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Avalanche photodiodes (APDs) are used in optical receivers of high-speed optical communication systems to improve signal-to-noise ratio over conventional photodiodes. Low excess noise characteristics are crucial for APDs to preserve the benefits associated with high internal gains. In this work, we presented room temperature data of avalanche gain and excess noise factors of Al0.85Ga0.15As0.56Sb0.44 APDs using pure and mixed carrier injection profiles. Using pure electron injection, the best possible excess noise performance for a given avalanche width was measured with an excess noise factor https://doi.org/10.15131/shef.data.15082455 )

InGaAs/GaAsSb type-II superlattice (T2SL) photodiodes grown on InP substrates are an alternative detector technology for applications operating in the short wavelength infrared band. Their cut-off wavelengths are heavily influenced by the thickness and material composition of InGaAs and GaAsSb used in the T2SL. We present a single band k.p. model performed using a finite difference approach in nextnano validated against two T2SL photodiode wafers and results from literature. These photodiode wafers cover both lattice matched and strained GaAs1−x Sb x compositions (x = 0.40, wafer A and 0.49, wafer B). The validation data covers temperature dependence of cut-off wavelengths (obtained from phase-sensitive photo response data) from 200 K to room temperature. The cut-off wavelengths were found to reduce at 1.32 nm K−1 for wafer A and 1.07 nm K−1 for wafer B. Good agreement was achieved between the validation data and nextnano simulations, after altering the GaAs1−x Sb x valance band offset (VBO) bowing parameter to −1.06 eV. Using this validated model, we show that the wavefunction overlap drops significantly if the GaAsSb barrier is thicker than the InGaAs well layer, hence defining the upper limit of the barrier layer. This validated model is then used to demonstrate that there is a linear dependence between the maximum achievable wavefunction overlap and cut-off wavelength of a lattice matched InGaAs/GaAsSb T2SL. We also found that the adoption of a 5 nm/3 nm InGaAs/GaAsSb T2SL structure offers an improved wavefunction overlap over the more common 5 nm/5 nm InGaAs/GaAsSb T2SL designs. The data reported in this paper is available from doi: 10.15131/shef.data.20310591.

Sensitivity of high-speed optical receivers is heavily influenced by the performance of the optical detectors used in the receivers, the data rate, and the target bit-error-rate (BER). A simulation model for sensitivity of optical receivers based on electron-avalanche photodiodes (e-APDs) is presented. It allows for the optimization of avalanche width and operating voltage to achieve the optimum receiver sensitivity for given bit rate and target BER. The effects modelled include inter-symbol interference (ISI), various dark current components (tunnelling, diffusion, and generation), current impulse duration, avalanche gain, and amplifier's noise. The model was demonstrated through simulations of Indium Arsenide (InAs) e-APDs. For $10^{-12}$ target BER, the receiver's sensitivities were found to be −30.6, −22.7, −19.2, and −16.6 dBm, for 10, 25, 40, and 50 Gb/s data rate, respectively. Desirable avalanche properties of InAs e-APDs are counteracted by detrimental effects of high dark currents. Hence InAs e-APDs with lower dark currents are required to be more competitive with other optical detector technologies for high-speed optical receivers. The data reported in this article is available from the ORDA digital repository (DOI: 10.15131/shef.data.9959468).

Avalanche photodiodes (APDs), and single photon avalanche diodes (SPADs), with InP avalanche regions and InGaAs absorption regions, are used for detecting weak infrared light at ∼1.55 μm wavelength. These devices are often cooled to below room temperature during operation yet both validated simulation models and impact ionization coefficients that accurately describe the avalanche characteristics of InP are lacking in the temperature range of interest (200 K to room temperature). In this article we present an accessible, validated temperature dependent simulation model for InP APDs/SPADs. The model is capable of simulating avalanche gain, excess noise, breakdown voltage, and impulse current at 150–300 K. Temperature dependent ionization coefficients in InP, which may be used with other APD/SPAD simulation models, are also presented. The data reported in this article is available from the ORDA digital repository (DOI: 10.15131/shef.data.c.4373006).

