Patent Publication Number: US-2023152081-A1

Title: Self-Mixing Interference Device for Sensing Applications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/913,645, filed Jun. 26, 2020, which is a nonprovisional of and claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 62/869,442, filed Jul. 1, 2019, the contents of which are incorporated herein by reference as if fully disclosed herein. 
    
    
     FIELD 
     The present disclosure generally relates to structures and configurations of vertical cavity surface emitting laser (VCSEL) diodes and associated photodetectors (PDs), such as resonant cavity photodetectors (RCPDs). The VCSEL diodes and associated PDs may be part of sensors or detectors that measure or determine displacement, distance, motion, speed, or velocity of a surface or an object. Such detectors or sensors may be directed to determining force or touch inputs on an electronic device. 
     The VCSEL diode and/or its associated RCPD may make use of self-mixing interference caused by reflections or backscatter of laser light emitted by the VCSEL diode. With self-mixing interference, the received reflections induce in the VCSEL diode a change or an altered state of the emitted laser light from the state of the emitted laser light without received reflections. The change in the emitted laser light may be correlated to a distance or motion of an object or target (such as an electronic device&#39;s input surface). 
     BACKGROUND 
     Electronic devices are commonplace in today&#39;s society. Example electronic devices include cell phones, tablet computers, personal digital assistants, and the like. Some of these electronic devices include one or more input elements or surfaces, such as buttons or touch screens, through which a user may enter commands or data by applying a touch or a press. The touch or press may be detected by components of the electronic device that detect displacement or motion of the input elements or surfaces. 
     Such detection components may make use of light sources in which a light beam, such as a laser, is emitted toward the input surface. Detection of displacement or movement of the input surface may be inferred from reflections or backscatter of the emitted laser light from the input surface. 
     A particular category of such detection components may include VCSEL diodes. A VCSEL diode may undergo self-mixing interference, in which reflections of its emitted laser light are received back into its lasing cavity and shift a property of the emitted laser light, such as wavelength, to a different state from what it would be in the absence of received reflections (i.e., free emission). In the case that the received reflections are from an input surface, the shift in the property may be correlated with the displacement or motion of the input surface. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     Disclosed herein are self-mixing interferometry sensors. The self-mixing interferometry sensors may include a vertical cavity surface emitting laser (VCSEL) diode and a resonant cavity photodetector (RCPD). The VCSEL diode and the RCPD may be formed from a common set of semiconductor layers. 
     In a first embodiment, a self-mixing interferometry sensor may include a VCSEL diode and an RCPD that is laterally adjacent to the VCSEL diode. The VCSEL and the RCPD may include a common set of semiconductor layers formed on a common substrate that includes an active region layer. The VCSEL diode and the RCPD may be at least partially separated by a trench through the common set of semiconductor layers, extending through the active region layer. The VCSEL diode may include additional semiconductor layers stacked with the common set of semiconductor layers. 
     The VCSEL diode may be configured to emit a laser light under forward bias and undergo self-mixing interference caused by first reflections or backscatters of the emitted laser light from an object, with the self-mixing interference altering a property of the emitted laser light. The RCPD may be configured to be reverse biased during emission of the laser light by the VCSEL diode and receive second reflections or backscatters of the emitted laser light from the object, and the RCPD may produce a measurable parameter related to the altered property of the emitted laser light. 
     In further embodiments, the additional semiconductor layers may be formed on the common set of semiconductor layers, and may include an etch stop layer, which may be Indium Gallium Phosphide, adjacent to the common set of semiconductor layers. A bias supply electrical contact may be connected to a layer of the additional semiconductor layers farthest from the common set of semiconductor layers. In various embodiments, the VCSEL diode may be configured to have a natural wavelength of 940 nanometers (nm), 850 nm, 1060 nm, or another natural wavelength. 
     In another embodiment, a self-mixing interferometry sensor may include a first and a second VCSEL diode, the second VCSEL diode being laterally adjacent to the first VCSEL diode. The first VCSEL diode and the second VCSEL diode include a common set of semiconductor layers formed on a common substrate including an active region layer. The self-mixing interferometry sensor may also include an RCPD positioned vertically adjacent to the second VCSEL diode on a side of the second VCSEL diode opposite to the common substrate. The first VCSEL diode and the second VCSEL diode are at least partially separated by a trench through the common set of semiconductor layers. 
     The first VCSEL diode may be configured to emit a laser light while forward biased and undergo self-mixing interference caused by receiving first reflections or backscatters from an object, with the self-mixing interference altering a property of the emitted laser light. The RCPD and the second VCSEL diode may be configured to be reverse biased during emission of the laser light by the first VCSEL diode. The RCPD may be configured to receive second reflections or backscatters of the emitted laser light from the object, and the altered property of the emitted laser light may be detectable using a measured parameter of the RCPD. 
     In some embodiments, the RCPD may include a tunnel junction layer containing a first semiconductor material, which may be Indium Gallium Arsenide, different from a second semiconductor material in the active region layer. There may be an etch stop layer, which may be Indium Gallium Phosphide, between the RCPD and the second VCSEL diode. In one example, the VCSEL diode may be configured to emit the laser light with a natural wavelength of 940 nanometers, but in other embodiments the VCSEL may have a natural wavelength of 850 nm, 1060 nm, or yet another natural wavelength. 
     The active region layer may include a first active region layer, a second active region layer and a tunnel junction layer between the first active region layer and the second active region layer. 
     In still another embodiment, a self-mixing interferometry sensor may include a VCSEL diode and an RCPD vertically adjacent to the VCSEL diode. The RCPD may include a first set of semiconductor layers formed on a substrate, and the VCSEL diode may include a second set of semiconductor layers formed on the first set of semiconductor layers opposite to the substrate. The VCSEL diode may be configured to emit laser light when forward biased and undergo self-mixing interference upon reception of reflections or backscatters of the emitted laser light from an object, the self-mixing interference altering a property of the emitted laser light. The RCPD may be configured to be reverse biased during emission of the laser light by the VCSEL diode and to detect the alteration in the property of the emitted laser light. There may be an etch stop layer, which may be Indium Gallium Phosphide, between the first set of semiconductor layers and the second set of semiconductor layers. The VCSEL diode may be configured to emit the laser light with a natural wavelength of 940 nm, 850 nm, 1060 nm or still another natural wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIG.  1 A  illustrates a first configuration using a vertical cavity surface emitting laser (VCSEL) diode for sensing an object, according to an embodiment. 
         FIG.  1 B  illustrates a second configuration using a VCSEL diode and an associated photodetector (PD) for sensing an object, according to an embodiment. 
         FIG.  1 C  illustrates a third configuration using a VCSEL diode and a vertically integrated PD for sensing an object, according to an embodiment. 
         FIG.  2 A  illustrates exemplary components of a laser diode, according to an embodiment. 
         FIG.  2 B  illustrates self-mixing interference within a laser, according to an embodiment. 
         FIG.  2 C  is a graph of variation in power of laser light from a laser diode undergoing self-mixing interference with respect to length of a feedback cavity, according to an embodiment. 
         FIG.  3 A  illustrates a structure of a VCSEL diode, according to an embodiment. 
         FIG.  3 B  illustrates a structure of another VCSEL diode, according to an embodiment. 
         FIG.  4    illustrates a configuration of components of a self-mixing interferometry (SMI) sensor, according to an embodiment. 
         FIG.  5 A  illustrates another configuration of components of a SMI sensor, according to an embodiment. 
