation using the finite element method. Experimental and simulated spectra show good agreement with each other, which justifies using the modeling for analysis of the experimental data. In particular, the origin of the peaks and the dot parameters, such as composition are deduced. Effects of dot composition and shape on the intraband absorption spectra are also predicted. Our model proves to be a useful tool in designing and analyzing advanced DWELL structures for a...

Development of high temperature AlGaAs soft X-ray photon counting detectors

New types of detectors based on the wide band gap material AlGaAs have been developed for soft X-ray spectroscopy applications. We report on the spectroscopic performance of simple p-i-n diodes and avalanche photodiodes (APDs). A number of diode types with different layer thicknesses have also been characterised. X-ray spectra from 55Fe and 109Cd radioactive sources show these diodes can be used for spectroscopy with promising energy resolution (1.0–1.25 keV) over a -30 to +90 °C temperature range. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al0.8Ga0.2As APDs was also investigated at temperatures from -20 to +80 °C. The temperature dependence of the pure electron initiated multiplication factor (Me) and the mixed carrier initiated avalanche multiplication factor (Mmix) were extracted from the X-ray spectra. The experimental results are compared with a spectroscopic Monte Carlo model for Al0.8Ga0.2As diodes from which the temperature dependence of the pure hole initiated multiplication factor (Mh) is determined. Monte Carlo simulations for the avalanche gain of absorbed X-ray photons have also been developed to study the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes. The model showed that the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and hence the photon energy), the relationship between the two ionization coefficients, and the overall mean gain. Our model showed that the conventional notion of APD gains degrading energy resolution significantly is incomplete. We compare the Monte Carlo simulations with experimental data from a number of different Al0.8Ga0.2As diodes.

Noise, Gain, and Responsivity in Low-Strain Quantum Dot Infrared Photodetectors With up to 80 Dot-in-a-Well Periods

We present a systematic study of noise, gain, responsivity, and specific detectivity D* , in a series of low-strain dot-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). The lattice-matched GaAs quantum wells and AlGaAs barriers in these devices prevent the accumulation of excessive strain and allow the growth of up to 80 DWELL periods. We show that the photoconductive gain in these QDIPs is inversely proportional to the number of periods, while the total quantum efficiency is proportional to the number of periods, meaning that the responsivity remains constant at a given mean electric field as the number of periods is varied. The dark current in each QDIP was also found to be constant at a given mean electric field.

Quantum dot infrared photodetectors with highly tunable spectral response for an algorithm based spectrometer

We report on low strain quantum dot infrared photodetectors (QDIP) with 80 dot in a well (DWELL) stacks. These QDIPs have been grown with lattice matched Al0.1Ga0.9As barriers and GaAs wells allowing a large number of stacks to be grown leading to an increased absorption volume. The QDIPs show a strong spectral response that varies significantly with applied bias, with four distinct peak wavelengths ranging from 5.5μm to 10.0μm. The highly tunable nature of the intrinsic responses makes these QDIPs very attractive as multispectral imagers in the MWIR and LWIR regions. The spectral diversity of these QDIPs has been exploited using an algorithm to produce a highly versatile algorithmic spectrometer. The algorithm assigns a specific weighting factor to each of the intrinsic responses and then sums these weighted responses to achieve any desired spectral shape. Triangular narrowband filters have been synthesised in this way with full width at half maximums (FWHM) as narrow as 0.2μm. The QDIPs can be used to image objects in the MWIR and LWIR regions by measuring the photocurrent generated at each specific bias and summing them using the calculated weighting factors for every wavelength of interest. This technique has been successfully used to capture the radiated power from a blackbody source through IR filters with different centre wavelengths and bandwidths as a function of wavelength in the LWIR and MWIR regions.

Investigation of the optical properties of InAs/InGaAs/GaAs quantum dot in quantum well multilayer structures for infrared photodetectors

A detailed study of InAs/InGaAs quantum dots in quantum well (DWELL) structures grown on GaAs substrates for infrared photodetectors was performed using surface photovoltage (SPV) spectroscopy. Three types of samples were investigated: as-grown, and annealed with dielectric coating SiO2 or SiN. The annealing resulted in intermixing of the material components. The amplitude and phase SPV spectra were measured at room temperature under various experimental conditions. The comparison of the SPV with the photoluminescence (PL) spectra allows one to conclude that the spectral features are due to optical transitions in the DWELL structure. The blueshift observed of these features in the intermixed samples implies that the energy levels responsible for the transitions change correspondingly due to the intermixing process. The interface band-bending in the samples and the mechanisms of the carrier dynamics were determined by a comparative analysis of the SPV amplitude and phase spectra, using our vector model for representation of the SPV signal.

Development of AlGaAs avalanche diodes for soft X-ray photon counting

We report on the performance of avalanche photodiodes (APDs) based on the wide band gap material AlGaAs which have been developed for soft X-ray spectroscopy applications. A number of diode types with different layer thicknesses have been characterised. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al<inf>0.8</inf>Ga<inf>0.2</inf>As APDs was investigated at temperatures from +80 °C to −20 °C. X-ray spectra from a ⁵⁵Fe radioactive source show these diodes can be used for spectroscopy with promising energy resolution (0.9–2.5keV) over a wide temperature range. The temperature dependence of the pure electron initiated multiplication factor (M<inf>e</inf>) and the mixed carrier initiated avalanche multiplication factor (M<inf>mix</inf>) were experimentally measured. The experimental results are compared with a spectroscopic Monte Carlo model for Al<inf>0.8</inf>Ga<inf>0.2</inf>As diodes from which the temperature dependence of the pure hole initiated multiplication factor (M<inf>h</inf>) is determined.

GaAs p-i-n diodes for room temperature soft X-ray photon counting

A study of leakage currents and X-ray photon counting using GaAs p-i-n diodes is presented. Different fabrication techniques have been investigated, namely He implantation, partial wet etching and full wet etching. It was found that the partially etched diodes showed well-defined spectral peaks when exposed to a 55Fe radioisotope source and uniformly low leakage currents ideal for X-ray detector arrays.