Quantum-dot-based avalanche photodiodes on silicon

A quantum-dot based avalanche photodiode (QD-APD) may include a silicon substrate and a waveguide on which a quantum dot (QD) stack of layers is formed having a QD light absorption layer, a charge multiplication layer (CML), and spacer layers. The QD stack may be formed within a p-n junction. The waveguide may include a mode converter to facilitate optical coupling and light transfer from the waveguide to the QD light absorption layer. The QD absorption layer and the CML layer may be combined or separate layers. The CML may generate electrical current from the absorbed light with more than 100% quantum efficiency when the p-n junction is reverse-biased.

BACKGROUND

Optical systems may be used to manipulate optical signals in various ways. For example, photodetectors may absorb an optical signal and convert it into an electrical current. As another example, laser diodes may be used to generate lasers by applying a voltage across the diode's p-n junction to make it forward-biased.

DETAILED DESCRIPTION

The terminology used herein is for the purpose of describing particular examples only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “plurality,” as used herein, is defined as two or more than two. The term “another,” as used herein, is defined as at least a second or more. The term “coupled,” as used herein, is defined as connected, whether directly without any intervening elements or indirectly with at least one intervening elements, unless otherwise indicated. Two elements may be coupled mechanically, electrically, or communicatively linked through a communication channel, pathway, network, or system. The term “and/or” as used herein refers to and encompasses any and all possible combinations of the associated listed items. It will also be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, these elements should not be limited by these terms, as these terms are only used to distinguish one element from another unless stated otherwise or the context indicates otherwise. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. The terms “about” and “approximately”, used interchangeably, mean up to 5% variation (plus or minus) from a given quantitative value following these terms. The term “adjacent,” when applied to two components, regions, or layers, means no other components, regions, or layers, respectively, are physically interposed between the two components.

Optical systems generally Include at least two types of components, those that generate or emit light, such as lasers, and those that detect light, such as photodetectors. Depending on the functions of the optical systems, the light transmitted within the system may represent a signal with certain predefined semantics, or represent optically encoded data. An example of light as a signal is an optical alarm system which may detect light or lack thereof and interpret the signal as an alarm that a restricted area has been breached. Examples of light as encoded data include fiber optic systems used in high-performance computer systems and wide-area or local-area optical networking, which use optical media or links to carry encoded digital data from one source computer or storage to another one. When light is used to carry data, the integrity of the data depends on the quality and sensitivity of light detection because unreliable detection of optical data may result in unreliable data (for example, a false positive or a false negative mistaking binary 1 for binary 0 or vice versa). Accordingly, a highly sensitive light detector may reduce data error, reduce power consumption, increase data density and bandwidth, and provides other benefits as further described below. A QD-APD device may be used to solve some or all of these problems.

Examples disclosed herein describe a QD-APD as a highly sensitive optical detector that may be built on a silicon waveguide to create a highly sensitive light detector. This may be done by creating or forming multiple layers on the passive silicon waveguide including one or more quantum dot (QD) light absorption layers and zero or more charge multiplication layers (CML), alternating with separator layers. In some example implementations, the QD light absorption and CML layers may be combined into one layer. A QD-APD includes three distinct sections along its length. One section is a silicon waveguide section, a second section is a mode conversion transition section, and a third section is a QD section in which the QD/CML is located. Light first enters the silicon waveguide section, then crosses the mode conversion transition section in which the silicon waveguide tapers down and becomes narrower, and the light absorption layer starts small and narrow and then gradually widens and becomes wider. At this point, in the QD section, the avalanche effect takes place.

The QD/CML layers may be reverse-biased to create an avalanche mode of operation with greater than 100% internal quantum efficiency. Internal quantum efficiency excludes the efficiency of the mode converter, described later in detail. In such configuration, one photon creates an electron-hole pair in the absorption layer. Hence, the output from one photon entering the QD-APD is greater than one charged particle (electron or hole). This allows a small amount of light to create a large and usable electrical current (signal) for electronic processing by a circuit or a computer.

Some example implementations described herein disclose a QD-APD operating in avalanche mode with gains resulting in internal quantum efficiencies greater than 100%, as further described below. This type of high-efficiency photodetector may detect minute amounts of light, compared with other implementations.

In some example implementations, the QD-APD includes several layers of QDs separated by spacer layers of material such as Gallium Arsenide (GaAs), and separate CML layers deposited near P-cladding in some examples, or N-cladding in other examples.