         FIG.  5 B  illustrates a configuration of components of a resonant cavity photodetector, according to an embodiment. 
         FIG.  6 A  illustrates another configuration of components of a SMI sensor, according to an embodiment. 
         FIG.  6 B  illustrates an energy band diagram for two active region layers and a tunnel junction within a VCSEL diode, according to an embodiment. 
         FIG.  6 C  illustrates an equivalent circuit for two active region layers and a tunnel junction within a VCSEL diode, according to an embodiment. 
         FIG.  7 A  illustrates a manufacturing and epitaxial grown process for a self-mixing interferometry sensor, according to an embodiment. 
         FIG.  7 B  illustrates a flip chip and substrate removal process for the self-mixing interferometry sensor of  FIG.  7 A , according to an embodiment. 
         FIG.  8    illustrates a configuration of components of a self-mixing interferometry sensor, according to an embodiment. 
         FIG.  9    illustrates a configuration of components of a self-mixing interferometry sensor, according to an embodiment. 
         FIG.  10    illustrates a quantum well configuration for an active region layer, according to an embodiment. 
     
    
    
     The use of cross-hatching or shading in the accompanying figures is generally provided to clarify the boundaries between adjacent elements and also to facilitate legibility of the figures. Accordingly, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristic, attribute, or property for any element illustrated in the accompanying figures. 
     Additionally, it should be understood that the proportions and dimensions (either relative or absolute) of the various features and elements (and collections and groupings thereof) and the boundaries, separations, and positional relationships presented therebetween, are provided in the accompanying figures merely to facilitate an understanding of the various embodiments described herein and, accordingly, may not necessarily be presented or illustrated to scale, and are not intended to indicate any preference or requirement for an illustrated embodiment to the exclusion of embodiments described with reference thereto. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The embodiments described herein are directed to self-mixing interferometry sensors, such as may be used for touch or input sensors, and to their structures. Such self-mixing interferometry sensors may use vertical cavity surface emitting laser (VCSEL) diodes and associated resonance cavity photodetectors (RCPDs). An electronic device may use such an self-mixing interferometry sensor as part of detecting a displacement, distance, motion, speed, or velocity of an input surface, such as a touch screen. Hereinafter, for convenience, all such possibly measured parameters will be referred to simply as “displacement or motion.” 
     Such self-mixing interferometry sensors detect the displacement or motion by causing one or more VCSEL diodes to emit a laser light toward the input surface or an object (hereinafter, just “object”) by applying a forward bias. Reflections of the emitted laser light from the input surface or object can be received back into the lasing cavity of the VCSEL diode. This can cause self-mixing interference in which there is a shift in the wavelength of the emitted laser light to a new altered value. This shift in the wavelength can be correlated to the displacement or motion of the object. 
     Other reflections of the emitted laser light may be concurrently detected by an RCPD, whose structure may include a p-n diode junction. Under a reverse voltage bias, no significant current flows through the p-n junction of the RCPD. But the received reflections of the emitted laser light can induce a photocurrent across the p-n junction. The intensity of the photocurrent, or another related interferometric property, can be related to the wavelength of the received reflections, which in turn can be related to the shifted wavelength of the emitted laser light. Processing circuitry (e.g., a processor or other circuit) included in the self-mixing interferometry sensor may then be able to infer the distance or motion values of the object. 
     Various embodiments described below describe structures or configurations of the one or more VCSEL diodes and RCPDs that are parts of such self-mixing interferometry sensors. A VCSEL diode and an RCPD may be formed from a common set of semiconductor layers that are formed epitaxially from a common substrate, and then electrically separated by an etched trench. Alternative methods of forming the common set of semiconductor layers may be used in addition to or instead of epitaxial growth. For simplicity of explanation, hereinafter the deposition or growth of the common set of semiconductor layers will be described as formed epitaxially, but one skilled in the art will recognize that alternative methods are within the scope of this disclosure. The RCPD is then laterally adjacent to the VCSEL diode. In some embodiments the self-mixing interferometry sensor may include two VCSEL diodes, both formed from a common set of semiconductor layers, with the first VCSEL diode operable to emit the laser light and the second reverse biased and having a vertically adjacent RCPD on top to receive reflections of the emitted laser light. In this configuration, further embodiments may include a tunnel junction within the first VCSEL diode. 
     Additional and/or alternative embodiments may make use of another configuration, in which a VCSEL diode is epitaxially formed on an RCPD. Some of the laser light of the VCSEL diode is also emitted into the RCPD and direct measurements are made of one or more interferometric properties, either of the VCSEL diode or of the RCPD. 
     These and other embodiments are described below with reference to  FIGS.  1 A- 10   . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
       FIGS.  1 A- 1 C  illustrate exemplary configurations in which a VCSEL diode can be used within a self-mixing interferometry sensor to detect displacement or motion of an object, such as may be caused by a user&#39;s touch or force input. The three configurations illustrate ways in which self-mixing interference can be caused in a VCSEL diode, and ways such self-mixing interference can be monitored. 
       FIG.  1 A  illustrates a configuration  100  for a self-mixing interferometry sensor that makes use of a VCSEL diode  102  configured to emit laser light  106 , which may be directed toward an object  110 . The VCSEL diode  102  may emit the laser light  106  under a forward voltage bias (or just “forward bias”) of its diode structure. During such forward bias, a bias current I BIAS    104 , flows through the VCSEL diode  102 . Charge carriers crossing the p-n junction of the VCSEL diode  102  may induce laser light emission from the VCSEL diode  102 . 
     There may be reflections  112  of the emitted laser light  106 , which may travel in multiple directions from the object  110 . Some of the reflections  112  may be received back into a lasing cavity of the VCSEL diode  102 , causing it to undergo self-mixing interference and altering a property of the emitted laser light  106  or of an electrical property of the VCSEL diode  102  itself. For example, a voltage monitor  108  may detect changes in a junction voltage of the VCSEL diode  102  that correlate with a distance or motion of the object  110 . 
       FIG.  1 B  illustrates a configuration  120  for an input or object detection sensor that uses a VCSEL diode  122 . Under a forward voltage bias, the VCSEL diode  122  emits the laser light  106  through an intervening glass layer  128  toward the object  110 . The glass layer  128  may cause some reflections  107  of the emitted laser light  106 . The forward voltage bias induces a bias current I BIAS    124  to flow through the VCSEL diode  122 . There may be reflections  112  of the emitted laser light  106  from an object or target, some of which may be received back into a lasing cavity of the VCSEL diode  122 , causing it to undergo self-mixing interference and altering a property of the emitted laser light  106 . The reflections  107  from the glass layer  128  may also affect how the VCSEL diode  122  undergoes self-mixing interference. Nevertheless, if the object is at least a certain minimum distance from the glass layer  128 , a spectral analysis of an electrical signal correlated with the self-mixing interference may allow separation of first effects in the electrical signal due to the self-mixing interference caused by the reflections  107  off the glass layer  128  from second effects due to the self-mixing interference caused by the reflections  112  off the object. In some embodiments, a separation of at least 100 μm between the object and the glass layer  128  suffices. 
     The configuration  120  may include a photodetector (PD)  126  that may operate to receive some of the reflections  112  from the object  110 . The alteration in the emitted laser light  106  may in turn alter an electrical property of the PD  126 , which may be detectable. For example, an alteration in a bias current I PD  of the PD  126  may be detected by a current monitor  130 . 
