Ionization Rate Ratio Research Articles

Field and spatial geometry dependencies of the electron and hole ionization rates in GaAs/AlGaAs multiquantum well APD's

Numerical calculations are presented of the electron and hole ionization rates in GaAs/AlGaAs multiquantum-well APDs (avalanche photodiodes) as a function of the applied electric field and the spatial geometries, i.e., the barrier- and well-layer widths, respectively. The model is calibrated to existing experimental data on bulk GaAs materials and then extrapolated to the multiquantum well structure. It is found that at high electric field strengths the net ionization rate approaches the weighted average of the constituent bulk rates; the potential discontinuity is relatively insignificant. The potential discontinuity most greatly affects the electron ionization rate at low applied electric field strengths within a spatially symmetric structure. It is further determined that the electron-to-hole ionization rate ratio is greatest at low applied electric fields with a spatially symmetric structure with equal well and barrier widths.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ratios of photoelectron to EUV ionization rates for aeronomic studies

This study reveals that the ratios of the photoelectron to EUV ionization rates are not constant but depend on the degree of attenuation of the solar EUV flux and on the transport of photoelectrons. At high altitudes in the absence of photoelectron transport, the O+ and N2+ ionization rate ratios are about 0.35, but they increase with increasing optical depth to such an extent that in the vicinity of the ionization peak, photoelectron impact ionization is as important as photoionization for O+ and N2+. The O2+ ratio is about half that of O+ at high altitudes and also increases with increasing optical depth but reaches a peak of about 0.4. We present simple formulae which mimic the attenuation behavior of the ionization ratios. Transport effects become important above about 250 km where the ratios vary by a factor of 2 depending on the presence or absence of photoelectrons from the conjugate ionosphere. In addition to the photoelectron to EUV ionization ratios, we present photodissociative branching ratios for O2 and N2. These photodissociative ratios are also a function of the degree of attenuation of the EUV flux. In the region where attenuation is not important, the N+ to N2+ ratio is 0.14, and the O+ to O2+ ratio is 0.22. There is a factor of 2 uncertainty in our calculated ratios on account of uncertainties in the solar EUV flux spectrum and also uncertainties in the electron impact cross sections.

MOVPE grown MQW pin diodes for electro-optic modulators and photodiodes with enhanced electron ionisation coefficient

Atmospheric pressure MOVPE has been used to prepare AlGaAs/GaAs MQW pin diodes exhibiting well-resolved room temperature exciton resonances and large reverse field breakdowns. The MQW i regions have been optimised by applying growth conditions which minimised the residual carbon concentration associated with the reagent TMA. Diodes with MQWs of well width 40 to 160 Å have been assessed for the quantum confined Stark effect (QCSE) by photocurrent spectra and as absorption modulators. A 75 period 50 Å Al0.3Ga0.7As/60 Å GaAs i region gave a transmission change of 20% at 828 nm for 16 V applied bias. Diodes have also characterised for hole and electron ionisation rates when biased as avalanche detectors. An electron-to-hole ionisation rate ratio enhancement of 14 was observed for a 45 period 58 Å Al0.3Ga0.7As/105 Å GaAs MQW.

The p-n heterojunction quantum well APD: A new high-gain low-noise high speed photodetector suitable for lightwave communications and digital applications

A new Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> As/Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</inf> In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</inf> As multiquantum well avalanche photodiode, the APD, is presented that provides comparable signal-to-noise performance compared to either the doped quantum well APD or the p-n junction quantum well APD, but without carrier trapping effects even at very low overall applied electric fields. The device is made of repeated unit cells consisting of a p-n junction formed between two dissimilar materials followed by a nearly intrinsic wide-bandgap layer. As in the doped quantum well device, the asymmetric unit cell selectively heats the electron distribution much more than the hole distribution within the narrow-gap Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> As layer leading to a greatly enhanced electron-to-hole ionization rates ratio. The most significant improvement over the doped and p-n junction quantum well devices is the lack of carrier trapping at the heterojunction without further engineering of the interface (compositional grading). Carrier trapping is avoided, thereby providing very high-speed performance even for low-voltage devices, by doping the narrow-gap layer. The resulting built-in field within the GaInAs layer is sufficiently large of itself that both electrons and holes are heated to energies large enough to overcome the potential barrier at the end of the quantum well. In this way, devices operating at 5 V bias can be built that will provide a gain of about 4 at large bandwidths, ~18 GHz.

