Avalanche photodiodes are semiconductor-based devices that act as photodetectors, converting light to electricity, with a built-in first stage of gain which increases the sensitivity over other types of semiconductor photodiodes. In operation, a reverse voltage may be applied to an avalanche photodiode and, generally, the higher the reverse voltage applied, the higher the gain.
Avalanche photodiodes have conventionally been fabricated with a multiplication region (which provides the gain) comprising silicon, however other materials such as Germanium, Gallium Nitride (GaN) or Indium Phosphide (InP) have also been used. The material used can influence the wavelength of radiation to which the avalanche photodiode is sensitive. For example silicon may be used for avalanche photodiodes to detect visible and near infrared radiation and GaN may be used for photodiodes to detect ultraviolet radiation. The thickness of the avalanche layer affects the ratio of ionization coefficients. It may be noted that in silicon, the electron ionization coefficient is much larger than the hole ionization coefficient at low electric fields.
Avalanche photodiodes are manufactured by growing or depositing materials on a substrate. Semiconductor processing techniques such as photolithography and electron beam lithography, large scale nano-imprint, electron beam and sputterer metal deposition, inductively coupled plasma etching, reactive ion etching, focused ion beam milling, rapid thermal annealing, dielectric deposition (Plasma-enhanced chemical vapor deposition (PECVD), Atomic layer deposition (ALD), evaporation) may be used to form the structure of the avalanche photodiode.
Avalanche photodiodes have many applications, for example in free-space optics (FSO), fibre-optic communications, laser rangefinders based on Light Detection and Ranging (LIDAR) and other sensing applications. In optical communications such as FSO or fibre-optic communications, an avalanche photodiode may act as an optical receiver. Avalanche photodiodes are particularly effective for use in such communications due to their 5-10 dB higher sensitivity relative to conventional photodetectors without internal gain. Many existing photodiodes for use in such applications operate at wavelengths of 1.3 μm or 1.55 μm.
Examples of known avalanche photodiodes are grown on InP substrates and use InP or InAlAs as the avalanche gain medium, or multiplication layer. However InP and InAlAs have higher noise performance and lower sensitivity compared to silicon avalanche photodiodes. Other avalanche photodiodes exist which have a better noise performance, such as InAs and AlInAsSb, however they are grown on a more expensive and low resistivity InAs or GaSb substrate.
According to a first aspect of the invention, there is provided an avalanche photodiode comprising a substrate and a structure. The structure comprises a first layer and a second layer, the first and second layers being arranged over and parallel to the substrate, wherein the first layer is between the substrate and the second layer (and in some examples is in contact with the substrate and/or the second layer). The first layer is an Aluminium Arsenide Antimonide (AlAsSb) multiplication layer, and the cross-sectional area parallel to the substrate of the first layer is smaller than that of the second layer, such that there is a recess in a sidewall of the structure. The relatively small cross-sectional area of AlAsSb layer provides confinement of the electric field. Furthermore, in use, dark current (which is proportional to the cross-sectional area of the layer) is reduced because of the reduced cross-sectional area of the AlAsSb layer. AlAsSb can also provide improved performance over silicon based avalanche photodiodes, with significantly reduced noise. Additionally silicon is unable to detect radiation at visible wavelengths, whereas AlAsSb (for example, when provided within a structure including an absorption layer such as an InGaAs absorption layer), is capable of detecting visible light.
In some examples the structure comprises “steep-sided” sidewall(s) above the recess, wherein the sidewall(s) may be steep-sided in that their angle is closer to vertical than to the plane of the substrate, and in some examples the sidewalls may be near vertical (e.g. up to around 10° from vertical, or up to around 5° from vertical) or vertical. As used herein ‘vertical’ is used to mean perpendicular relative to the substrate. Such steep-sided (in some examples, vertical or near vertical) sidewalls may be defined using photolithography and formed using conventional semiconductor processing techniques such as plasma etching.
In some examples the steep-sided sidewall(s) of the structure and the recess are coated with a dielectric material. Such layers are known as passivation layers and can include materials such as Al2O3, SiO2, SiNx, benzocyclobutene (BCB) and SU-8. Passivation layers such as Al2O3 may be deposited by atomic layer deposition. The passivation layer may improve mechanical stability of the structure, preserve the surface states leading to improved reliability of the photodiode, and/or reduce leakage currents.
