Avalanche photodiodes (APD) can improve the signal to noise ratio in applications such as LIDAR, range finding and optical time domain reflectometry. However, APDs operating at eye-safe wavelengths around 1550 nm currently limit the sensitivity because the APDs’ impact ionization coefficients in the avalanche layers are too similar, leading to poor excess noise performance. The material AlGaAsSb has highly dissimilar impact ionization coefficients (with electrons dominating the avalanche gain) so is an excellent avalanche material for 1550 nm wavelength APDs. We previously reported a 1550 nm wavelength AlGaAsSb SAM APD with extremely low excess noise factors, 1.93 at a gain of 10 and 2.94 at a gain of 20. Using a more optimized design, we have now realized an AlGaAsSb SAM APD with a lower dark current (7 nA at a gain of 10 from a 230 μm diameter APD), a higher responsivity (0.97 A/W) and a lower excess noise (1.9 at a gain of 40), compared to our previous SAM APD. Noise-equivalent-power (NEP) measurements of our APD with a simple transimpedance amplifier circuit produced an NEP 12 times lower than a state-of-the-art APD under identical test conditions, confirming the advantage of low-noise AlGaAsSb SAM APDs.
The optical detector used in pulsed LIDAR, range finding and optical time domain reflectometry systems is typically the limiting factor in the system’s sensitivity. It is well-known that an avalanche photodiode (APD) can be used to improve the signal to noise ratio over a PIN detector, however, APDs operating at the eye-safe wavelengths around 1550 nm are limited in sensitivity by high excess noise. The underlying issue is that the impact ionization coefficient of InAlAs and InP used as the avalanche region in current commercial APDs are very similar at high gain, leading to poor excess noise performance. Recently, we have demonstrated extremely low noise from an Al(Ga)AsSb PIN diode with highly dissimilar impact ionization coefficients due to electron dominated impact ionization. In this paper, we report on the first low noise InGaAs/AlGaAsSb separate absorption, grading and multiplication APDs operating at 1550 nm with extremely low excess noise factor of 1.93 at a gain of 10 and 2.94 at a gain of 20. Furthermore, the APD’s dark current density was measured to be 74.6 μA/cm2 at a gain of 10 which is competitive with commercial devices. We discuss the impact of the excess noise, dark current and responsivity on the APDs sensitivity and, project a noise-equivalent power (NEP) below 80 fW/Hz0.5 from a 230 μm diameter APD and commercial transimpedance amplifier (TIA). The prospects for the next generation of extremely low noise APDs for 1550 nm light detection are discussed.
A series of AlAsSb p+-i-n+ and n+-i-p+ diodes with varying i-region thickness from 0.08μm to 1.55μm have been used to determine the temperature dependent impact ionization coefficients by performing avalanche multiplication measurements from 210K to 335K. The increase in electron and hole ionization coefficients as the temperature decreases is much smaller when compared to InAlAs and InP. This leads to a much smaller avalanche breakdown variation of 13mV/K in a 1.55μm p+- i-n+ diode. For a 10Gb/s InGaAs/AlAsSb separate absorption and multiplication avalanche photodiode (SAM-APD), the variation in breakdown voltage is predicted to be only 15.58 mV/K.
This paper presents the electron and hole avalanche multiplication and excess noise characteristics based on bulk AlAs0.56Sb0.44 p-i-n and n-i-p homojunction diodes lattice matched to InP, with nominal avalanche region thicknesses of 0.6 -1.5 μm. From these, the bulk electron and hole impact ionization coefficients (α and β respectively), have been determined over an electric field range of 220-1250 kV/cm for α and from 360-1250 kV/cm for β for the first time. Excess noise characteristics suggest an β/α ratio as low as 0.005 for an avalanche region of 1.5 μm in this material, close to the theoretical minimum and significantly lower than AlInAs, InP, or even silicon. This material can be easily integrated with InGaAs for networking and sensing applications, with modeling suggesting that a sensitivity of -32.1 dBm at a bit-error rate (BER) of 1×10-12 at 10 Gb/s at 1550 nm can be realized. This sensitivity can be improved even further by optimizing the dark currents and by using a lower noise transimpedance amplifier.