There are several advantages provided by such high-efficiency photodetection. One advantage is the output power of a laser can be reduced, thus increasing the efficiency of the optical link.

Another advantage of QD-APDs is that they have inherent gain due to their avalanche mode behavior. Hence, a QD light absorption layer may act as an electrical signal amplifier.

Still another advantage of QD-APDs and a solution to some manufacturing complexity problems is that the same material stack as a diode laser may be used to manufacture the QD-APD. The same design and manufacturing techniques may be used to manufacture both the QD-APDs and QD comb laser and ring laser devices resulting in high-performance photodetectors and lasers. More specifically, simple integration of lasers and photodetectors manufactured this way, using a single bonding step with no selective area bonding, and no high temperature growth steps is possible. Hence, in the same manufacturing process, different devices may be manufactured less expensively that may be integrated more easily in various applications.

Another advantage of QD-APDs integrated on silicon described herein is efficient coupling between passive silicon waveguides, which reduces cost and complexity of systems and system data interfaces.

Still another advantage of QD-APDs is lower dark current than bulk or quantum well photodetectors. Dark current is the random charges, or electric current, generated inside the photodetector without any input light. It is due to random charge generation and recombination of electrons and holes. Dark current is generally not desirable because it distorts the output current or signal of the photodetector and may also result in a false output signal. Dark current in a QD-APD is less sensitive to etching defects and high temperatures in usage.

Turning now to the drawings,FIG. 1shows an example dense wavelength division multiplexing (DWDM) optical system100. The DWDM system100is an example optical system that includes a laser light generator or emitter and also light detectors, both of which together create an optical data transmission system. The DWDM optical system100includes a transmitter101having a laser device102coupled with an optical fiber103and a plurality of modulators104. The optical fiber103is further coupled with a receiver105having a plurality of demultiplexing rings106. The demultiplexing rings106are in turn coupled with a plurality of QD-APDs107.

In some example implementations, the laser device102may be a comb laser generating multiple carrier light beams each at a different frequency, or equivalently, a different wavelength. Data may be modulated onto the carrier light beams via the modulators104. Each of the modulators104modulates a different data stream and encodes it onto a different carrier beam. Those skilled in the art will appreciate that a data stream is a series of related data belonging to one data set and serially following each other from a source to a destination. This way, the same optical media, for example, the optical fiber103, may carry multiple data streams simultaneously without interfering with each other. For short distances, for example on the order of a few inches, the media used may not be a solid or even a material medium but air or a vacuum.

The receiver105may receive the modulated carrier beams and then demultiplex them via the demultiplexing rings106, each demultiplexing ring106being tuned to a particular frequency of carrier beam. Thus, each demultiplexing ring106in effect filters and separates the carrier to which it is tuned from the multiple streams multiplexed together in the optical fiber103. The demultiplexing rings106are further coupled with QD-APDs107that receive a particular frequency of light and convert them, along with the data modulated onto the carrier. This process is further described with respect toFIGS. 4-7below.

FIGS. 2A-2Gshow examples of structural and layer details of a QD-APD200that may be included in the DWDM system100ofFIG. 1. Accordingly, the QD-APD200may be the same as or similar to one or more of the QD-APDs107illustrated inFIG. 1. In some example implementations, with reference toFIG. 2A, QD-APD200includes a passive waveguide201that extends to a mode converter202position or location within the QD-APD structure, as further defined below, and narrows down to a narrower waveguide204, overlaid with a QD stack203.

In some example implementations, the passive waveguide201may be made of silicon, which may also function as a substrate for other layers, as further described below. The passive waveguide201tapers down in the proximity of the QD stack203widening. The taper angles with respect to Y-axis may be between about 1° and about 90°. This proximity where the dual tapers take place forms the mode converter202in which the light is directed to the QD stack203from the passive waveguide201, as further described with respect toFIGS. 2B-2Cbelow.FIG. 2Ais shown in the X-Y plane as indicated by the X-Y-Z reference frame. The QD-APD200includes three distinct areas or sections along the Y-axis, including a first section having the passive waveguide201(on left ofFIG. 2A) at full width, which takes light as its input and carries the light to the mode converter202. The second section, the mode converter202, is defined by a narrowing of the passive waveguide201, which creates a narrowed waveguide section, and the starting and widening of the QD stack203, which creates a widened QD stack section. So, the mode converter202includes portions of the passive waveguide201having a tapered region leading to a narrow section204of the waveguide, and portions of the QD stack203having a sloped region (or having a reverse taper—a taper in the opposite direction—with respect to the tapered region of the waveguide201) leading to a widened section of the QD stack203, as shown inFIG. 2A. The third section includes the narrowed waveguide204and full width, without taper, QD stack203. The cross sections of these three areas, in X-Z plane, are shown inFIGS. 2B-2Gthat follow.