       FIG.  1 C  illustrates a configuration  140  for a self-mixing interferometry sensor that uses a VCSEL diode  142 . Under a forward voltage bias, the VCSEL diode  142  may emit the laser light  106  toward the object  110 . The forward voltage bias induces a bias current I BIAS    144 , to flow through the VCSEL diode  142 . 
     Some of the reflections  112  of the emitted laser light  106  may be received within the VCSEL diode  142 , causing it to undergo self-mixing interference and altering a property of the emitted laser light  106 . The VCSEL diode  142  may include an integrated PD  146 . The PD  146  may detect an operational change in the VCSEL diode  142  due to the self-mixing interference. For example, the VCSEL diode  142  may also emit some laser light downwards into the PD  146 . Any alterations in the emitted laser light  106  due to self-mixing interference may also occur in the downward emitted laser light, and cause an operational change in the PD  146 . For example, a bias current I PD  of the PD  146  may be detected by a current monitor  132 . 
       FIGS.  2 A- 2 C  illustrate properties of self-mixing interference of emitted laser light in a laser diode. The explanations are intended only to describe certain aspects of self-mixing interference needed to understand the disclosed embodiments. Other aspects of self-mixing interference will be clear to one skilled in the art. 
       FIG.  2 A  illustrates an exemplary configuration of a laser diode  200 , such as any of the VCSEL diodes  102 ,  122 , and  142  described above, that may be used as part of a self-mixing interferometry sensor. In any type of laser, an input energy source causes a gain material within a cavity to emit light. Mirrors  202  and  204  on opposite ends of the cavity feed the light back into the gain material to cause amplification of the light and to cause the light to become coherent and (mostly) have a single wavelength. An aperture in one of the mirrors allows transmission of the laser light (e.g., transmission toward an object or input surface). 
     In the laser diode  200 , there are two mirrors  202  and  204  on opposite ends of the cavity. The lasing occurs within the cavity  206 . In the case of VCSEL diodes, the two mirrors  202  and  204  may be implemented as distributed Bragg reflectors, which are alternating layers with high and low refractive indices. The cavity  206  contains a gain material, which may include multiple doped layers of III-V semiconductors. Specific details of the semiconductor materials will be presented below for the various embodiments. The emitted laser light  210  can be emitted through the topmost layer or surface of the laser diode  200 . In some VCSEL diodes, the coherent light may also be emitted through the bottom layer. 
       FIG.  2 B  shows a functional diagram of self-mixing interference (or also “optical feedback”) within a laser. In  FIG.  2 B , the cavity  206  is shown reoriented so that the emitted laser light  210  is emitted from the cavity  206  to the right. The cavity  206  has a fixed length established at manufacture. The emitted laser light  210  travels away from the cavity  206  until it intersects or impinges on an input surface or another object, such as the object  110  described above in relation to  FIGS.  1 A- 1 C . The gap of distance L from the emission point through the mirror  204  of the emitted laser light  210  to the target  216  is termed the feedback cavity  208 . The length L of the feedback cavity  208  is variable as the target  216  can move with respect to the laser diode  200 . 
     The emitted laser light  210  is reflected back into the cavity  206  by the target  216 . The reflected light  212  enters the cavity  206  to coherently interact with the original emitted laser light  210 . This results in a new state illustrated with the new emitted laser light  214 . The new emitted laser light  214  at the new state may have characteristics (e.g., a wavelength or power) that differ from what the emitted laser light  210  would have in the absence of reflection and self-mixing interference. 
       FIG.  2 C  is a graph  220  showing the variation in power of the new emitted laser light  214  as a function of the length L of the feedback cavity  208 , i.e., the distance from the emission point through the mirror  204  of the emitted laser light  210  to the target  216 . The graph depicts a predominantly sinusoidal variation with a period of λ/2. Theoretical considerations imply that the variation is given by the proportionality relationship: Δ∝ cos(4πL/λ). This relationship generally holds in the absence of a strong specular reflection. In the case of such strong specular reflection, the cosine becomes distorted, i.e., higher harmonics are present in the relationship. However, the peak-to-peak separation stays at λ/2. For an initially stationary target  216 , this relationship can be used to determine that a deflection has occurred. In conjunction with other techniques, such as counting of the completed number of periods, the range of the deflection may also be determined. 
     Though the graph  220  shows the variation in power of the new emitted laser light  214  as a function of the length L of the feedback cavity  208 , similar results and/or graphs may hold for other interferometric properties of a VCSEL diode or other type of laser diode that are measured by a self-mixing interferometry sensor. Measurements of one or more such interferometric parameters by a self-mixing interferometry sensor can be used to infer displacements or motions of the target  216  from the laser diode  200 . 
     Further details of structures for VCSEL diodes will now be presented in relation to  FIGS.  3 A- 3 B . 
       FIGS.  3 A- 3 B  show two exemplary configurations of the structure and operations of VCSEL diodes. Embodiments to be described below may make use of these structures, or variations thereof. One skilled in the art will recognize that other configurations and variations are within the scope of this disclosure. 
       FIG.  3 A  illustrates a first configuration  300  for a VCSEL diode  302  under forward bias and emitting a laser light  306   a  toward the object  110 . Under the forward bias, a bias current  304 , I BIAS , flows into the VCSEL diode  302 , with some or all of it returning to a ground layer or contact  312 . 
     As previously described, some reflections  112  of the emitted laser light  306   a  may be received back into the VCSEL diode  302  to induce self-mixing interference. Some of the reflections  112  may also become reflected light  314   a  directed toward a photodetector (not shown), as described above in relation to  FIG.  1 B . 
     VCSEL diode  302  may include an emission side (or “top side”) distributed Bragg reflector  303   a  that functions as a first (or “emission side”) mirror of a laser structure. The emission side distributed Bragg reflector  303   a  may include a set of pairs of alternating materials having different refractive indices. Each such pair of alternating materials will be termed herein a Bragg pair. One or more of the materials in the emission side distributed Bragg reflector  303   a  are doped to be p-type and so form a part of the anode section of a p-n diode junction. An exemplary pair of materials that may be used to form the emission side distributed Bragg reflector  303   a  are aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs). 
     VCSEL diode  302  may include a base side distributed Bragg reflector  303   b  that functions as a second (or “base side”) mirror of a laser. The base side distributed Bragg reflector  303   b  may also include a set of Bragg pairs of alternating materials having different refractive indices. One or more of the materials in the base side distributed Bragg reflector  303   b  are doped to be n-type and so form a part of the anode section of a p-n diode structure. An exemplary pair of materials that may be used to form the base side distributed Bragg reflector  303   b  are aluminum arsenide and GaAs. 
     VCSEL diode  302  may include an active region layer  318  that functions in part as the lasing cavity. In laser diodes, such as VCSEL diode  302 , an active region layer may include one or more quantum wells. The active region layer  318  of VCSEL diode  302  may be adjacent to an oxide layer  316 , having an aperture through which escapes the emitted laser light  306   a.    
     The VCSEL diode  302  may be formed by epitaxial growth of the layers for each of the emission side and base side distributed Bragg reflectors  303   a ,  303   b , the active region layer  318  and the oxide layer  316 , and possibly other layers. These various layers may be formed by epitaxial growth on a substrate layer  308 , with the ground layer or contact  312  formed afterwards. Electrical supply contacts  305   a ,  305   b  may be formed on the outermost (i.e., emission side) layer of the VCSEL diode  302 . While shown as separated in  FIG.  3 A , the electrical supply contacts  305   a ,  305   b  may be connected, such as to form, for example, a ring or horseshoe connection on the top side of the VCSEL diode  302 . 