In some examples the avalanche photodiode has maximum responsivity at a wavelength of 1.3 μm or 1.55 μm. These wavelengths are typically used in existing optical networks, so providing an avalanche photodiode with high responsivity at these wavelengths allows compatibility with existing infrastructure. The device structure and materials may be selected to ensure responsivity at these wavelengths, for example a PIPIN structure (described in greater detail below) allows such operation.
In some examples the thickness of the first layer is 0.6 μm to 1.5 μm. The thickness of the first layer contributes towards determining the operational properties of the avalanche photodiode. Such a relatively thick AlAsSb layer provides improved performance relative to silicon based photodiodes.
In some examples the structure is a PI PIN structure. A PIPIN structure allows use of materials which can absorb radiation with wavelengths of 1.3 μm and 1.55 μm which allows for interoperability with existing infrastructure, for example with existing telecom infrastructure.
In some examples the PIPIN structure comprises layers, in the following order: a P-type contact layer at the top of the structure; an intrinsic absorption layer; an intrinsic grading layer; a P-type charge layer; the first layer; and an N-type contact layer, adjacent to the substrate. The second layer is at least one of the P-type contact layer, the intrinsic absorption layer, the intrinsic grading layer or the P-type charge layer.
In some examples the avalanche photodiode is a top illuminated photodiode. When the avalanche photodiode is top illuminated, the entire backside of the substrate may form a contact, which is therefore easier to align as no backside patterning is required. A top illuminated avalanche photodiode may have a reduced cost of manufacture relative to a bottom illuminated avalanche photodiode due to this ease of alignment.
In some examples the avalanche photodiode further comprises at least one of a top and bottom contact. In some examples there may be a top contact and no bottom contact. A bottom contact may be mounted on the substrate and a top contact may be mounted on a layer distal to the substrate.
In some examples the first layer is lattice matched to the substrate, which may be an Indium Phosphide (InP) substrate. Lattice matching when growing a layer of semiconductor with different bandgap on InP avoids introducing a change in the crystal structure. If an avalanche photodiode was constructed without lattice matching, then there could be strain and/or defects at the interface and the resulting device performance may be compromised.
In some examples the first layer is an AlAs0.56Sb0.44 layer. AlAs0.56Sb0.44 provides has a relatively large indirect band-gap of around 1.65 eV and is lattice matched to InP.
In some examples the second layer comprises an InGaAs absorption layer, which may be lattice matched to the substrate.
According to a second aspect of the invention there is provided a method of fabricating an avalanche photodiode. The method comprises depositing a first layer comprising Aluminium Arsenide Antimonide over a substrate; depositing a second layer over the first layer; and removing a portion of the first layer so that the first layer has a smaller cross-sectional area than the second layer in a plane parallel to the substrate. Removing a portion of the first layer reduces the dark current because dark current is proportional to the cross-sectional area. Furthermore the relatively wide second layer provides for easier alignment and manufacture because any features which are to be defined on top of the structure have a larger target area. Furthermore resistance is inversely proportional to the cross-sectional area, so the resistance of the wider second layer is reduced by its relatively large cross-sectional area.
AlAsSb layers have been suggested for use in laser devices. However due to the relatively small thickness of such a layer in a laser device, the process would be difficult to control and the conditions required for the process may not be commercially viable due to the high temperature (500° C.) and long processing time required.
In some examples removing a portion of the first layer comprises performing a wet oxidation process on the first and second layers. AlAsSb has a relatively high oxidation rate. Therefore it will oxidise to a greater extent than the other layers forming a thicker oxide. The oxidation process thereby causes a reduction in the cross-sectional area of the first layer.
In some examples removing a portion of the first layer further comprises removing the oxide formed by the wet oxidation process. Removing the oxide forms a recess in the first layer, so that the second layer overhangs the first layer, providing the above advantages.
In some examples, prior to removing a portion of the first layer, an etch process is performed on the first layer and the second layer to form a structure comprising the first layer and the second layer with (at least substantially, or nearly) vertical sidewalls.
In some examples a dielectric material is deposited on the sides of the first layer and second layer. The dielectric material acts as a passivation layer, provides additional mechanical support to the structure and/or can also improve the electrical properties of the photodiode for example by reducing leakage currents and improving reliability.
