Patent Application
20120236891
Lasers are commonly used in many modern communication components for data transmission. One use that has become more common is the use of lasers in data networks. Lasers are used in many fiber optic communication systems to transmit digital data on a network. In one exemplary configuration, a laser may be modulated by digital data to produce an optical signal, including periods of light and dark output that represents a binary data stream. In actual practice, the lasers output a high optical output representing binary highs and a lower power optical output representing binary lows. To obtain quick reaction time, the laser is constantly on, but varies from a high optical output to a lower optical output.
Optical networks have various advantages over other types of networks such as copper wire based networks. For example, many existing copper wire networks operate at near maximum possible data transmission rates and at near maximum possible distances for copper wire technology. On the other hand, many existing optical networks exceed, both in data transmission rate and distance, the maximums that are possible for copper wire networks. That is, optical networks are able to reliably transmit data at higher rates over further distances than is possible with copper wire networks.
One type of laser that is used in optical data transmission is a Vertical Cavity Surface Emitting Laser (VCSEL). As its name implies, a VCSEL has a laser cavity that is sandwiched between and defined by two mirror stacks. A VCSEL is typically constructed on a semiconductor wafer such as Gallium Arsenide (GaAs). The VCSEL includes a bottom mirror constructed on the semiconductor wafer. Typically, the bottom mirror includes a number of alternating high and low index of refraction layers. As light passes from a layer of one index of refraction to another, a portion of the light is reflected. By using a sufficient number of alternating layers, a high percentage of light can be reflected by the mirror.
An active region that includes a number of quantum wells is formed on the bottom mirror. The active region forms a PN junction sandwiched between the bottom mirror and a top mirror, which are of opposite conductivity type (e.g. a p-type mirror and an n-type mirror). Notably, the notion of top and bottom mirrors can be somewhat arbitrary. In some configurations, light could be extracted from the wafer side of the VCSEL, with the “top” mirror totally reflective--and thus opaque. However, for purposes of this invention, the “top” mirror refers to the mirror from which light is to be extracted, regardless of how it is disposed in the physical structure. Carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected minority carriers form a population inversion in the quantum wells that produces optical gain. Optical gain occurs when photons in the active region stimulate electrons to recombine with holes in the conduction band to the valance band which produces additional photons. When the optical gain exceeds the total loss in the two mirrors, laser oscillation occurs.
The active region may also include an oxide aperture formed using one or more oxide layers formed in the top and/or bottom mirrors near the active region. The oxide aperture serves both to form an optical cavity and to direct the bias current through the central region of the cavity that is formed. Alternatively, other means, such as ion implantation, epitaxial regrowth after patterning, or other lithographic patterning may be used to perform these functions.
A top mirror is formed on the active region. The top mirror is similar to the bottom mirror in that it generally comprises a number of layers that alternate between a high index of refraction and a lower index of refraction. Generally, the top mirror has fewer mirror periods of alternating high index and low index of refraction layers, to enhance light emission from the top of the VCSEL.
Illustratively, the laser functions when a current is passed through the PN junction to inject carriers into the active region. Recombination of the injected carriers from the conduction band to the valence band in the quantum wells results in photons that begin to travel in the laser cavity defined by the mirrors. The mirrors reflect the photons back and forth. When the bias current is sufficient to produce a population inversion between the quantum well states at the wavelength supported by the cavity, optical gain is produced in the quantum wells. When the optical gain is equal to the cavity loss, laser oscillation occurs and the laser is said to be at threshold bias and the VCSEL begins to ‘lase’ as the optically coherent photons are emitted from the top of the VCSEL.
It has been determined that the composition of the active region of a VCSEL can determine the functionality of the laser. Particularly, if the compositions of quantum wells and the barrier layers that surround quantum wells are not prepared carefully, the traps can form at the interface between the quantum well and quantum well barrier. Such formation of traps can coagulate into micro-dislocation loops which may grow into larger loops and cause non-radiative recombination. This can cause the VCSEL to fail. Thus, it can be advantageous to design an active region to prevent such formation of traps at the interface between quantum wells and quantum well barriers.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology where some embodiments described herein may be practiced.