     The VCSEL diode  302  may alternatively be formed by epitaxial growth from a substrate starting with the layers for the emission side distributed Bragg reflector  303   a . The substrate may then be separated from a substrate, such as by etching or cleaving, and a flip chip process used to attach the VCSEL diode  302  to another substrate or circuit, so that the emission side distributed Bragg reflector  303   a  is configured to emit laser light  306   a.    
       FIG.  3 B  illustrates a second configuration  320  for a VCSEL diode  322  under forward bias and emitting a laser light  306   b  toward the object  110 . Some reflections  112  of the emitted laser light  306   b  may be received back into the VCSEL diode  322  to induce self-mixing interference. Still others of the reflections  112  may also become reflected light  314   b  directed along a direction toward a photodetector (not shown), as described above in relation to  FIG.  1 B . 
     The configuration  320  for VCSEL diode  322  has many features in common with the configuration  300  for VCSEL diode  302 , such as having multiple semiconductor layers extending from substrate layer  308 , to which is attached a ground layer or contact  312 . Also as described for VCSEL diode  302  are electrical supply contacts  305   a ,  305   b , which may be formed on the outermost (i.e., emission side) layer of the VCSEL diode  322 , and which may be as described as for the configuration  300 . The VCSEL diode  322  may also include an active region layer  318  and an oxide layer  316 , which may be as described as for the configuration  300 . The VCSEL diode  322  may include a base distributed Bragg reflector  323   b , which may as described above in regard to the base side distributed Bragg reflector  303   b.    
     The VCSEL diode  322  may include an emission side (or “top side”) distributed Bragg reflector  323   a  that differs from that of VCSEL diode  302 . The emission side distributed Bragg reflector  323   a  may include a layer that serves as an etch stop layer during manufacture. One skilled in the art will recognize that various materials may serve as an etch stop layer. One such material is indium gallium phosphide (InGaP), which may have a composition of the form In x Ga 1-x P (for 0≤x≤1). The etch stop layer can enable a wet chemical etching to be stopped at the etch stop layer, due to a significant difference of etching rates between the etch stop layer and GaAs/AlGaAs layers. After stopping at the etch stop layer, a different chemical solution can be used to remove only the etch stop layer without significantly etching off the underlying GaAs or AlGaAs layer. By using this method, the etch stop layer may be inserted anywhere in the epitaxy layer stack as an etch stop layer so that etching can be stopped at the layer underneath the etch stop layer, and an intra-cavity contact put on that layer. One skilled in the art will see that such an etch stop layer can be InGaP or another material, as long as its etching rate under certain chemical solutions is significantly different from the layer (such as GaAs and/or AlGaAs layer) at which to stop. 
     In some embodiments, the VCSEL diodes  302  and  322  may be configured to emit respective laser light  306   a  and  306   b  at a natural wavelength of approximately 940 nanometers (nm), the natural wavelength being the wavelength of the respective emitted laser light  306   a ,  306   b  when there is no alteration due to self-mixing interference caused by reflections of the respective emitted laser light  306   a ,  306   b . In other embodiments, the natural wavelength may be 850 nm, 1060 nm, or another natural wavelength. Hereinafter, for simplicity of explanation only, embodiments with VCSELs having a natural wavelength of 940 nm will be discussed. One skilled in the art will recognize that the embodiments may be implemented with a VCSEL having another natural wavelength. The natural wavelength may be varied within a range about 940 nm due to a deliberately applied modulation of I BIAS  about a constant (DC) value. For example, in some embodiments, a triangle or a sinusoidal modulation may be applied to I BIAS . Such biasing may be applied by self-mixing interferometry sensors as part of determining a displacement or motion of the object  110 . As one example, the amplitude of such a modulation of I BIAS  may be on the order of 1 mA, and cause a self-mixing interference shift in wavelength on the order of 0.5 nm from the natural wavelength of 940 nm. In other embodiments, the VCSEL diodes  302  and  322  may be configured to emit respective laser light  306   a  and  306   b  at a different natural wavelength, and use a different modulation amplitude to produce a different self-mixing interference wavelength shift from the natural wavelength. 
     The particular configurations  300  and  320  are exemplary; variations within the scope of this disclosure will be recognized by one skilled in the art. The structures of the VCSEL diodes  302  and  322 , and their variations, may be part of the self-mixing interferometry sensors now to be described. 
       FIG.  4    illustrates an embodiment of at least a part of configuration  400  of a self-mixing interferometry sensor. The self-mixing interferometry sensor may include at least two components: a VCSEL  402  operable to emit laser light  306   b  toward an object  110  when forward biased, and a resonant cavity photodetector (RCPD)  406  that may operate to detect the reflected light  314   b  of all the reflections  112  of the emitted laser light  306   b . Some of the reflections  112  of the emitted laser light  306   b  may be received back into the VCSEL  402  and cause self-mixing interference that alters, for example, the wavelength of the emitted laser light  306   b.    
     In the embodiments shown in  FIG.  4   , the VCSEL diode  402  has the configuration of the VCSEL diode  322  of  FIG.  3 B . However, the VCSEL diode  402  may have another configuration, such as that of VCSEL diode  302  of  FIG.  3 A . In the particular embodiment of  FIG.  4   , the VCSEL diode  402  may include a base side distributed Bragg reflector  403   b , an emission side distributed Bragg reflector  403   a  that includes an etch stop layer  424 , which may be InGaP, an active region layer  418 , and an oxide layer  416 . 
     The embodiment shown in  FIG.  4    may include the RCPD  406  positioned laterally adjacent to the VCSEL diode  402 . The VCSEL diode  402  and the RCPD  406  are both positioned on a common substrate  408 . The common substrate  408  may include a ground contact or layer  412 . Further details concerning the structures and fabrication of the VCSEL diode  402  and the RCPD  406  will be presented below. 
     In the embodiment of  FIG.  4   , the RCPD  406  has a structure similar to that of the VCSEL diode  302  of  FIG.  3 A . In particular, the RCPD  406  may include an n-type doped base side distributed Bragg reflector  407   b . The active region layer  418  and the oxide layer  416  extend into the RCPD  406 , as explained further below. The RCPD  406  may also include a p-type doped top side distributed Bragg reflector  407   a , so that the RCPD  406  forms a p-n junction. In the embodiment of  FIG.  4   , the emission side distributed Bragg reflector  407   a  has fewer layers than the emission side distributed Bragg reflector  403   a  of the VCSEL diode  402 . 
     Each of the VCSEL diode  402  and the laterally adjacent RCPD  406  have respective electrical supply contacts. For the VCSEL diode  402 , the electrical supply contacts  405   a  and  405   b  are assumed to be electrically connected, and are attached to the topmost (farthest from the common substrate  408 ) of the semiconductor layers forming the VCSEL diode  402 . A forward bias voltage +V BIAS  applied to the electrical supply contact  405   b  may cause a current I BIAS  to flow through the p-n diode junction formed by the p-type emission side distributed Bragg reflector  403   a  and the n-type base side distributed Bragg reflector  403   b , and be collected at the ground contact or layer  412 . For the RCPD, the electrical supply contacts  417   a  and  417   b  are assumed to be electrically connected, and are attached to the topmost of the semiconductor layers forming the RCPD  406 . 