AlAs0.56Sb0.44 is a promising avalanche material which can be grown lattice-matched to InP and therefore use InGaAs as the absorption region in a Separate Absorption and Multiplication APD (SAM-APD). The electron and hole ionisation coefficients in this material are very dissimilar and our experiments show that this leads to AlAs0.56Sb0.44 having the lowest excess noise performance of any InP based material system (F = 2.2 at M = 40) reported to date. Simulations suggest that operation at 1550 nm and 25 GB s-1 with a sensitivity of -25.7 dBm is possible in a normal incidence SAM-APD.
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.
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.
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.
KEYWORDS: Indium arsenide, Photodiodes, Sensors, Black bodies, Temperature metrology, Radiation thermometry, Signal to noise ratio, Diodes, Photodetectors, Optical amplifiers
We report on the evaluation of InAs photodiodes and their potential for low temperature sensing. InAs n-i-p photodiodes were grown and analyzed in this work. Radiation thermometry measurements were performed at reference blackbody temperatures of 37 to 80°C to determine photocurrent and temperature error. The uncooled InAs photodiodes, with a cutoff wavelength of 3.55 μm, detect a target temperature above 37°C with a temperature error of less than 0.46°C. When the photodiode was cooled to 200 K, the temperature error at 37°C improves by 10 times from 0.46 to 0.048°C, suggesting the potential of using InAs for human temperature sensing.
An InAsBi photodiode has been grown, fabricated and characterized to evaluate its performance in the MWIR
region of the spectrum. Spectral response from the diode has been obtained up to a diode temperature of 225 K.
At this temperature the diode has a cut off wavelength of 3.95 μm, compared to 3.41 μm in a reference InAs
diode, indicating that Bismuth has been successfully incorporated to reduce the band gap of InAs by 75 meV.
Similar band gap reduction was deduced from the cut off wavelength comparison at 77 K. From the dark current
data, R0A values of 590 MΩcm2 and 70 MΩcm2 at temperatures of 77 and 290 K respectively, were obtained in
our InAsBi photodiode.
InAs/GaSb type-II superlattices (T2SLs) are attractive due to their potentially low dark currents and high responsivity. These low dark currents arise due to reduced Auger recombination caused by the spatial separation between the electrons and holes. Coupling these two aspects together leads to the potential of high operating temperature and high D*. An additional attraction of T2SLs is their wavelength tunability; the wavelength can be tuned between 3 to 12 μm, making them attractive for the militarily important MWIR and long-wave infrared (LWIR) bands. InAs/GaSb T2SLs are traditionally grown upon GaSb substrates due to lattice matching of the type-II material on GaSb. However, GaSb substrates are relatively small and expensive compared with GaAs, leading to increased cost. Additionally, the high absorption coefficient of GaSb requires the substrate to be removed prior to use in FPAs. We present an InAs/GaSb T2SL grown upon a GaAs substrate which operates at room temperature. A room temperature spectral response could be measured for the layer, with responsivity and shot and thermal noise limited specific detectivity (D*) of 0.45 A/W and 8.0x108 cmHz1/2/W, respectively, at a bias voltage of -0.3 V. This uncooled operation D* is the best to date compared with the literature for a p-i-n or n-i-p MWIR structure grown upon a GaAs substrate.
Two of the key challenges in the realisation of focal plane arrays based on type-II InAs/GaSb superlattices (T2SL) are
the difficulty in achieving a good sidewall profile and the increased dominance of surface leakage current as the device
dimensions shrink. We report the electrical and morphological results of test pixels for mid-wave infrared T2SL
photodiodes etched using a Cl2/Ar based inductively coupled plasma reactive ion etching (ICP-RIE) process and
passivated using SU-8 epoxy photoresist. The etch rate and sidewall surface morphology of GaSb, InAs, and InAs/GaSb
T2SL materials are compared after dry etching under the same conditions, leading to the determination of an optimal
etch rate. The effect of surface treatment using selected wet chemical etchants before passivation on the surface leakage
current is presented. Limitations of the dry etching recipe and further improvement of the sidewall verticality and
smoothness are also discussed. Good sidewall profiles and
bulk-limited dark currents are demonstrated for T2SL
photodiodes etched to depths between 1.5 and 3.5 μm with a pitch size down to 12 μm.