Thin avalanche layers have been adopted to achieve low excess noise and high-gain bandwidth products in InP and InAlAs avalanche photodiodes. In this paper, we report the excess noise characterization in a series of $\text{Al}_{\mathbf{1{-}}{\boldsymbol x}}$ Ga $_{\boldsymbol{x}}$ As $_{\mathbf{0.56}}$ Sb $_{\mathbf{0.44}}$ ( $x\,= \,0,\,0.05$ , 0.1, 0.15) diodes with avalanche layer thickness of 110–116 nm. These alloys, lattice matched to InP, showed lower excess noise than InP and InAlAs. Dark current, most probably originating from surface leakage, was observed to be lower in composition with higher Ga concentration. Avalanche gain and excess noise measurements using lasers of 543 and 633 nm wavelengths indicated that at a given electric field, the electron ionization coefficient is larger than the hole ionization coefficient. Using the 543 nm laser, low excess noise data corresponding to an effective ionization coefficient ratio of $k\,= \,0.1$ in the conventional excess noise theory was measured in $\text{Al}_{\mathbf{1{-}}{\boldsymbol x}}$ Ga $_{\boldsymbol{x}}$ As $_{\mathbf{0.56}}$ Sb $_{\mathbf{0.44}}$ ( $x\,= \,0.05$ , 0.1, 0.15), although pure electron injection was not achieved. Our results demonstrated the potential of using $\text{Al}_{\mathbf{1{-}}{\boldsymbol x}}$ Ga $_{\boldsymbol{x}}$ As $_{\mathbf{0.56}}$ Sb $_{\mathbf{0.44}}$ ( $x\,= \,0.05$ , 0.1, 0.15) as replacement for InP and InAlAs for high speed and low excess noise avalanche photodiodes. The data reported in this paper is available from the ORDA digital repository ( https://doi.org/10.15131/shef.data.5155822 ).

Indium Arsenide (InAs) infrared photodiodes provide high quantum efficiency in the wavelength range of 1.0-3.0 μm. Planar diode configuration has been adopted to reduce surface leakage. In this work, new fabrication procedures for planar InAs avalanche photodiodes (APDs) are reported. Beryllium (Be) ions were implanted into InAs at a relatively low energy of 34 keV. Effects of duration of post implant annealing on the electrical characteristics of InAs APDs were investigated. It was found that a combination of post implant annealing at 500 °C for 15 min and a shallow surface etch produces planar APDs with good characteristics (room temperature dark current density of 0.52 A/cm 2 at -0.2 V and external quantum efficiency of 51% at 1520 nm at -0.3 V). These represent a 3 times reduction in dark current and 1.4 times increase in responsivity, compared to earlier Be-implanted planar InAs APDs. The APDs' avalanche gain characteristics remain similar to those from earlier reports, with a gain of 4 at a relatively low operating bias of 5 V. This suggests the potential of integrating InAs APDs with low voltage readout integrated circuits (ROIC) for development of infrared imaging arrays. The data reported in this paper is available from the ORDA digital repository (DOI: 10.15131/shef.data.6955037).

An avalanche photodiode with a ratio of hole-to-electron ionization coefficients, k = 0, is known to produce negligible excess noise irrespective of the avalanche gain. The low noise amplification process can be utilized to detect very low light levels. In this work, we demonstrated InAs avalanche photodiodes with high external quantum efficiency of 60% (achieved without antireflection coating) at the peak wavelength of 3.48 µm. At 77 K, our InAs avalanche photodiodes show low dark current (limited by 300 K blackbody background radiation), high avalanche gain and negligible excess noise, as InAs exhibits k = 0. They were therefore able to detect very low levels of light, at 15-31 photons per 50 µs laser pulse at 1550 nm wavelength. These correspond to the lowest detected average power by InAs avalanche photodiodes, ranging from 19 to 40 fW. The measurement system’s noise floor was dominated by the pre-amplifier. Our results suggest that, with a lower system noise, InAs avalanche photodiodes have high potential for optical detection of single or few-photon signal levels at wavelengths of 1550 nm or longer.