     The RCPD  406  may be reverse biased during at least part of the time that VCSEL diode  402  is forward biased. The reverse bias may be applied by a negative voltage, indicated by −V PD , applied at the electrical supply contact  417   b . While reverse biased, the RCPD  406  may operate as a photodetector. While so operating, the reflected light  314   b  may impinge on the topmost layer of the RCPD  406 , with some becoming secondary reflections  414   b , and others becoming absorbed light  414   a . Under the applied reverse bias, no significant current flows across the p-n junction of the RCPD  406 . But the absorbed light  414   a  can induce charge carriers to cross the junction and produce a photocurrent I PD . Variations in the emitted laser light  306   b  from the VCSEL diode  402  may produce corresponding detectable variations in the photocurrent I PD . The self-mixing interferometry sensor may then use those detectable variations to infer displacement or motion of the object  110 . 
     The VCSEL diode  402  and the RCPD  406  may both be formed from a common set of semiconductor layers epitaxially formed on the common substrate  408 . That is, the various semiconductor layers for the base side distributed Bragg reflectors  403   b  and  407   b , the active region layer  418 , the oxide layer  416 , and the layers for the emission side distributed Bragg reflectors  403   a  and  407   a  may be sequentially applied to the common substrate  408 , which may be a semiconductor material or another type of substrate, such as sapphire, glass or another material. 
     After epitaxial formation of all the semiconductor layers, various layers at various locations may be removed, such as by etching or another process. Hereinafter, etching will denote any of the known methods of removing material from a semiconductor during fabrication. In the embodiment of  FIG.  4   , a trench  410  is etched through the semiconductor layers, including through the active region layer  418 . In some embodiments, the trench  410  may extend through to the common substrate  408 , though this is not required. The trench  410  can provide electrical separation between the VCSEL diode  402  and the RCPD  406 , allowing for the alternate biasing of each. 
     In the embodiment of  FIG.  4   , some of the semiconductor layers for the emission side distributed Bragg reflector  403   a , such as the etch stop layer  424  and above, are initially epitaxially formed at the location of the RCPD  406  as well, but then removed by etching. A first purpose for removing those semiconductor layers is to provide higher absorption of the reflected light  314   b . A second purpose is to allow for a wider angle of acceptance of the reflected light  314   b : absorbed light  414   a  may enter the RCPD  406  at a larger angle from the normal and still pass into the active region layer. Other advantages are also possible. 
     One skilled in the art will recognize that the VCSEL diode  402  and associated RCPD  406  shown in  FIG.  4   , as well as the various VCSEL diodes and RCPDs illustrated in each of the embodiments disclosed herein, may be a section of an array of such elements used within a self-mixing interferometry sensor. The epitaxial formation of the common set of semiconductor layers may allow for more efficient formation of such an array. 
     In some embodiments, during formation of a particular layer of the common set of semiconductor layers, techniques such as masking and doping may be used to alter a doping or chemical composition of a certain layer. For example, during epitaxial formation of the active region layer  418 , its doping type or chemical composition may be altered at the location of the RCPD  406 . This may allow for an improved detection of the absorbed light  414   a.    
       FIGS.  5 A- 5 B  illustrate certain details of a configuration for a self-mixing interferometry sensor  500 . The self-mixing interferometry sensor may include a first VCSEL diode  502 , a similarly structured, laterally adjacent second VCSEL diode  522 , formed on a common substrate  508 , and a resonant cavity photodetector (RCPD)  532  vertically adjacent to the second VCSEL diode  522 . A ground contact or layer  512  may be attached to a side of the common substrate  508  opposite to the first VCSEL diode  502 . 
     The first VCSEL diode  502  may be structured similarly to the VCSEL diode  302  of  FIG.  3 A . In the embodiment of  FIG.  5 A , the first VCSEL diode  502  may include a base side distributed Bragg reflector  503   b , which may be as those described previously. The first VCSEL diode  502  may include an active region layer  518 , and an oxide layer  516  above it (i.e., on a side of the active region layer  518  opposite to the common substrate  508 ). The first VCSEL diode  502  may include an emission side distributed Bragg reflector  503   a , which may be as those described previously. Electrical supply contacts  505   a  and  505   b , which may be connected are attached to the emission side of the first VCSEL diode  502 . As previously described, the emission side distributed Bragg reflector  503   a  may have p-type doping, and the base side distributed Bragg reflector  503   b  may have n-type doping to form a diode p-n junction structure. 
     A forward voltage bias, shown as +V BIAS , may be applied at the electrical supply contact  505   b . With a forward voltage bias to the first VCSEL diode  502 , a bias current I BIAS  will flow through the p-n junction and be received at the ground contact or layer  512 . During an applied forward bias, the first VCSEL diode may emit laser light  306   a , such as toward an object (not shown), such as object  110  previously described. 
     The second VCSEL diode  522  may be similarly structured to the first VCSEL diode  502 , such as by formation from a common set of semiconductor layers, as described below. In the embodiment of  FIGS.  5 A- 5 B , the second VCSEL diode  522  may include a base side distributed Bragg reflector  523   b , a separated extension of the active region layer  518 , a separated extension of the oxide layer  516 , and an emission side distributed Bragg reflector  523   a . Electrical supply contacts  535   a  and  535   b , which may be connected, are connected to a semiconductor layer parallel to the topmost (emission side) semiconductor layer of first VCSEL diode  502 . 
     The self-mixing interferometry sensor  500  may include RCPD  532  formed on the emission side distributed Bragg reflector  503   a  of the second VCSEL diode  522 . An etch stop layer  533 , which may be InGaP, may be between the RCPD  532  and the second VCSEL diode  522 . During at least part of the time in which the first VCSEL diode  502  is forward biased to emit laser light  306   a , the second VCSEL diode  522  may be reverse biased by applying a negative voltage at the electrical supply contacts  535   a ,  535   b . This can reduce undesired contributions to the photodetector current I PD    536 , as will now be explained in relation to  FIG.  5 B . 
       FIG.  5 B  illustrates further details of the RCPD  532 . The RCPD  532  may be structured similar to a VCSEL diode. In the embodiment of  FIG.  5 B , the RCPD  532  may include a base side distributed Bragg reflector  534   b , a photodetector active region layer  538 , and an emission side distributed Bragg reflector  534   a . The base side distributed Bragg reflector  534   b  may have p-type doping, and the emission side distributed Bragg reflector  534   a  may have n-type doping to form a diode p-n junction. 
     The RCPD  532  has electrical supply contacts  537   a  and  537   b  on the topmost layer of its emission side distributed Bragg reflector  534   a . The electrical supply contacts  537   a  and  537   b  may be connected, such as by being parts of the same electrical supply contact, as explained previously. 
     During times when the first VCSEL diode  502  is forward biased to emit laser light  306   a , the second VCSEL diode  522  maybe reverse biased by a negative voltage −V PD  being applied to the electrical supply contacts  535   a  and  535   b . The RCPD  532  may also be reverse biased by setting the electrical supply contacts  537   a  and  537   b  to ground (0V). When reflected light  314   a  impinges on the RCPD  532 , some may be absorbed and cause changes to the photodetector current I PD . These changes to the photodetector current I PD  may correlate with alterations in the reflected light  314   a  caused by self-mixing interference occurring in the first VCSEL diode  502 . Processing circuitry (not shown) may measure these changes to the photodetector current I PD  and infer displacement or motion of the object causing the reflected light  314   a , as described previously. 