Near-single carrier-type multiplication in a multiple graded-well structure for a solid-state photomultiplier

We report the observation of very large ionization rate ratios (β/α) in multiple graded-well Al 0.48 In 0.52 As/Ga 0.47 In 0.53 As avalanche photodiodes (APD's) grown by molecular beam epitaxy (MBE). The multiplication effects can be explained by impact ionization across the valance band-edge discontinuity of thermally generated holes which are dynamically stored in the wells. Since electrons are not confined in the graded structure, there is no multiplication of electrons by this process. This is the first observation of near-single carrier-type multiplication in a III-V semiconductor material.

Generation function in uniformly multiplying avalanche diode

A simple method of finding generating function in uniformly multiplying avalanche diodes has been presented. Explicit expression for such functions has been obtained for some useful values of the ionization rate ratios. The application of the result in finding the pdf of a filtered avalanche process has been discussed.

Properties of GaAs/Al0.53Ga0.47As Avalanche Photodiode with Superlattice Fabricated by Molecular Beam Epitaxy

A GaAs/AlGaAs APD with superlattice, exhibiting low dark current and large photocurrent multiplication characteristics, has been successfully fabricated by MBE. Dark currents were as low as 2.1×10-6 A/cm2 for a multiplication factor of 30. A study of the influence of oval defects on APD characteristics showed that no one-to-one correlation was found between oval defects and fatal diode deterioration. The ionization rate ratio, β/α, estimated from multiplication noise measurement, was less than 0.6–0.8. MBE is a useful technique for fabricating high performance APDs.

Gain-bandwidth-limited response in long-wavelength avalanche photodiodes

The gain-bandwidth(GB)-limited response of In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</inf> As/ InP heterostructure avalanche photodiodes (APD's) and related devices used in long-wavelength digital optical receivers is calculated. We find that these diodes, as currently designed, are useful at bit rates <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B \lsim 2</tex> Gbit/s when employed in conjunction with high-sensitivity optical receivers. Response at higher bit rates may be obtained depending on the details of device design. On the other hand, use of poor-quality receivers that require moderate-to-high values of optimum gain can significantly degrade the performance of heterostructure APD's at high bit rates due to GB limitations. We also show that APD receiver bandwidth can be expressed in terms of the sensitivity obtained using the receiver in conjunction with a p-i-n photodiode. It is found that the response speed of optimized receivers is lowest for an APD effective ionization rate ratio of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k = 0.5</tex> .

New device applications of bandedge discontinuities in multilayer heterojunction structures

Recent work on new device applications of band edge discontinuities in molecular beam epitaxial multilayer heterostructures is discussed. Band discontinuties can be used to control the “real space” transfer of electrons between sets of layers (negative resistance, switching and storage devices). Another application deals with the possibility of enhancing the ionization rates ratio via the asymmetry between conduction and valence band discontinuities (superlattice photodetectors). Finally a multistage graded gap structure, the solid-state analog of a photomultiplier (staircase APD), has been disclosed.

Band-gap engineering via graded gap, superlattice, and periodic doping structures: Applications to novel photodetectors and other devices

Recent new multilayer structures and their device applications are reviewed. These new concepts allow one to radically modify the conventional energy band diagram of a pn junction and thus tailor the high field transport properties to an unprecedented degree (band-gap engineering). This approach has been used to propose and implement a new class of avalanche photodiodes with enhanced ionization rates ratio and the solid state analog of a photomultiplier (staircase detector). Other device applications such as repeated velocity overshoot structures are also discussed.

Staircase solid-state photomultipliers and avalanche photodiodes with enhanced ionization rates ratio

The theory of the staircase avalanche photodiode (APD) is presented and recent results on a new class of APD's with enhanced ratio of ionization coefficients are reviewed. The staircase APD consists of a multistage graded gap structure where only electrons ionize; the entire ionization energy is provided by large conduction band steps (dynodes). A general expression for the excess noise factor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</tex> in terms of the number of stages and the multiplication per stage is presented. For high ionization yields per dynode the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</tex> factor is near unity independently of the number of stages, implying virtually noise free multiplication at high gain similar to a photomultiplier. This cannot be achieved in a conventional APD at high gain even if one of the ionization coefficients is zero. A comparison between the noise behavior of the staircase APD and that of a phototube is also presented. A microscopic theory of the ionization yield γ is discussed; to obtain a high γ electrons must approach the dynode with an energy in the order of ten times the optical phonon energy. The possible problem of residual hole-initiated ionization is also discussed. Formulas for the electron and hole initiated multiplications are derived; from a measurement of these quantities one can directly obtain the ionization yield and the residual hole ionization coefficient. Experimental and theoretical results on other structures (superlattice, channeling, graded gap APD's) with high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha/\beta</tex> ratio are also reviewed and design considerations for a long-wavelength multilayer APD are presented.