In some examples the first layer is deposited using a digital alloy process. Digital alloy deposition comprises sequentially depositing alternating thin layers of the metals of the alloy. This allows precise lattice-matched alloys to be grown with high crystalline quality.
Non-limiting examples will now be described with reference to the accompanying drawings, in which:
The structure 204 comprises a first layer 206 and a second layer 208, both of which are over and parallel to a major plane of the substrate 202. The first layer 206 is arranged between the substrate 202 and the second layer 208.
The first layer 206 is an Aluminium Arsenide Antimonide (AlAsSb) multiplication layer, also known as an avalanche layer. AlAsSb may be lattice matched to substrates such as InP substrates. AlAsSb also has high temperature stability compared with some other semiconductor materials such as InAlAs, InP or GaAs.
The cross-sectional area parallel to the substrate 202 of the first layer 206 is smaller than that of the second layer 208, thereby forming a recess 210 in a sidewall of the structure 204. The recess may be at least 5 μm deep, and can be up to tens of microns deep. The relatively small cross-section of the first layer 206 provides a reduction in size of the high field area but without influencing the absorption or contact layers, which are described in greater detail below. The first layer may have a reduced width, wherein the width may measure from 10 μm up to hundreds of microns. This results in reduced dark current relative to a first layer 206 with a larger cross-sectional area because dark current is proportional to the cross-sectional area. Such photodiodes may have a low excess noise factor of ˜2 and may have a bandwidth of around ≥7 Ghz at an avalanche gain of 50, and so would be capable of operating in 2.5 Gps and 10 Gps FSO links. Examples of the formation of the recess in the first layer 206 are described in more detail below.
The first layer 206 is a multiplication layer and may have a thickness in the range from 0.6 μm to 1.5 μm to provide the bulk property. A thicker first layer 206 provides lower noise, however a thinner first layer 206 provides increased speed of operation. Therefore the thickness of the layer may vary depending on the intended application of the avalanche photodiode to balance the noise and speed requirements for that application. For example, high speed telecommunications, such as 5G, call for high speed operation so a thickness of 0.6 μm may be preferred whereas for applications such as LIDAR, low noise is highly desirable and lower speed of operation may be acceptable so a larger thickness of 1.5 μm may be preferred. The second layer 208 is an intrinsic absorption layer and may have a thickness up to 2 μm, the intrinsic grading layer 214 may have a thickness in the range from 0.1 μm to 0.2 μm, and the P-type charge layer 216 may have a thickness in the range from 0.1 μm to 0.2 μm.
The second layer 208 is the absorption layer and comprises InGaAs lattice matched to the substrate. The intrinsic grading layer 214 may comprise AlGaAs and the P-type charge layer 216 may comprise InAlAs.
As shown in
The thickness of each layer is selected to achieve a photodiode which operates efficiently at preferred wavelengths. In one example the thickness of the first layer is 1500 nm and the thickness of the second layer is 1000 nm. Responsivity is a measure of the input-output gain of a photodiode, in particular a measure of the electrical output per optical input. Preferably a photodiode will have maximum responsivity at a wavelength at which it is designed to operate. Many existing optical systems operate with wavelengths of 1.3 μm or 1.55 μm. Therefore in some example embodiments, the avalanche photodiode may have a maximum responsivity at a wavelength of 1.3 μm or 1.55 μm.
The avalanche photodiode is a top illuminated photodiode. This means that the photodiode is sensitive to optical radiation entering the top of the structure i.e. the surface of the structure furthest from the substrate. Such a photodiode may comprise second contacts 220b on the upper surface of the substrate 202 as shown in
In the avalanche photodiode, the first layer 206 comprises AlAsSb. In particular, in this example the AlAsSb layer is AlAs0.56Sb0.44, which is lattice matched to the substrate 202 which is an Indium Phosphide (InP) substrate. AlAs0.56Sb0.44 provides low tunnelling current with a thin avalanche region width of 80 nm and has a relatively large indirect band gap of ˜1.65 eV. Furthermore it has a relatively high temperature stability. Therefore use of AlAs0.56Sb0.44, provides a photodiode with very low excess noise.
The above methods are described with greater detail with respect to
After formation of the structure shown in
While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.
The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and features of the dependent claims may be combined in any practical combination.