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.
Important avalanche breakdown statistics for Single Photon Avalanche Diodes (SPADs), such as avalanche breakdown
probability, dark count rate, and the distribution of time taken to reach breakdown (providing mean time to breakdown
and jitter), were simulated. These simulations enable unambiguous studies on effects of avalanche region width,
ionization coefficient ratio and carrier dead space on the avalanche statistics, which are the fundamental limits of the
SPADs. The effects of quenching resistor/circuit have been ignored. Due to competing effects between dead spaces,
which are significant in modern SPADs with narrow avalanche regions, and converging ionization coefficients, the
breakdown probability versus overbias characteristics from different avalanche region widths are fairly close to each
other. Concerning avalanche breakdown timing at given value of breakdown probability, using avalanche material with
similar ionization coefficients yields fast avalanche breakdowns with small timing jitter (albeit higher operating field),
compared to material with dissimilar ionization coefficients. This is the opposite requirement for abrupt breakdown
probability versus overbias characteristics. In addition, by taking band-to-band tunneling current (dark carriers) into
account, minimum avalanche region width for practical SPADs was found to be 0.3 and 0.2 μm, for InP and InAlAs,
respectively.
In this work we report on InAs avalanche photodiodes (APDs) that exhibit high gain with extremely low excess
avalanche noise. Our measurements showed that InAs has significantly larger electron ionization coefficient than most
compound III-V semiconductors and extremely small hole ionization coefficient. This large electron to hole ionization
coefficient ratio leads to excess noise factor, F~2 when electrons initiated the multiplication process in our APDs.
Significantly larger excess noise factors were measured when both electrons and holes were injected into the avalanche
region to initiate the multiplication process. Our InAs APDs demonstrated ionization characteristics similar to those
observed in Cadmium Mercury Telluride (CMT) in the short wave infrared (SWIR). Measurements of temperature
dependence of leakage current provided early indications of potentially higher operating temperature than CMT. The low
excess noise behaviour and higher operating temperatures, demonstrate that InAs APDs have potential to be developed
into low cost high performance photon counting APD arrays to rival CMT.
We report on the characterisation of impact ionisation in InAs and the development of practical InAs avalanche
photodiodes (APDs) to exploit the properties identified. Avalanche multiplication measurements show that the hole
ionisation coefficient is negligible in InAs. We have demonstrated that this results in extremely low excess
multiplication noise, F<2, for electron initiated gain. Indeed the excess noise measured was comparable to the excellent
results reported for HgCdTe APDs and notably lower than those reported for other III-V based APDs. It could be
desirable to exploit this extremely low excess noise characteristic, now demonstrated in a III-V material, in a number of
applications. In this work we consider in particular the exploitation in an InAs APD focal plane array, to achieve
enhanced sensitivity imaging in the SWIR. The greatest barrier to such exploitation is the requirement to sufficiently
suppress the reverse leakage current, typically present in InAs diodes under high reverse bias at room temperature. We
report on initial developments in this respect, presenting leakage current results for the temperature ranges accessible by
both thermoelectric and Stirling engine cooling. These demonstrate that InAs APDs have the potential to operate at
higher temperatures than the sMWIR sensitive composition of HgCdTe currently used in emerging APD focal plane
array applications. We also show that InAs APDs are capable of operating at bias voltages below 10V, supporting easy
integration with fine pitch read out ICs, without band to band tunnelling contributing significantly to the leakage current,
even at low operating temperatures.