Avalanche Photodiodes with Enhanced Ionization Rates Ratio: Towards a Solid State Photomultiplier

Recent results on a new class of low excess noise avalanche photodiodes (APDs) are discussed. A significant enhancement of the ionization rates ratio has been demonstrated in AlGaAs/GaAs superlattice and graded gap APDs. In addition two novel APDs where only electrons ionize (channeling and staircase APDs) have been disclosed. The staircase APD has lower bias voltage (5-10V) than conventional APDs and virtually noise free multiplication similarly to a phototube.

IIB-9 Multiplication Noise in (111)InP Avalanche Photodiodes

The effective hole to electron ionization rate ratio (k) of ~3 has been obtained in (111)InP, which is greater than that of (100)InP.

A new method to control impact ionization rate ratio by spatial separation of avalanching carriers in multilayered heterostructures

We propose a new scheme to control the impact ionization rate ratio α/β of electrons and holes by utilizing multilayered heterostructures (–A–B–A–B–). In the structure, avalanching carriers, which accelerated mainly along the layer plane, are spatially separated by the action of band edge discontinuities in such a way that the impact ionization process of electrons takes place mainly in layer A and that of holes in layer B. An analysis is made to show that a proper choice of constituent materials allows the control of α/β over an extremely wide range (10−2∼103).

Multiplication noise in planar InP/InGaAsP heterostructure avalanche photodiodes

Planar InP/InGaAsP avalanche photodiodes have been fabricated and multiplication noise is discussed. The effective hole to electron ionization rate ratio is found to be 1.9 from the wavelength dependence of multiplication noise.

Enhancement of electron impact ionization in a superlattice: A new avalanche photodiode with a large ionization rate ratio

The first superlattice avalanche photodiode (APD) is reported. The high field region of this p-i-n structure consists of 50 alternating Al0.45Ga0.55As (550 Å) and GaAs (450 Å) layers. A large ionization rate ratio has been measured in the field range (2.1–2.7)×105 V/cm, with α/β≃10 at a gain of 10 giving a McIntyre noise factor Fn = 3. The ionization rate ratio enhancement with respect to bulk GaAs and AlGaAs is attributed to the large difference in the band edge discontinuities for electrons and holes at the heterojunction interfaces. The superlattice APD is a new device concept which can be used to develop low noise APD’s in a variety of III-V materials including long wavelength 1.3–1.6-μm semiconductors.

Excess-noise and receiver sensitivity measurements of In 0.53 Ga 0.47 As/InP avalanche photodiodes

We report the results of excess-noise and normalised receiver sensitivity measurements for In0.53Ga0.47As/InP avalanche photodiodes for use in the λ=0.95 μm to 1.65 μm spectral region. The excess-noise measurements are consistent with a hole-to-electron ionisation rate ratio in InP of ∼ 0.4. The receiver sensitivity, measured at 45 Mbit/s and 10−9 bit-error-rate, was −53.2 dBm at a gain of 20 assuming unity quantum efficiency for the detector. This sensitivity is the highest reported at λ=1.3 μm and represents an improvement over a PIN detector using the same amplifier.

Multiplication noise of InP avalanche photodiodes

An InP avalanche photodiode with a guard ring structure is fabricated. The maximum avalanche gain obtained is 210 at a primary photocurrent of 0.2 μA, and uniform photoresponse without local enlargement is obtained at an avalanche gain of 10. Multiplication noise characteristics are investigated, and the effective ionization rate ratio of holes to electrons is found to be 1.7–1.8.

Temperature-dependent characteristics of a reach-through avalanche photodiode (RAPD) in Ge, Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs reach-through avalanche photodiodes (RAPD). The temperature coefficient of avalanche breakdown voltage in a depletion region is studied. The response time of a reach-through APD in these materials is also discussed. Finally a comparison of the characteristics between PIN APD and RAPD is presented. The theoretical data have also been substantiated experimentally by Kanedaet al.

Temperature dependent characteristics of a PIN avalanche photodiode (APD) in Ge, Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs PIN avalanche photodiodes. The temperature coefficient of avalanche breakdown voltage in a high field region is studied. Finally the response time of a PIN APD in these materials is discussed.