     In various embodiments, the number of Bragg pairs in the emission side distributed Bragg reflector  534   a  is less than the number of Bragg pairs in the base side distributed Bragg reflector  523   b . This may provide the RCPD  532  with an increased acceptance angle for the reflected light  314   a . This may lead to an improved signal-to-noise ratio, such as in I PD . In one embodiment 4 Bragg pairs are used for the emission side distributed Bragg reflector  534   a  and 11 Bragg pairs are used for the base side distributed Bragg reflector  534   b.    
     Various advantages accrue to the configuration for the self-mixing interferometry sensor  500 . Under reverse bias the RCPD  532  and the second VCSEL diode  522  can be formed to act as a selective wavelength filter for the wavelength of the emitted laser light  306   a . Another advantage is that the photodetector active region layer  538  may be or include InGaAs. InGaAs has a lattice structure that is not compatible with the lattice structure of AlGaAs which may be a material used as part of the distributed Bragg reflectors adjacent to the photodetector active region layer  538 . Since the InGaAs in the photodetector active region layer  538  has a more limited extent than the various semiconductor layers in the first VCSEL diode  502  and the second VCSEL diode  522 , there is less mechanical stress. In still another advantage, the photodetector active region layer  538  may be formed with different doping or chemical composition than that of the active region layer  518 . 
     The configuration for the self-mixing interferometry sensor  500  may be formed as described above for the self-mixing interferometry sensor of  FIG.  4   . The first VCSEL diode  502  and the second VCSEL diode  522  may both be formed from a common set of semiconductor layers epitaxially formed on the common substrate  508 . The further semiconductor layers for the RCPD  532  may then be epitaxially formed on those layers. Etching may then be used to form the trench  510  and to remove the further semiconductor layers added for the RCPD  532  from above the location for the first VCSEL diode  502 , from above the locations for the electrical supply contacts  535   a  and  535   b , and possibly from other desired locations. 
       FIGS.  6 A- 6 C  illustrate variations for the embodiments illustrated in  FIGS.  5 A- 5 B . The embodiments described in relation to  FIGS.  6 A- 6 C  can provide a strong feedback signal, such as measured by the signal-to-noise ratio in I PD  as described above. The embodiments described in relation to  FIGS.  6 A-C  are directed to reducing the internal reflectance of the emission side distributed Bragg reflector that functions as one mirror of the laser. Reducing the internal reflectance, such as by increasing the amount of emitted laser light, allows for stronger reflections from objects, and hence stronger reflections being absorbed by the associated RCPD. But a reduced internal reflectance may work against the VCSEL diode reaching the threshold for lasing. The embodiments described in relation to  FIGS.  6 A- 6 C  allow for reduced reflectance of the emission side distributed Bragg reflector by decreasing the threshold for lasing. This is achieved by including a tunnel junction with or in the active region layer or layers. These considerations will now be described in detail. 
       FIG.  6 A  illustrates a configuration of a self-mixing interferometry sensor  600  similar to the configuration described for the self-mixing interferometry sensor  500 . The self-mixing interferometry sensor  600  may include a first VCSEL diode  602 , and a second VCSEL diode  622  laterally adjacent to the first VCSEL diode  602 , with both formed from a common set of semiconductor layers on a common substrate  608 . A ground layer or contact  612  is formed on a side of the common substrate  608  opposite to the first VCSEL diode  602 . The self-mixing interferometry sensor  600  may also include a resonance cavity photodetector (RCPD)  632  vertically adjacent to a side of the second VCSEL diode  622  opposite to the common substrate. 
     The first VCSEL diode  602  may include an emission side distributed Bragg reflector  603   a  and a base side distributed Bragg reflector  603   b . The first VCSEL diode  602  may include electrical supply contacts  605   a  and  605   b , which may be connected, adjacent to the topmost of the common set of semiconductor layers, or on a topmost layer of the emission side distributed Bragg reflector  603   a.    
     The first VCSEL diode  602  may contain both a first active region layer  618   a  and a second active region layer  618   b  positioned between the emission side distributed Bragg reflector  603   a  and the base side distributed Bragg reflector  603   b , with the first active region layer  618   a  proximate to the emission side distributed Bragg reflector  603   a . The first VCSEL diode  602  may include an oxide layer  616  between the first active region layer  618   a  and the emission side distributed Bragg reflector  603   a . The first VCSEL diode  602  may contain a first tunnel junction layer  624   a  that is n-doped, and a second tunnel junction layer  624   b  that is p-doped. The first and second tunnel junction layers  624   a ,  624   b  are positioned between the first active region layer  618   a  and the second active region layer  618   b . Further details of the structure and operability of the tunnel junction layers  624   a ,  624   b  will be presented below in regard to  FIGS.  6 B-C . 
     The second VCSEL diode  622  may include an emission side distributed Bragg reflector  623   a  and a base side distributed Bragg reflector  623   b . The second VCSEL diode  622  may include electrical supply contacts  635   a  and  635   b , which may be connected, adjacent to the topmost of the common set of semiconductor layers. 
     The self-mixing interferometry sensor  600  may also include a RCPD  632  formed from additional semiconductor layers adjacent to, and extending vertically from, the emission side distributed Bragg reflector  623   a  of the second VCSEL diode  622 . The RCPD  632  may include an etch stop layer  633 , which may be InGaP, adjacent to the topmost of the semiconductor layers of the emission side distributed Bragg reflector  623   a . The RCPD  632  may include an active region layer  638 . The RCPD  632  may include electrical supply contacts  637   a  and  637   b , which may be connected, positioned on the topmost of the additional semiconductor layers. 
     The self-mixing interferometry sensor  600  may operate by applying a forward voltage bias +V BIAS  to the electrical supply contacts  605   a ,  605   b  to induce lasing and the emission of laser light  306   a . Some of the emitted laser light  306   a  may reflect from an object (not shown) and be received back into the first VCSEL diode  602  and induce self-mixing interference therein, which may induce a change in the emitted laser light  306   a  to a new wavelength. Other reflections of the emitted laser light  306   a  become the reflected light  314   b  directed toward the RCPD  632 . The RCPD  632  and the second VCSEL diode  622  are reverse biased for at least part of the time that the first VCSEL diode  602  emits laser light  306   a . The reverse bias may be applied as a negative voltage −V PD  applied at the electrical supply contacts  635   a ,  635   b . The reflected light  314   b  may enter the RCPD  632  and induce a photodetector current I PD . As described previously, the photocurrent may then be an input to a processor or processing circuitry that can infer a value for the new wavelength and/or a distance or motion of the reflecting object. 
     The self-mixing interferometry sensor  600  may be formed as described previously. A common set of semiconductor layers for the first VCSEL diode  602  and the second VCSEL diode  622  is epitaxially formed on the common substrate  608 . The additional semiconductor layers for the RCPD  632  may then be epitaxially formed on the common set of semiconductor layers. Etching may then remove any of the additional semiconductor layers formed over the location for the first VCSEL diode  602 . Etching may also remove a section of the common set of semiconductor layers to form the trench  610 . The trench  610  may extend through the first and second active region layers  618   a ,  618   b.    