With continued reference toFIG. 2A, the QD-APD200disclosed herein includes a QD light absorber material. The QD light absorber material may be or may Include various III-V semiconductors (with reference to Groups III-V of the periodic table of elements), for example, Indium Arsenide (InAs), GaAs, Indium Phosphorus (InP), and the like, and may be bonded on top of the passive waveguide201, as further described in more detail below. Light may be coupled from the passive waveguide201to enter the QD stack203using the mode converter202.

Light absorption in the QD stack203, further described below, and charge amplification may occur in the same epitaxial layer, in some example implementations, and may happen in different epitaxial layers in other example implementations. As described later in more detail with reference toFIGS. 2B-2G, the QD stack203is a layered light absorption region, which may include two or three distinct light absorption layers including a QD layer, a CML layer, and an spacer layer separating the other layers. In some example implementations the QD layer and the CML layer may be combined into a QD-CML layer. Multiple such light absorption layers may be formed in the QD stack203. In some example implementations, the charge amplification may be dominated by electrons. In this case, a CML may be formed separately and distinctly from the QD layer adjacent to a P-cladding206as shown and described later with respect toFIG. 2E. The CML may be made of AlxGa1-xAs (0<x<1) or other equivalent material compositions. In some implementations, the QD light absorption layers themselves may be or may include the CML. A typical epitaxial stack may contain between 1 and 10 QD light absorption layers. In other example implementations, more than 10 QD light absorption layers may be formed.

According to one or more implementations,FIG. 2Bshows an example cross-section in X-Z plane at the first section of the QD-APD200, as shown inFIG. 2A. The layers shown include a substrate219having a lower silicon layer215at the bottom, a buried oxide (BOX) layer216in the middle, and an upper silicon layer217at the top, arranged with respect to the X-Z reference frame shown. The passive waveguide201is created on top of the silicon substrate219. The number and arrangement of these components is an example only and provided for purposes of illustration. Other arrangements and numbers of components may be utilized without departing from the examples of the present disclosure. A light mode profile208shows a spatial light distribution of a certain size at this cross-section, which is restricted to the silicon waveguide. The light mode profile changes as the QD-APD200is traversed along the Y-axis.

Continuing on to the second cross-section of the three QD-APD200sections, according to one or more implementations,FIG. 2Cshows a cross-section at the mode converter202ofFIG. 2A. The substrate219cross-section is as described with respect toFIG. 2Babove. The passive waveguide201is layered on top of the silicon substrate219. The width of the passive wavegulde201, along the X-axis, remains the same as the first section before the taper starts. The next layer on top of the passive waveguide201is an N-cladding218. Next, the QD stack203is added on top of the N-cladding218. The structure of the QD stack203is further described below with respect toFIGS. 2E and 2F. A P-cladding206is the next layer on top of the QD stack203. A metal electrical contact layer205is added on top of the P-cladding206to inject current into the QD-APD structure. A mode profile209of the light in this section is different from the mode profile208of the first section. In this mode profile, a light outflow211is spatially expanded into the absorption region along Z-axis defined and occupied by the QD stack203. The light outflow211thus entering the QD stack203is the input to the p-n junction that when reverse-biased causes the generation of electric charges, namely, electrons and holes, forming an electrical current that may be further detected and processed by electronic circuits and computers.

With reference toFIGS. 2A to 2C, the width of passive waveguide201included in the QD-APD200along the X-axis ofFIGS. 2B and 2Cmay range from about 300 nm to about 2 μm and the thickness of passive waveguide201along the Z-axis may range from about 200 nm to about 500 nm. The width of the QD stack203, except in the sloped or tapered region, may range from about 1 μm to about 10 μm and the thickness of the QD stack203may range from about 100 nm to about 500 nm. The width of the QD stack203along the X-axis may generaly be greater than the width of passive waveguide201at any of its varying widths.