The usefulness of avalanche photodiodes (APDs) resides in their ability to produce internal gain via impact ionization without generating excessive noise. This process is stochastic and the gain values fluctuate around a mean value, giving rise to the so-called excess noise. In this work, we evaluate the gain fluctuations in APDs using a multi-channel analyzer (MCA). Two Al0.85Ga0.15As0.56Sb 0.44 APDs, one p-i-n and one n-i-p were used. Illuminated with a pulsed light source, the APDs were connected to a charge-sensitive amplifier, counting the number of charges created by each avalanche event initiated by the light pulse. The signal was subsequently sent to an MCA, recording the gain values and outputting a gain spectrum. Both APDs were investigated for mean gains up to ~ 9. For a given mean gain, the gain distribution for the n-i-p diode was found to be significantly broader than for the p-i-n diode, as expected from the excess noise values previously measured in those devices. The coefficient of variance (CoV), defined as the ratio of standard deviation to mean value of the gain peaks, was found to be low for the p-i-n APD, consistent with the low excess noise values in this material. For higher mean gain values, the CoV of the n-i-p APD gave higher values than for the p-i-n APD, again corroborating the conventional excess noise measurements.

Avalanche photodiodes are widely used in optical receivers for high-speed communication systems. We have been studying the material AlGaAsSb (lattice-matched to InP substrates) experimentally in recent years, evaluating its potential as an alternative avalanche material for these avalanche photodiodes. Our experimental studies cover key characteristics relevant to high-speed avalanche photodiodes. The data obtained show that the material AlGaAsSb has higher gain-bandwidth product, lower excess noise factors, and weaker temperature dependence, compared to current avalanche materials such as InP and InAlAs.

Linear mode and Geiger mode Avalanche Photodiodes are widely used to detect weak optical signals, with the latter able to detect a single photon at a time. Practical simulators for these devices should accurately produce relevant device characteristics and not be overly computationally intensive. The Simple Monte Carlo Simulator, written in C++, offers such a combination and can simulate avalanche photodiodes made with Silicon, Gallium Arsenide and Indium Gallium Phosphide, with the potential to include other semiconductor materials. The software is available on The University of Sheffield Research Data Catalogue and Repository at https://doi.org/10.15131/shef.data.5683939 and on GitHub at https://github.com/jdpetticrew/Simple-Monte-Carlo-Simulator.

With increasing interest over the past decade in space-related remote sensing and communications using near-infrared (NIR) wavelengths, there is a need for radiation studies on NIR avalanche photodiodes (APDs), due to the high radiation environment in space. In this work, we present an experimental study of proton radiation effects on performance parameters of InAs APDs, whose sensitivity extends from visible light to ~3.5 μm. Three irradiation energies (10.0, 31.4, and 58.8 MeV) and four fluences (10sup9/supto 10sup11/supp/cmsup2/sup) were used. At the harshest irradiation condition (10.0 MeV energy and 10sup11/supp/cmsup2/supfluence) the APDs' avalanche gain and leakage current showed a measurable degradation. However, the responsivity of the APDs was unaffected under all conditions tested. The data reported in this article are available from the figshare digital repository (DOI: https://dx.doi.org/10.15131/shef.4560562).

Multiplication and avalanche excess noise measurements have been undertaken on a series of AlInP homo-junction p-i-n and n-i-p diodes with $i$ region widths ranging from 0.04 to 0.89 $\mu \text{m}$ , using 442 and 460 nm wavelength light. Low dark currents of $^{\mathrm {-2}}$ at 95% of breakdown voltage were obtained in all the devices because of its wide bandgap and there was no tunneling dark current present even at high fields >1000 kV/cm. For a given multiplication factor, the excess noise decreased as the avalanche width decreased due to the dead-space effect. Using 460 nm wavelength light, measurements showed that a separate absorption multiplication avalanche photodiode with a nominal multiplication region width of 0.2 $\mu \text{m}$ had an effective $k$ (hole to electron ionization coefficient ratio) of $\sim 0.3$ .