       FIG.  6 B  illustrates an energy band diagram  640  for the first and second VCSEL active regions  618   a ,  618   b  and the two tunnel junction layers  624   a ,  624   b  in the first VCSEL diode  602 . The energy band diagram  640  shows the relative positions of the conduction band, denoted E c  and the valence band, denoted E v . The first tunnel junction layer  624   a , which is proximate to the first active region layer  618   a , may be a highly doped (e.g., doping level greater than 1×10 19  cm 3 ) n-type semiconductor, such as the shown highly doped n-type (n++) GaAs. The second tunnel junction layer  624   b , which is proximate to the second active region layer  618   b , may be a highly doped p-type semiconductor, such as the shown highly doped p-type (p++) GaAs. In other embodiments, the tunnel junction layers may use semiconductor materials other than GaAs. A non-limiting set of such semiconductor materials includes Al x Ga 1-x As (for 0≤x≤1), InP, In x Ga 1-x As (for 0≤x≤1), In 1-x-y Al x Ga y As (for 0≤x≤1, 0≤y≤1), and In x Ga 1-x As y P 1-y  (for 0≤x≤1, 0≤y≤1). 
     The first active region layer  618   a  may include at least three sections: a top section of p-type doped aluminum gallium arsenide (AlGaAs)  642   a  proximate to the emission side distributed Bragg reflector  603   a , a bottom section of n-type doped aluminum gallium arsenide (AlGaAs)  642   b  proximate to the first tunnel junction layer  624   a , and a central section of alternating pairs of intrinsic InGaAs and AlGaAs  642   c  forming quantum wells. 
     The second active region layer  618   b  may include at least three sections: a top section of p-type doped AlGaAs  644   a  proximate to the second tunnel junction layer  624   b , a bottom section of n-type doped aluminum gallium arsenide (AlGaAs)  644   b  proximate to the base side distributed Bragg reflector  603   b , and a central section of alternating pairs of intrinsic InGaAs and AlGaAs  644   c  forming quantum wells. . 
       FIG.  6 C  illustrates an electrical circuit model  650  indicating the functionality of the various layers of the first VCSEL diode  602 , which is shown horizontally for explanation purposes. The first and second tunnel junction layers  624   a ,  624   b  together operate as a resistor separating the first active region layer  618   a  and the second active region layer  618   b . The first and second active region layers  618   a ,  618   b  each are modeled as diodes. A single active region layer, as in the embodiments of  FIG.  5   , may not be inadequate by itself to provide sufficient gain for lasing when the reflectance of the emission side mirror, such as emission side distributed Bragg reflector  603   a , is reduced in order to produce more emitted laser light. The resistance provided by the first and second tunnel junction layers  624   a ,  624   b  allows for the first and second active region layers  618   a ,  618   b  to provide an increased total gain sufficient to produce lasing even with a reduced reflectance of the emission side mirror. 
     As an example, in a single conventional VCSEL diode having a single active region layer, the bottom mirror may need a reflectance with R B &gt;99.99%, and the top mirror may need a reflectance with R T &gt;99.3%. In certain of the embodiments of  FIGS.  6 A-C , the top reflectance, as provided by the emission side distributed Bragg reflector  603   a , may be able to be reduced to satisfy just R T &gt;98.5%. The decrease in reflectance of the emission side distributed Bragg reflector  603   a  may, in some embodiments, be implemented by it having fewer Bragg pairs. 
     In the electrical circuit model  650 , the electrical supply contact  605   a  functions as the anode at which a positive bias voltage is provided, and the ground layer or contact  612  functions as the cathode. In the embodiments of self-mixing interferometry sensor  600  of  FIGS.  6 A-C , the positive bias voltage may need to be higher to produce lasing than for embodiments in which there is only one active region layer, because there is a need to supply the diode voltage drops for both the first and second active region layers  618   a ,  618   b . In the electrical circuit model  650 , the emission side distributed Bragg reflector  603   a  and the base side distributed Bragg reflector  603   b  each function as resistors. 
     The embodiments of self-mixing interferometry sensors in  FIG.  4   ,  FIGS.  5 A-B , and  FIGS.  6 A-C  have a laterally adjacent RCPD designed to receive reflections of the emitted laser light that are distinct from the reflections that produce the self-mixing interference in the lasing VCSEL diode. Additional and/or alternative embodiments of self-mixing interferometry sensors described with respect to  FIG.  8    and  FIG.  9    make use of RCPDs that are vertically adjacent to the lasing VCSEL diode. 
       FIGS.  7 A and  7 B  illustrate a manufacturing and epitaxial growth process for a self-mixing interferometry sensor  700  as depicted and described herein. As part of the epitaxial growth for the self-mixing interferometry sensor  700 , a first etch stop layer  706  (e.g., an InGaP etch stop layer) may be epitaxially formed on a substrate  701 . Semiconductor layers for a VCSEL diode  702  may be epitaxially formed on the first etch stop layer  706 . 
     A second etch stop layer  704  (e.g., an InGaP etch stop layer) may be epitaxially grown on a top portion of the semiconductor layers comprising the VCSEL diode  702 . The second etch stop layer may separate semiconductor layers for the VCSEL diode  702  from semiconductor layers for the RCPD  722 . As depicted in  FIG.  7 A , the RCPD  722  may be epitaxially formed from the second etch stop layer  704 . A substrate layer  708  may be formed or otherwise placed above the semiconductor layers for the RCPD  722 . 
     The VCSEL diode  702  may include an emission side distributed Bragg reflector  703   a  proximate to the first etch stop layer  706 , an oxide layer  716  (e.g., a high-aluminium Al x Ga 1-x As layer, where X is approximately equal to 0.98), an active region layer  718 , and a base side distributed Bragg reflector  703   b  proximate to the second etch stop layer  704 . In some embodiments, the active region layer  718  may be a number of forward biased quantum wells (e.g., forward biased quantum wells configured to generate light). In some embodiments, the number of quantum wells comprising the active region layer  718  may be less than six and/or may have a quantum well thickness of less than 10 nm. The base side distributed Bragg reflector  703   b  and the emission side distributed Bragg reflector  703   a  may function as previously described. 
     The RCPD  722  may include an emission side distributed Bragg reflector  723   a , an active region layer  724 , and a base side distributed Bragg reflector  723   b . The emission side distributed Bragg reflector  723   a  and the base side distributed Bragg reflector  723   b  may function as previously described. The active region layer  724  may be a photon absorption layer (e.g., an InGaAs photon absorption layer). In some embodiments, the active region layer  724  has a thickness between 15 nm and 60 nm. The active region layer  724  may, in some embodiments, be formed of quantum wells. A substrate layer  708  may be grown and/or positioned above the RCPD  722  (e.g., proximate to the base side distributed Bragg reflector  723   b ). 
       FIG.  7 B  illustrates the self-mixing interferometry sensor  700  after a flip chip process has occurred. In  FIG.  7 B , the substrate  701  and the first etch stop layer  706  may have been removed during a removal process. The substrate layer  708  may be positioned proximate to a ground contact layer  712 . As illustrated in  FIG.  7 B , the self-mixing interferometry sensor  700  may be flipped upside down (e.g., in a flip chip process) so that, for example, the VCSEL diode  702  is positioned above the RCPD  722 . After the flip chip process, the self-mixing interferometry sensor  700  may undergo further substrate removal processes and may transform into the self-mixing interferometry sensor  800  depicted and described below with respect to  FIG.  8   . 