In this work, we present the study on In0.53Ga0.47As/GaAs0.51Sb0.49 type-II heterojunction PIN diodes and
Separate Absorption, Charge and Multiplication (SACM) APDs utilising In0.52Al0.48As as the multiplication layer and
In0.53Ga0.47As/GaAs0.51Sb0.49 type-II heterostructures as the absorption layer. In0.52Al0.48As lattice matched to InP has been
shown to have superior excess noise characteristics and multiplication with relatively low temperature dependence
compared to InP. Furthermore, the type-II staggered band line-up leads to a narrower effective bandgap of approximately
0.49 eV corresponding to the APD cut off wavelength of 2.4 μm. The device exhibited low dark current densities near
breakdown. The device also exhibited multiplication in excess of 100 at 200 K.
There are many applications where the ability to detect optical signals in the 1.65 - 3 μm wavelength range
would be of considerable interest. In this paper we discuss two technologies that offer considerable promise for high
speed, high sensitivity detection in this region utilising avalanche gain. InGaAs/GaAsSb Type II superlattices as the
absorption region and InAlAs as the multiplication region can be combined to form a separate absorption and
multiplication (SAM) avalanche photodiode (APD), all grown lattice matched on InP substrates. Detection at room
temperature up to 2.4 μm can be readily achieved as can gains in excess of 40. InAs homojunction p-i-n diodes are
capable of detecting light with wavelengths > 3 μm, even at 77 K. Although controlling the surface leakage current is a
major challenge in mesa devices of InAs, gains in excess of 40 have also been obtained in these devices at room
temperature. InAs is also the only III-V semiconductor material that appears to show excess noise-free avalanche gain
when electrons are used to initiate the avalanche multiplication. We will discuss recent developments in these two
material systems to date and the current state of the technology.
The InAs/GaSb Type II strained layer superlattice (SLS) is promising III-V material system for infrared (IR) devices due to the ability to engineer its bandgap between 3-30 μm and potentially have many advantages over current technologies such as high uniformity smaller leakage current due to reduced Auger recombination which are crucial for large IR focal plane arrays. However, an issue with this material system is that it relies on growth on GaSb substrates. These substrates are significantly more expensive than silicon, used for HgCdTe detectors, lower quality and are only available commercially as 3" diameters. Moreover it has to go through thinning down before it could be hybridized to readout integrated circuits. GaAs substrate is a possible alternative. We report on growth and characterisation of Type-II InAs/GaSb SLS photodiodes grown on GaAs substrates for mid-wave infrared with peak responses of 3.5 μm at 77K and 4.1 μm at 295K. Comparisons with similar structure grown on GaSb substrates show similar structural, optical and electrical characteristics. Broadening of X-ray rocking curves were observed on the structure grown on GaAs substrate. A full width half maximum (FWMH) of 25.2 arc sec. for the superlattice was observed near ~30.4 degree for the structure on GaSb substrate compared to near ~30.4 degree for structure grown on GaAs. However peak responsivity values of ~ 1.9 A/W and ~ 0.7 A/W were measured at 77K and 295K for devices grown on GaAs substrate. Room temperature responsivity suggests that these photodiodes are promising as high temperature IR detectors.
We report on studies of avalanche multiplication in InAs APDs. A range of p-i-n and n-i-p photodiodes have been characterised with both avalanche multiplication and the accompanying excess noise being measured. By using a number of laser wavelengths the injection of optically generated carriers into the multiplication region has been varied, allowing the relative magnitude of the ionisation coefficients to be determined. The results of multiplication measurements show that, contradictory to the only other published experimental results for InAs, the electron ionisation coefficient is much greater than the hole ionisation coefficient. This large ionisation coefficient ratio should result in low excess multiplication noise for electron initiated gain, a prediction which has been confirmed by the measurement of multiplied photocurrent noise. The excess noise measured on InAs APDs was extremely low, unlike that reported for other wider bandgap III-Vs, and comparable with that measured on so called electron APDs fabricated from HgCdTe. These characteristics make InAs an interesting option for the fabrication of high sensitivity APD focal plane arrays in the III-V material system.
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.