Now, with reference toFIG. 2D, a cross-section of the third section of the QD-APD200is the same asFIG. 2C, except for the width of the passive waveguide201, which is now reduced, as shown inFIG. 2Aalso. Accordingly, a mode profile210of the light is also changed to a spatially more expanded form and further inside the QD stack203area.

FIGS. 2E-2Gshow some example implementations of the QD stack203in the QD-APD200ofFIG. 2D.FIG. 2Eshows one example implementation of the QD stack203. In this example implementation, a CML layer256is created immediately on top of or adjacent to the N-cladding218, where no other layer is interposed between the CML layer256and the N-cladding218. The spacer layers254, made of GaAs in some implementations, separate QD light absorption layers255from other layers, as shown. The QD light absorption layers255absorb light and start the charge generation process as described below with respect toFIGS. 3A-3C. The CML layer256multiplies the charge in avalanche mode as described below with respect toFIGS. 3D-3F. The QD light absorption layers255may include an absorption region to absorb photons from the received optical signal. In some implementations, the entire QD light absorption layer255may be the absorption region, whereas in other implementations a portion of the QD light absorption layer255may make up the absorption region.

FIG. 2Fshows another example implementation of the QD stack203. In this example implementation, the QD light absorption layers255and the CML layer256(shown separately inFIG. 2E) are combined together into QD-CML layers257. The spacer layers254separate the QD-CML layers257.

FIG. 2Gshows another example implementation of the QD stack203. In this example implementation, the QD light absorption layers255are separated from each other by the spacer layers254, and the CML layer256is formed next to or adjacent to the P-cladding206, as shown in the figure, where no other layer is Interposed between the CML layer256and the P-cladding206.

FIGS. 3A-3Cshow examples of a p-n junction of the QD-APD200in non-avalanche, regular mode operation.FIG. 3Ashows an example p-n junction310, with a positively doped (p-type semiconductor with excess holes) region311and a negatively doped (n-type semiconductor with excess electrons) region314separated by an absorption region319. Light, in the form of photons312, enters the absorption region319and generates an electron-hole pair, including an electron, which then impacts an atom313.

With reference toFIG. 3B, photo-generated carriers based on photon312(FIG. 3A) may ionize the atom313(FIG. 3A) into an electron-hole pair including hole315and electron316.

With reference toFIG. 3C, in a reverse-biased p-n junction, the electron316is attracted towards the negatively doped region314and the hole315is attracted towards the positively doped region311. With many such carrier-atom impacts, an electrical current is set up in the p-n junction310.

The maximum quantum efficiency of non-avalanche mode of operation is 100%, indicating that each photon312(considered as an input to the p-n junction) generates at most one electron-hole pair as the electrical charge (considered as an output from the p-n junction).

FIGS. 3D-3Fshow examples of a p-n junction in avalanche mode operation. With reference toFIG. 3D, the avalanche mode of operation of a p-n junction310is described. The structure of the p-n junction is the same as described earlier with respect toFIGS. 3A-3C. The operation at this point is identical or similar toFIG. 3A, in which a photon312enters the absorption region between the positively doped region311and the negatively doped region314and impacts an atom313.

FIG. 3Eshows an example ionization of an atom313(FIG. 3D) into an electron-hole pair, as described with respect toFIG. 3B. In the avalanche mode of operation, the initial or early electron-hole pairs created are called parent pairs because they may generate other electron-hole pairs, which are called child pairs. Subsequently, each child pair may generate their own child pairs, and so on.

FIG. 3Fshows an example avalanche process, which is the main difference between the avalanche mode of operation and the non-avalanche mode shown inFIG. 3C. At this stage, the parent electron-hole charge(s) may impact and ionize other atoms313and generate additional electron-hole pairs, as their children. This process may continue starting with each photon312(FIG. 3D) and continuing with each electron-hole pair generated as a result, generating even more children. With continued reference toFIG. 3F, the parent hole315and parent electron316may impact other atoms in turn and generate their own (without a photon) child holes317and child electrons318. Hence, the quantum efficiency of the avalanche mode of operation may well exceed 100%, because one photon312(input) may eventually generate more than one electron-hole pair (output).

