Abstract:
In one aspect, an avalanche photodiode, includes an absorber, a first superlattice structure directly connected to the absorber and configured to multiply holes and a second superlattice structure directly connected to the first superlattice structure and configured to multiply electrons. The first and second superlattice structures include III-V semiconductor material. The avalanche photodiode is a dual mode device configured to operate in either a linear mode or a Geiger mode. In another aspect, a method includes fabricating the avalanche diode.

Description:
BACKGROUND 
       [0001]    A superlattice is a periodic structure of layers of at least two materials. In some examples, a layer is several nanometers thick. In some examples, a layer is a semiconductor material. In one particular example, a layer is a group III-V semiconductor material. 
       SUMMARY 
       [0002]    In one aspect, an avalanche photodiode, includes an absorber, a first superlattice structure directly connected to the absorber and configured to multiply holes and a second superlattice structure directly connected to the first superlattice structure and configured to multiply electrons. The first and second superlattice structures include III-V semiconductor material. The avalanche photodiode is a dual mode device configured to operate in either a linear mode or a Geiger mode. 
         [0003]    In another aspect, a method includes performing k·p simulations to establish bandgap and electronic band structure on a proposed configuration of the avalanche photodiode; performing Monte-Carlo simulations on the proposed configuration of the avalanche photodiode if the band-offsets are conducive for carrier transport; and determining layer thicknesses and doping levels of the avalanche photodiode. The avalanche photodiode includes an absorber; a first superlattice structure directly connected to the absorber and configured to multiply holes and a second superlattice structure directly connected to the first superlattice structure and configured to multiply electrons. The first and second superlattice structures include III-V semiconductor material. The avalanche photodiode is a dual mode device configured to operate in either a linear mode or a Geiger mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a diagram of an example of a dual mode superlattice avalanche photodiode. 
           [0005]      FIG. 2  is a graph of an example of a gain versus bias curve of the dual mode superlattice avalanche photodiode of  FIG. 1 . 
           [0006]      FIG. 3  is a graph of an example of an electron multiplication superlattice structure bandstructure. 
           [0007]      FIG. 4  is a graph of an example of a hole multiplication superlattice structure bandstructure. 
           [0008]      FIG. 5  is a flowchart of an example of a process to fabricate a dual mode III-V superlattice avalanche photodiode. 
           [0009]      FIG. 6  is a computer on which any portion of the process of  FIG. 5  may be implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Described herein are techniques to fabricate an avalanche photodiode using superlattice structures that enable the avalanche photodiode to be a dual mode device (i.e., the avalanche photodiode may be operated in either a linear mode or a Geiger mode). 
         [0011]    Referring to  FIG. 1 , a circuit  100  includes an avalanche photodiode  102 . The avalanche photodiode (APD)  102  includes an absorber  104 , a first superlattice structure  108  and a second superlattice structure  112 . In some examples, the first and second superlattice structures  108 ,  112  are each a type II strain layer superlattice (SLS). In one particular example, the absorber  104  is a p-type absorber graded for efficient carrier injection. The absorber  104  is configured to absorb photons at a desired wavelength (e.g., a laser wavelength used in an active imaging or ranging system). 
         [0012]    When biased by a source  120  (e.g., a DC power source), the absorber  104  receives a photon to generate electron hole pairs (e.g., electron hole pair  114 ). The first superlattice structure  108  is configured to multiply holes and the second superlattice structure  112  is configured to multiply electrons. For example, when an electron from the electron hole pair  114  is received in the second superlattice structure  112 , the electron is multiplied (see stage I) to form two electrons in total. At second stage II, the two electrons are multiplied to become four electrons and at stage III the four electrons are multiplied to become eight electrons. When a hole is received by the first superlattice structure  108  from the second superlattice structure  112 , the hole is multiplied to become two holes in stage IV. 