       FIG.  8    illustrates a configuration of a self-mixing interferometry sensor  800  that may include a VCSEL diode  802  vertically adjacent to RCPD  822 . The self-mixing interferometry sensor  800  may be manufactured and epitaxially grown by the process described above with respect  FIGS.  7 A and  7 B  or may be epitaxially grown from the substrate layer  808  upward as described herein. The semiconductor layers for the RCPD  822  may be epitaxially formed on a substrate  808 . A ground contact layer  812  may be formed in or on the substrate, such as on a side of the substrate  808  opposite to the RCPD  822 . 
     An etch stop layer  804 , which may be InGaP, may separate the semiconductor layers for the RCPD  822  from further layers forming the VCSEL diode  802 . The VCSEL diode  802  may include a base side distributed Bragg reflector  803   b  proximate to the etch stop layer  804 , an active region layer  818 , an oxide layer  816 , and an emission side distributed Bragg reflector  803   a . The base side distributed Bragg reflector  803   b  and the emission side distributed Bragg reflector  803   a  function as described previously. Affixed to the topmost layer of the VCSEL diode  802  are electrical supply contacts  805   a  and  805   b , which may be connected. 
     The RCPD  822  may itself be structured with an active region layer  824  positioned between an emission side distributed Bragg reflector  823   a  and a base side distributed Bragg reflector  823   b . The base side distributed Bragg reflector  823   b  and the emission side distributed Bragg reflector  823   a  function as described previously. The semiconductor layers for the RCPD  822  may laterally extend beyond the semiconductor layers forming the VCSEL diode  802 . Such a configuration may be formed by epitaxially forming all the layers included in the self-mixing interferometry sensor initially, and then etching part of the layers of the VCSEL diode  802 . An electrical supply contact  825  is affixed to topmost layer of the RCPD at such a position that extends laterally beyond the VCSEL diode  802 . 
     In operation, a forward voltage bias +V BIAS  is applied to the electrical supply contacts  805   a  and  805   b , which induces a bias current I BIAS  to flow into the VCSEL diode  802  and induce emission of laser light  306   a , as described previously. For at least part of the time that the VCSEL diode  802  is emitting laser light, the RCPD  822  may be reverse biased by the application of a negative voltage −V PD  at the electrical supply contact  825 , to cause the flow of a photodetector current I PD . In addition to the emitted laser light  306   a , some of laser light produced in the active region layer  818  may be directed downward and be absorbed or received in the reverse biased RCPD  822 . 
     If some of the emitted laser light  306   a  is reflected from an object (not shown) and is received in the VCSEL diode  802 , self-mixing interference may occur that induces an alteration in the wavelength of the emitted laser light  306   a . Some of such altered laser light may then be received in the RCPD  822 , producing a measurable change in the photodetector current I PD , or another electrical or interferometric property of the RCPD  822 . Such a measurable change may be used to infer a distance or a motion of the reflecting object. 
       FIG.  9    illustrates a configuration for a self-mixing interferometry sensor  900 . The self-mixing interferometry sensor may include a VCSEL diode  902  and a laterally adjacent RCPD  932 . The RCPD  932  is formed as part of a common set of semiconductor layers epitaxially formed on a substrate  908 . The VCSEL diode  902  is formed vertically above the common set of semiconductor layers. A trench  910  may be etched at least partially through the common set of semiconductor layers to provide electrical separation between the VCSEL diode  902  and the RCPD  932 . 
     The VCSEL diode  902  may include sequentially from the common set of semiconductor layers: an etch stop layer  904 , semiconductor layers forming a base side distributed Bragg reflector  903   b , an active region layer  918 , an oxide layer  916 , and semiconductor layers forming an emission side distributed Bragg reflector  903   a . These parts of the VCSEL diode  902  are as described for VCSEL diode  802  of  FIG.  8   . Contacting the topmost layer of the VCSEL diode  902  are electrical supply contacts  905   a  and  905   b , which may be connected. 
     The RCPD  932  may include sequentially from the substrate  908 : semiconductor layers forming a base side distributed Bragg reflector  933   b , an active region layer  934 , and semiconductor layers forming an emission side distributed Bragg reflector  933   a . The various layers for these three structures have respective lateral extensions, separated by the trench  910 , beneath the VCSEL diode  902 : layers  923   b , layer  924 , and layers  923   a . These layers may be structured as an RCPD  922 , as described previously. Further, such an RCPD  922  may also be used for detection of self-mixing interference in the laser light emitted by the forward biased VCSEL diode  802 , as described for the embodiment of  FIG.  8   . There is an etch stop layer  904 , which may be InGaP, between the layers  923   a  and the semiconductor layers of the base side distributed Bragg reflector  903   b . Contacting the topmost layer of the RCPD  932  are electrical supply contacts  925   a  and  925   b , which may be connected. 
     The self-mixing interferometry sensor  900  may be operated analogously to previous embodiments. A forward voltage bias +V BIAS  is applied to the electrical supply contacts  905   a  and  905   b , which induces a bias current I BIAS  to flow into the VCSEL diode  902  and induce emission of laser light  306   a , as described previously. If some of the emitted laser light  306   a  is reflected from an object (not shown) and is received in the VCSEL diode  902 , self-mixing interference may occur that induces an alteration in the wavelength of the emitted laser light  306   a.    
     For at least part of the time that the VCSEL diode  902  is emitting laser light, the RCPD  932  may be reverse biased by the application of a negative voltage −V PD  at the electrical supply contact  925 . Other reflections of the altered laser light may then be received in the RCPD  932 , producing a measurable change in the photodetector current I PD , or another electrical or interferometric property of the RCPD  922 . Such a measurable change may be used to infer a distance or a motion of the reflecting object. 
       FIG.  10    illustrates a quantum well configuration for an active region layer  1000  of a self-mixing interferometry sensor. The active region layer  1000  may include quantum wells  1002   a - 1002   c  and may be bounded by a first barrier  1004  and a second barrier  1006 . In some embodiments, the active region layer  1000  may correspond to the active region layer  718  of the VCSEL diode  702  as discussed above with respect to  FIGS.  7 A and  7 B . In such embodiments, the active region layer  1000  may be configured to emit light (e.g., photons) and the quantum wells  1002   a - 1002   c  may be forward biased quantum wells. In alternative or additional embodiments, the active region layer  1000  may correspond to the active region layer  724  of the RCPD  722  as discussed above with respect to  FIGS.  7 A and  7 B . In such embodiments, the active region layer  1000  may be configured to absorb light (e.g., photons) and the quantum wells  1002   a - 1002   c  may be reverse biased. At the reverse bias, the Quantum Confined Stark Effect (QCSE) may make the quantum wells  1002   a - 1002   c  photon absorptive and may convert absorbed light to a photo-current signal. 
     In some embodiments, three quantum wells  1002   a - 1002   c  may be positioned in an active region layer  1000 . In some embodiments, there may be up to six quantum wells present in the active region layer  1000 . In further embodiments, there may be six or more quantum wells present in the active region layer  1000 . In some embodiments, the thickness of each quantum well  1002   a - 1002   c  may be less than 10 nm. In some embodiments, the thickness of each quantum well  1002   a - 1002   c  may be equal to or greater than 10 nm. The quantum wells  1002   a - 1002   c  may be formed during the epitaxial growth process illustrated and described with respect to  FIGS.  7 A and  7 B  or may be formed during a reverse epitaxial growth process described with respect to  FIG.  8   . 
     Other examples and implementations are within the scope and spirit of the disclosure and appended claims. For example, features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items prefaced by “at least one of” indicates a disjunctive list such that, for example, a list of “at least one of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Further, the term “exemplary” does not mean that the described example is preferred or better than other examples. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.