We report measurements on a series of quantum dot infrared photodetectors grown with different combinations of
monolayer thicknesses (2.2. 2.55 and 2.9 ML) and quantum dot layer sheet doping densities (6×1010 cm-2 and 12×1010
cm-2). The dark current and noise current were higher in devices grown with sheet doping density of 12×1010 cm-2. At a
given bias voltage the dark current and the noise current was found to be lowest in devices having 2.55 ML and sheet
doping density of 6×1010 cm-2. This combination gives a sheet doping density to dot density ratio of approximately unity.
Highest gain was achieved in devices with 2.55 ML and sheet doping density of 6×1010 cm-2.
We report the findings of work undertaken to develop InAs photodiodes with low reverse leakage current, for detection
of mid-wave infrared wavelengths up to 3.5μm. Good quality epitaxial growth of InAs and the lattice matched ternary
AlAs0.16Sb0.84 was developed using molecular beam epitaxy. A photodiode structure was designed, grown and
characterized using an AlAs0.16Sb0.84 layer to block the diffusion of minority electrons. Further reductions in the reverse
leakage current were achieved through studies of wet etching using a range of etchants. A sulphuric acid based etchant
provided the lowest surface leakage current for a single etch step, however the surface leakage current was further
reduces when a two steps etching process was employed, starting with a phosphoric acid based etchant and finishing off
with a sulphuric acid based etchant. Surface profile analysis showed that higher etching rates were obtained in the
direction parallel to the <100> direction. The atomic composition of the etched surface was investigated using Auger
analysis. By etching a test pixel array, the potential for fabricating small pitch focal plane arrays by wet etching was
evaluated.
The avalanche multiplication noise characteristics of AlxGa1-xAs (x equals 0-0.8) have been measured in a wide range of PIN and NIP diodes. The study includes determining the effect of the alloy fraction, x, as it varies from 0 to 0.8 while the effect of the avalanche width, w, is investigated by varying it from 1 micrometers down to 0.05 micrometers . For x equals 0-0.6, the ratio of the electron to hole ionization coefficients, 1/k, decreases from 3 (for x equals 0) to 1 (for x equals 0.6), leading to higher noise in a local prediction as x increases. Measurements for x equals 0-0.6 in nominally 1um thick diodes indicates that the excess noise factor can be approximately predicted by the local model. However, as the avalanche width reduces, a lower than expected noise factor was measured. This behaviour is associated with the effect of deadspace, whereby carriers have insufficient energy to initiate ionization for a significant region of the device. The presence of deadspace leads to a more deterministic process, which acts to reduce excess noise. For x equals 0.8 however, its 1/k value is surprisingly high in a bulk structure, leading to noise performance that is primarily determined by the 1/k value and is comparable to that of silicon. Similar to the results of thin AlxGa1-xAs (x equals 0-0.6) diodes, thinner Al0.8Ga0.2As structures exhibit excess noise factor that is significantly reduced by the nonlocal deadspace effects.
Avalanche photodiodes with thin, sub-micron avalanching regions are found to give avalanche noise lower than predicted by conventional noise theory. Measurements of the excess noise on a range of sub-micron GaAs, InP and Si homojunction p-i-n diodes show that the noise decreases as the avalanching width decreases, even though the electron and hole ionization coefficients remain very similar at high electric fields. Simple Monte Carlo modeling of the ionization process suggests that this reduction is due to the increasing importance of the `dead-space', the minimum distance over which carriers need to travel in order to gain the ionization threshold energy. As this dead-space becomes more significant and the subsequent ionization coefficient increases, the ionization process becomes more deterministic and hence the avalanche noise decreases. Modeling also predicts that reductions in avalanche noise can be obtained in p-n junctions where the electric-field varies rapidly and this has now been observed experimentally.
We have measured avalanche multiplication and noise in Si p- i-n diodes with avalanche widths, w, of 0.12 micrometers , 0.18 micrometers and 0.32 micrometers , both for pure electron and mixed carrier injection. Multiplication and excess noise measurements were also performed with hole injection on a n+-i-p+ diode with w equals 0.84 micrometers . Pure electron initiated avalanche noise results were found to be almost indistinguishable in all three layers. The excess noise factor increases dramatically with increasing w when the injection is mixed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.