We report an evaluation of an epitaxially grown uncooled InAs photodiode for the use in radiation thermometry. Radiation thermometry measurements was taken using the photodiode covered blackbody temperatures of 50 °C–300 °C. By determining the photocurrent and signal-to-noise ratio, the temperature error of the measurements was deduced. It was found that an uncooled InAs photodiode, with the peak and cutoff wavelengths of 3.35 and 3.55 $\mu \text{m}$ , respectively, measured a temperature of 50 °C, with an error of 0.17 °C. Many plastics have $C$ – $H$ molecular bond absorptions at 3.43 $\mu \text{m}$ and hence radiate thermally at this wavelength. Our results suggest that InAs photodiodes are well suited to measure the temperature of plastics above 50 °C. When tested with a narrow bandpass filter at 3.43 $\mu \text{m}$ and blackbody temperatures from 50 °C–300 °C, the InAs photodiode was also found to produce a higher output photocurrent, compared with a commercial PbSe detectors.

Avalanche photodiodes (APDs) with thin avalanche regions have shown low excess noise characteristics and high gain-bandwidth products, so they are suited for long-haul optical communications. In this work, we investigated how carrier injection profile affects the avalanche gain and excess noise factors of Al5787318).

Avalanche photodiodes (APDs) with thin avalanche regions have shown low excess noise characteristics and high gain-bandwidth products, so they are suited for long-haul optical communications. In this work, we investigated how carrier injection profile affects the avalanche gain and excess noise factors of Al0.85Ga0.15As0.56Sb0.44 (lattice-matched to InP substrates) p-i-n and n-i-p diodes with total depletion widths of 145-240 nm. Different carrier injection profiles were achieved by using light with wavelengths of 420, 543 and 633nm. For p-i-n diodes, shorter wavelength light produces higher avalanche gains for a given reverse bias and lower excess noise factors at a given gain, compared to longer wavelength light. Thus, using 420 nm light on the p-i-n diodes, corresponding to pure electron injection conditions, gave the highest gain and lowest excess noise. In n-i-p diodes, pure hole injection yields significantly lower gain and higher excess noise, compared to mixed carrier injection. These show that the electron ionization coefficient, α, is higher than the hole ionization coefficient, β. Using pure electron injection, excess noise factor characteristics with effective ionization ratios, keff, of 0.08-0.1 were obtained. This is significantly lower than those of InP and In0.52Al0.48As, the commonly used avalanche materials combined with In0.53Ga0.47As absorber. The data reported in this paper is available from the ORDA digital repository (DOI: 10.15131/shef.data.5787318).

Avalanche gain and breakdown voltage in most wide bandgap semiconductor materials are dependent on temperature and most instruments utilizing APDs rely on temperature stabilization or voltage compensation circuitry to maintain a constant avalanche gain. The complexity in operation circuitry can be reduced by incorporating material with inherently superior temperature stability in its avalanche gain and breakdown voltage. In state of the art APDs, the temperature dependence of avalanche breakdown voltage is quantified by the temperature coefficient of avalanche breakdown, Cbd. We report on the temporal and temperature stability of avalanche gain and breakdown voltage of 100 nm thick avalanche layers of Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb). The Cbd (1.60 mV/K) is smaller compared to state of art InP and InAlAs APDs for similar avalanche layer thickness. The temporal stability of avalanche gain for the AlGaAsSb APD was also evaluated in temperature ranges of 294 K to 353 K. The APD was biased at room temperature gain of 10 and maximum fluctuation of ±0.7% was recorded at 294 K which increases to ±1.33% when the temperature was increased to 353K. The promising temperature stability of gain indicates the potential of AlGaAsSb lattice matched to InP in achieving higher tolerance to temperature fluctuations and reduction of the operational complexity of circuitry. The dark currents are robust and do not show significant thermal degradation after gain measurements at elevated temperatures.

Results characterising a set of nine prototype Al0.8Ga0.2As p+–i–n+ mesa photodiodes (400 µm diameter, 1.7 µm i layer) are presented. The results show the performance of the devices as room temperature spectroscopic photon counting soft X-ray detectors. The responses of the photodiodes to illumination with an 55Fe radioisotope X-ray source were measured using a low noise charge sensitive preamplifier; the energy resolutions measured with the devices were consistent with each other and had a mean FWHM at 5.9 keV of 1.27 keV. The devices are the thickest (highest detection efficiency) AlGaAs X-ray spectroscopic mesa photodiodes reported in the literature to date. They also have better energy resolution than all previously reported non-avalanche AlGaAs X-ray detectors of the same area.