         [0013]    In one particular example, the first superlattice structure  108  and the second superlattice structure  112  are each noiseless. That is, the first superlattice structure  108  only multiplies holes (not electrons) and the second superlattice structure  112  only multiplies electrons (not holes). 
         [0014]    The first superlattice structure  108  comprises III-V semiconductor material. In one example, the first superlattice structure  108  includes a layer of indium arsenide (InAs), a layer of indium gallium antimonide (InGaSb), a layer of aluminium antimonide (AlSb) and a layer of gallium antimonide (GaSb), which are repeated through the first superlattice structure  108 . Each layer of indium arsenide (InAs) is about 0.8 nanometers thick, each layer of indium gallium antimonide (InGaSb) is about 1.9 nanometers thick, each layer of aluminium antimonide (AlSb) is about 0.3 nanometers thick and each layer of gallium antimonide (GaSb) is about 0.1 nanometers thick. The first superlattice structure  108  is about two thousand Angstroms thick. 
         [0015]    The second superlattice structure  108  comprises III-V semiconductor material. In one example, the second superlattice structure  112  includes a layer of indium arsenide (InAs), a layer of aluminum gallium antimonide (AlGaSb), and a layer of gallium antimonide (GaSb), which are repeated through the second superlattice structure  112 . Each layer of indium arsenide (InAs) is about 1.5 nanometers thick, each layer of aluminum gallium antimonide (AlGaSb) is about 3.8 nanometers thick and each layer of gallium antimonide (GaSb) is about 0.9 nanometers thick. The second superlattice structure  112  is about 1500 Angstroms thick. 
         [0016]    Referring to  FIG. 2 , a graph  200  depicts a curve  202  of the gain versus bias relationship of the avalanche photodiode  102 . Separate minibands within the conduction and valence bands in the avalanche photodiode allow for decreased impact ionization rates in certain energy bands as depicted in the curve  102 . Thus, the avalanche photodiode  102  can operate in either a linear mode  208  or a Geiger mode  212 . In the linear mode  208 , as the bias over the avalanche photodiode  112  is increased from zero, the avalanche photodiode  112  reaches a gain saturation region  218 , where the gain is relatively constant. With continued increased bias, the avalanche photodiode  112  reaches avalanche breakdown  206 , which starts the Geiger mode  212 . In the Geiger mode  212  there is a sharp rise in the gain with increased gain. 
         [0017]      FIGS. 3 and 4  are examples of bandstructures for electron multiplication and hole multiplication, respectively that are configured to enable the avalanche photodiode  102  to be able to function in either the Geiger mode or the linear mode depending on bias. In  FIGS. 3 and 4 , the black circles represent electrons while the white circles represent holes. 
         [0018]      FIG. 3  is a graph of an example of an electron multiplication superlattice structure bandstructure of the second superlattice structure  112 . The second superlattice structure  112  is configured to generate a Eg&lt;Delta resonance in the conduction band. As the reverse bias across the avalanche photodiode  102  is increased, photoelectrons, generated in the absorber  104  via photon absorption, get injected into the second superlattice structure  112  and gain energy in conduction band  1  (CC 1 ) and after gaining the threshold energy (Eth), the electrons (with finite momentum, k) start impact ionizing and generate electron-hole pairs. In the process, the electron loses energy and comes back to conduction band edge at the zone center (i.e., momentum (k)=0) (see arrows  302   a ,  302   b ). The impact ionization generated electrons and holes move in opposite directions and undergo impact ionization in their respective multiplication regions (electrons impact ionize only in the electron multiplication region (e.g., second superlattice structure  112 ) while holes only multiply in the hole multiplication region (e.g., first superlattice structure  108 )). This process generates a positive feedback between the hole and electron multiplication regions. The gain in the avalanche photodiode  102  increases rapidly. If the gain is not stabilized, the avalanche photodiode  102  will exhibit runaway behavior. The electron initiated impact ionization becomes less and less efficient as the electron energy is increased beyond Eth. This is due to the strict energy and momentum conservation conditions required for impact ionization. The gain in the avalanche photodiode  102  increases but starts to saturate due to the inefficient impact ionization of electrons. This is the linear mode operation in the avalanche photodiode  102  and gain stabilization is implemented utilizing the gain saturation behavior. With proper electronic bandstructure configuration, the Eg&lt;Delta resonance in the conduction band can be configured to occur at zone center. This configuration allows for carriers in CC 1  (at or above Eth) to scatter to the zone center in conduction band  2  (CC 2 ) as depicted by (see arrow  312 ). The scattered electrons at the zone center (k=0) in CC 2  can very efficiently impact ionize due to relaxed momentum conservation conditions. The transition matrix elements at zone center are very strong and the impact ionization process at k=0 (see arrows  308   a ,  308   b  is very efficient. This leads to a rapid increase in the impact ionization rate for electrons and the gain in the avalanche photodiode  102  starts to increase rapidly again, which is the Geiger mode like behavior (depicted in  FIG. 2 ) in the avalanche photodiode  102  implemented using electrons in CC 2  zone center for impact ionization. 