FIG. 4shows examples of evanescent coupling between a waveguide401and several demultiplexing ring waveguides404a-404dof various sizes. As discussed with respect toFIG. 1, a laser device102may be used to transmit data modulated thereon. Several light frequencies, or equivalently, wavelengths, may be generated by the laser device, for example, a comb laser, and combined and transmitted over one optical media, such as an optical fiber103. A wavelength vs. power graph402shows several distinct carriers with different wavelengths403being transmitted via a waveguide401. A number of ring waveguides404a-404deach having a different radius R1-R4, respectively, are coupled with the waveguide401via evanescent coupling through small air gaps408a-d, respectively, on the order of about 0.2 μm or more or less as desired. The waveguide rings404a-404dare further coupled with QD-APD waveguides405a-d, via other air gaps407a-407d, respectively. Each of the QD-APD waveguides405a-dreceives the corresponding data stream with distinct carrier wavelengths406a-d, from the respective waveguide rings404a-404d.

Optical evanescent coupling is used to transfer light between an optical transport, such as a waveguide, to semiconductor devices, such as photodetectors that use or process the light. The waveguide rings404a-404dinherently operate in a narrow bandwidth by resonance. As a result, the rings may be tuned to particular frequencies. In practice, the tuning is done by choosing the appropriate radius for the waveguide rings and can be further fine-tuned by a heater and/or a MOS-tuner (metal oxide semiconductor tuner). This way, each ring separates a particular carrier with the frequency tuned to the radius of the ring, which is subsequently passed on to the corresponding QD-APD waveguide.

FIG. 5shows examples of evanescent coupling between a waveguide and several QD-APDs via demultiplexing tuning ring waveguides of various sizes. The example evanescent coupling arrangement500, shows a waveguide501carrying an optical signal and coupled with the demultiplexing tuning ring waveguides502aand502b, tuned to different wavelengths1and2via air gaps503aand503b, respectively. The demultiplexing tuning ring waveguides502aand502bare in turn coupled with QD-APDs505aand505bvia air gaps504aand504b, respectively.

With this arrangement, each of several optical signals transmitted by optical media, such as an optical fiber, are separated by the demultiplexing tuning ring waveguides502aand502band transferred to the appropriate QD-APD for detection and subsequent electrical charge generation.

FIG. 6shows an example evanescent coupling arrangement with a waveguide coupled with a partial QD-APD demultiplexing ring waveguide. The example evanescent coupling arrangement600shows a waveguide601carrying an optical signal and coupled with a tuning ring waveguide603via an air gap602. The tuning ring waveguide603is implemented as a QD-APD with QD stack604covering less than the entire tuning ring waveguide603. The optical signal may travel multiple times around the waveguide ring603, thereby having multiple opportunities to be absorbed in the absorption region of QD Stack604, which improves the overall absorption and efficiency of the QD-AFD.

In this arrangement, the tuning ring waveguide603is implemented as a QD-APD and performs both the function of tuning and separating the optical signal from the waveguide601and the function of detecting the light signal. This way the optical data transmission system may be simplified and be produced at lower cost.

FIG. 7shows an example waveguide coupled with a full QD-APD demultiplexing ring waveguide. The example evanescent coupling arrangement700shows a waveguide701carrying an optical signal and coupled with a tuning ring waveguide703via an air gap702. The tuning ring waveguide703is implemented as a fully formed QD-APD with QD stack covering the entire tuning ring waveguide703. The optical signal may travel multiple times around the tuning ring waveguide703, thereby having multiple opportunities to be absorbed in the absorption region of QD light absorption layers, which improves the overall absorption and efficiency of the QD-AFD.

In this arrangement, the tuning ring waveguide703is implemented as a QD-APD and performs both the function of tuning and separating the optical signal from the waveguide701and the function of detecting the light signal. This way the optical data transmission system may be simplified and be produced at lower cost.

The foregoing disclosure describes a number of example implementations of a QD-APD. For purposes of explanation, certain examples are described with reference to the components illustrated inFIGS. 1 to 7. The functionality of the illustrated components may overlap, however, and may be present in a fewer or greater number of elements and components. Further, all or part of the functionality of illustrated elements may co-exist or be distributed among several geographically dispersed locations. Moreover, the disclosed examples may be Implemented in various environments and are not limited to the illustrated examples. Thus, the present disclosure merely sets forth possible examples of implementations, and many variations and modifications may be made to the described examples. All such modifications and variations are intended to be included within the scope of this disclosure and protected by the following claims.