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.

We demonstrate an ${\rm Al}_{0.52}{\rm In}_{0.48}{\rm P}$ homo-junction Separate Absorption Multiplication Avalanche Photodiode as a detector with narrow spectral response in the blue-green part of the optical spectrum. Due to its wide band-gap, this device has a dark current density of ${\rm cm}^{-2}$ at 99.9% of the breakdown voltage at room temperature. This device has a peak responsivity at 483 nm of 0.15 A/W when punched-through and is capable of an avalanche gain higher than 100.

The structural and optical properties of GaAs1−xBix alloys for x up to 0.108 have been investigated by high resolution X-ray diffraction and photoluminescence (PL). At room temperature (RT), the PL intensity of the GaAs0.97Bi0.03 sample was found to be ∼300 times higher than a GaAs control sample grown at the same temperature (400 °C). PL measurements carried out at 10 K show that when excitation power, Pex was increased from 0.11 to 1140 W cm−2, the PL peak energy blue-shifts by 80 meV while the full-width-at-half-maximum reduces from 115 to 63 meV. However, the PL peak emission energy becomes independent of the excitation power at RT. The results indicate the presence of localized energy states in the GaAs0.97Bi0.03 sample, which trap carriers at low temperatures and that the majority of the carriers become delocalized at RT. Furthermore, the temperature dependent PL also shows an S-shape behavior, which is a signature of localization effects. A theoretical model, which was derived by solving a rate equation was employed. The model successfully reproduces the observed S-shape behavior and the theory fits well with the experimental data. The RT band gap of GaAs1−xBix for x up to 0.108 has been plotted and compared with the literature.

The analytical calculation of the bit error rate (BER) of digital optical receivers that employ avalanche photodiodes (APDs) is challenging due to 1) the stochastic nature of the avalanche photodiode's impulse-response function and 2) the presence of intersymbol interference (ISI). At ultrafast transmission rates, ISI becomes a dominant component of the BER, and its effect on the BER should be carefully addressed. One solution to this problem, termed the bit-pattern-dependent (PD) approach, is to first calculate the conditional BER given a specific bit pattern and then average over all possible bit patterns. Alternatively, a simplifying method, termed the bit-pattern-independent (PI) approach, has been commonly used whereby the average bit stream is used to calculate the distribution of the receiver output, which, in turn, is used to calculate the BER. However, when ISI is dominant, the PI approximation is inaccurate. Here, the two approaches are analytically compared by analyzing their asymptotic behavior and their bounds. Conditions are found to determine when the PI method overestimates the BER. The BER found using the PD method exponentially decays with the received optical power, whereas for the PI approach, the BER converges to a constant, which is unrealistic. For an InP-based APD receiver with a 100-nm multiplication layer, the PI method is found to be inaccurate for transmission rates beyond 20 Gb/s.

Increasing reliance on the Internet places greater and greater demands for high-speed optical communication systems. Increasing their data transfer rate allows more data to be transferred over existing links. With optical receivers being essential to all optical links, bandwidth performance of key components in receivers, such as avalanche photodiodes (APDs), must be improved. The APDs rely on In3827460.v1).

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 investigated the temperature dependence of dark current, avalanche gain and breakdown voltage in Al 0.85 Ga 0.15 As 0.56 Sb 0.44 (hereafter AlGaAsSb) avalanche photodiodes (APDs) with avalanche region widths, w = 90 and 178 nm. There is negligible band to band tunnelling currents and a very weak temperature dependence of gain observed in both didoes. The temperature coefficient of breakdown voltage, C bd = 0.41 mV/K in AlGaAsSb diode with w = 90 nm is the lowest value reported for different semiconductor materials with similar avalanche region width.

We investigate a novel In 0.53 Ga 0.47 As/Al 0.85 Ga 0.15 As 0.56 Sb 0.44 (hereafter InGaAs/AlGaAsSb) avalanche photodiode (APD) for high speed operation. The fabricated APD with 20 µm diameter exhibits a maximum responsivity of 5.33 A/W at 1550 nm, a maximum 3-dB bandwidth (f 3dB ) of 14 GHz, and a clear eye diagram at 10 Gb/s. A GBP value as high as 407.4 GHz was extracted. The results demonstrate the potential of using InGaAs/AlGaAsSb APDs for telecom/data communication system operating at or above 10 Gb/s.