         [0019]      FIG. 4  is a graph of an example of a hole multiplication superlattice structure bandstructure. 
         [0020]    Referring to  FIG. 5 , a process  500  is an example of a process to fabricate a dual mode III-V superlattice avalanche photodiode (e.g., APD  102 ). The process  500  includes choosing an operating wavelength ( 502 ) and performing 14-band k·p simulations to establish a bandgap and electronic bandstructure on a particular superlattice structure (SLS) avalanche photodiode (APD) ( 508 ). 
         [0021]    Process  500  determines whether the band-offsets are conducive for carrier transport ( 512 ). If the band-offsets are not conducive for carrier support, process  500  iterates the SLS configuration ( 514 ) 
         [0022]    If the band-offsets are conducive for carrier support, Monte-Carlo simulations are performed ( 516 ). Process  500  determines if the electron-to-hole ionization coefficient ratio is different than 1 ( 530 ). If the electron-to-hole ionization coefficient ratio is 1, process  500  iterates the SLD configuration ( 514 ). If the electron-to-hole ionization coefficient ratio is different than 1, process  500  determines whether the avalanche initiating carrier ionization rate is saturating at the desired bias ( 524 ). If the avalanche initiating carrier ionization rate is not saturating at the desired bias, process  500  iterates the SLD configuration ( 514 ). 
         [0023]    If the avalanche initiating carrier ionization rate is saturating at the desired bias, Process  500  determines separate adsorption and multiplication (SAM) APD layer thickness and doping levels ( 528 ) and simulates gain versus bias for SLS SAM APD with dead-space multiplication theory (DSMT) code ( 534 ). 
         [0024]    Referring to  FIG. 6 , in one example, a computer  600  includes a processor  602 , a volatile memory  604 , a non-volatile memory  606  (e.g., hard disk) and the user interface (UI)  608  (e.g., a graphical user interface, a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory  606  stores computer instructions  612 , an operating system  616  and data  618 . In one example, the computer instructions  612  are executed by the processor  602  out of volatile memory  604  to perform all or part of the processes described herein (e.g., process  500 ). 
         [0025]    The processes described herein (e.g., process  500 ) are not limited to use with the hardware and software of  FIG. 6 ; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a non-transitory machine-readable medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information. 
         [0026]    The system may be implemented, at least in part, via a computer program product, (e.g., in a non-transitory machine-readable storage medium such as, for example, a non-transitory computer-readable medium), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se. 
         [0027]    The processes described herein are not limited to the specific examples described. For example, the process  500  is not limited to the specific processing order of  FIG. 5 . Rather, any of the processing blocks of  FIG. 5  may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
         [0028]    The processing blocks (for example, in the process  500 ) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate. 
         [0029]    Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.