Electrical characterization of two GaAs p+-i-n+ mesa X-ray photodiodes over the temperature range 0 °C to 120 °C together with characterization of one of the diodes as an X-ray detector over the temperature range 0 °C to 60 °C is reported as part of the development of photon counting X-ray spectroscopic systems for harsh environments. The randomly selected diodes were fully etched and unpassivated. The diodes were 200 μm in diameter and had 7 μm thick i layers. The leakage current density was found to increase from (3 ± 1) nA/cm−2 at 0 °C to (24.36 ± 0.05) μA/cm−2 at 120 °C for D1 and from a current density smaller than the uncertainty (0.2 ± 1.2) nA/cm−2 at 0 °C to (9.39 ± 0.02) μA/cm−2 at 120 °C for D2 at the maximum investigated reverse bias (15 V). The best energy resolution (FWHM at 5.9 keV) was achieved at 5 V reverse bias, at each temperature; 730 eV at 0 °C, 750 eV at 20 °C, 770 eV at 40 °C, and 840 eV at 60 °C. It was found that the parallel white noise was the main source of the photopeak broadening only when the detector operated at 60 °C, at 5 V, 10 V, and 15 V reverse bias and at long shaping times (>5 μs), whereas the sum of the dielectric noise and charge trapping noise was the dominant source of noise for all the other spectra.

A random path length model for X-ray avalanche photodiodes (APDs) has been used to assess the effects of carriers' dead spaces and injection position on the avalanche gain distributions. Significant carrier's dead space, typically found in submicrometer avalanche regions, is found to reduce spread in the avalanche gain distribution, which is consistent with the reduced excess noise factors observed in conventional APDs. Mixed carrier-type injection increases the spread in the gain distribution along the avalanche region. The model was validated by comparing the results with experimental X-ray spectra from a GaAs/ Al0.8Ga0.2As separate absorption and multiplication APD. Good agreement was achieved between measured and simulated data. Avalanche multiplication shifted the detected photo peak away from the system noise, thereby improving its energy resolution and signal-to-noise ratio.

The growth of III–V bismuthides is complicated by the low incorporation efficiency of Bi in GaAs, leading either to the formation of metallic Bi droplets or low layer composition fractions. Typically growth is performed between 280 and 350 °C and at near stoichiometric Ga:As fluxes in order to encourage Bi incorporation. However most work reported to date also utilises As2 as the As overpressure constituent. It is found in this work that growth with As4 allows high Bi composition films with the standard 1:20 Ga:As4 beam equivalent pressure ratio (BEPR) utilised for higher temperature buffer layer growth. The Bi fraction versus Bi:As4 BEPR is found to be initially linear, until a maximum value is obtained for a given temperature after which the continued oversupply of Bi results in the formation of droplets.

Circular mesa GaAs p+–i–n+ diodes for photon counting soft X-ray spectroscopy have been fabricated and characterised over a temperature range of +80 to –30 °C. The spectroscopic performance of the diodes, as measured by the FWHM of the Mn Kα X-ray line from an 55Fe radioisotope, is reported. In addition, we compare the GaAs diodes with previously fabricated and characterised Al0.8Ga0.2As p+–i–n+ diodes of similar geometry.

Realistic Monte Carlo simulations for the avalanche gain of absorbed X-ray photons were carried out in a study of the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes (APDs). The work explored how 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 ionization coefficients, and the mean gain itself. We showed that the conventional notion of APD gains significantly degrading energy resolution is incomplete. If the X-ray photons are absorbed outside the avalanche region, then high avalanche gains with little energy resolution penalty can be achieved using dissimilar ionization coefficients. However, absorption of X-ray photons within the avalanche region will always result in broad gain distribution (degrading energy resolution), unless electrons and holes have similar ionization coefficients.

An avalanche photodiode (APD) using a InAlAs multiplication region and a type-II InGaAs/GaAsSb superlattice as the absorber (both lattice matched to InP) is reported. An optical and electrical characterization of the photodiode is performed. The APD exhibited an absorption cutoff wavelength of 2.5 μm, which is expected from the InGaAs/GaAsSb superlattice. A responsivity of 0.47 A/W (without gain) for the APD at a 2004-nm wavelength was demonstrated. The APD breakdown voltage showed a weak temperature dependence of ~40 mV/K, as a result of the excellent temperature stability in InAlAs.

The reverse-bias current-voltage characteristics of a series of Ga1-xlnxNyAs1-y diodes with bandgap of between 0.87 and 1.04 eV are reported. At low bias, diffusion and generation-recombination currents are dominant at high and low temperatures, respectively. At high reverse bias, the dark current is insensitive to changes in temperature, which is indicative of tunneling current mechanisms. We also observe an exponential dependence of the dark current with the electric field in the mid-bias range for all our diodes, which may be explained by the Poole-Frenkel effect.

Simple analytical expressions for temperature coefficients of breakdown voltage of avalanche photodiodes (APDs) utilizing InP or InAlAs are reported. The work is based on measurements of temperature dependence of avalanche breakdown voltage in a series of InP and InAlAs diodes at temperatures between 20 and 375 K. While avalanche breakdown voltage becomes more temperature sensitive with avalanche region thickness for both materials, the InAlAs diodes are less sensitive to temperature changes compared to InP diodes.

Calculations based on a rigorous analytical model are carried out to optimize the width of the indium phosphide avalanche region in high-speed direct-detection avalanche photodiode-based optical receivers. The model includes the effects of intersymbol interference (ISI), tunneling current, avalanche noise, and its correlation with the stochastic avalanche duration, as well as dead space. A minimum receiver sensitivity of -28 dBm is predicted at an optimal width of 0.18 mum and an optimal gain of approximately 13, for a 10 Gb/s communication system, assuming a Johnson noise level of 629 noise electrons per bit. The interplay among the factors controlling the optimum sensitivity is confirmed. Results show that for a given transmission speed, as the device width decreases below an optimum value, increased tunneling current outweighs avalanche noise reduction due to dead space, resulting in an increase in receiver sensitivity. As the device width increases above its optimum value, the receiver sensitivity increases as device bandwidth decreases, causing ISI to dominate avalanche noise and tunneling current shot noise.

We study the breakdown characteristics and timing statistics of InP and In0.52Al0.48As single-photon avalanche photodiodes (SPADs) with avalanche widths ranging from 0.2 to 1.0 mum at room temperature using a random ionization path-length model. Our results show that, for a given avalanche width, the breakdown probability of In0.52Al0.48As SPADs increases faster with over bias than InP SPADs. When we compared their timing statistics, we observed that, for a given breakdown probability, InP requires a shorter time to reach breakdown and exhibits a smaller timing jitter than In0.52Al0.48As. However, due to the lower dark count probability and faster rise in breakdown probability with over bias, In0.52Al0.48As SPADs with avalanche widths les 0.5 mum are more suitable for single-photon detection at telecommunication wavelengths than InP SPADs. Moreover, we predict that, in InP SPADs with avalanche widths les 0.3 mum and In0.52Al0.48As SPADs with avalanche widths les 0.2 mum, the dark count probability is higher than the photon count probability for all applied biases.

Hole initiated multiplication characteristics of 4H-SiC Separate Absorption and Multiplication Avalanche Photodiodes (SAM-APDs) with a n- multiplication layer of 2.7 µm were obtained using 325nm excitation at temperatures ranging from 300 to 450K. The breakdown voltages increased by 200mV/K over the investigated temperature range, which indicates a positive temperature coefficient. Local ionization coefficients, including the extracted temperature dependencies, were derived in the form of the Chynoweth expression and were used to predict the hole multiplication characteristics at different temperatures. Good agreement was obtained between the measured and the modeled multiplication using these ionization coefficients. The impact ionization coefficients decreased with increasing temperature, corresponding to an increase in breakdown voltage. This result agrees well with the multiplication characteristics and can be attributed to phonon scattering enhanced carrier cooling which has suppressed the ionization process at high temperatures. Hence, a much higher electric field is required to achieve the same ionization rates.