In one embodiment, a VCSEL can include: one or more quantum wells having (Al)InGaAs; two or more quantum well barriers having Al(In)GaAs bounding the one or more quantum well layers; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier, wherein the quantum wells, barriers and transitional monolayers are substantially devoid of traps. In one aspect, the one or more transitional monolayers include GaP, GaAs, and/or GaAsP. In another aspect, the two or more transitional monolayers includes AlInGaAs with a barrier-side monolayer having lower In and higher Al compared to a quantum well side monolayer that has higher In and lower Al. In another aspect, the one or more transitional monolayers is devoid of Al and In.
In one embodiment, a method for preparing a VCSEL can include using molecular beam epitaxy (MBE) for growing a crystalline structure having: one or more quantum wells; two or more quantum well barriers bounding each of the one or more quantum wells; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier. The method can include forming the VCSEL to include: one or more quantum wells having InGaAs, with In ranging from about 2% to about 11% and Ga ranging from about 89% to about 98%, or one or more quantum wells having AlInGaAs, with Al ranging from about 6% to about 11% with In ranging from about 8% to about 20% and Ga ranging from about 69% to about 86%; one or more quantum well barriers having AlGaAs or Al(In)GaAs, with Al ranging from about 25% to about 40% and with In ranging from about 0 to about 2% and with Ga ranging from about 60% to about 75%; and one or more transitional layers between the quantum wells and quantum well barriers, the transitional layers having GaAs, GaAsP, or GaAsPSb
In one embodiment, a method for preparing an active region of a VCSEL can include: (a) growing a quantum well barrier having Al(In)GaAs; (b) growing an transitional layer having one or more of GaP, GaAsP, or GaAs; (c) growing a quantum well layer having (Al)InGaAs; (d) growing another transitional layer having one or more of GaP, GaAsP, or GaAs; (e) repeating processes (a) through (d) over a plurality of cycles; and (f) growing a quantum well barrier having Al(In)GaAs.
In one embodiment, a VCSEL can include: one or more quantum wells having high In and low Al; two or more quantum well barriers having high Al and low In bounding the one or more quantum well layers; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier, wherein the quantum wells, barriers and transitional monolayers are substantially devoid of traps. In this embodiment, the one or more quantum wells are characterized by the following: high In ranges from about 2% to about 6%; and low Al ranges from about 0 to about 12%; in the one or more quantum well barriers: high Al ranges from about 25% to about 40%; and low In ranges from about 0 to about 2%.
For example, the one or more transitional monolayers can include GaP, GaAs, and/or GaAsP. Alternatively, the two or more transitional monolayers includes AlInGaAs with a barrier-side monolayer having lower In and higher Al compared to a quantum well side monolayer that has higher In and lower Al.
The foregoing and following information as well as other features of this disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings, in which:
all arranged in accordance with at least one of the embodiments described herein, and which arrangement may be modified in accordance with the disclosure provided herein by one of ordinary skill in the art.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein.
The semiconductor devices of the present invention can be manufactured from any type of semiconductor. Examples of suitable materials include III-V semiconductor materials (e.g., prepared from one or more Group III material (boron (B), aluminium (Al), gallium (Ga), indium (In), thallium (Tl), and ununtrium (Uut)) and one or more Group V materials (nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi) and ununpentium (Uup) (unconfirmed))) and optionally some type IV materials.
The semiconductor device can include an active region having one or more quantum wells and one or more quantum well barriers. The quantum wells and quantum well barriers can be separated by one or more transitional layers therebetween. The transitional layers may also be referred to as interfacial layers as they are located at the interface between the quantum wells and quantum well barriers. Electrical confining layers can sandwich the active region and provide optical gain efficiency by confining carriers to the active region. The confining layers can have a region of high energy band gap which in the case of many III-V compounds translates to high aluminum content (e.g., 60%-100% Al for the type III material). The aluminum content can be selected to give the material a relatively wide band gap, as compared to the band gap in the quantum well barriers of the active region. The wide band gap material can give the confining layer good carrier confinement and increases the efficiency in the active region. In an exemplary embodiment, the high aluminum region may also include an increase in doping. The confining layer can be doped with a p-type or n-type dopant depending on whether the confinement barrier is on the n-side or p-side of the active region.
The quantum wells can include AlInGaAs with low amounts of Al and high amounts of In. The quantum well barriers can include AlInGaAs with low amounts of In and high amounts of Al. The transitional layers can be included between the quantum wells and quantum well barriers, which transitional layers can include GaP, GaAs, GaAs, or a gradient of AlInGaAs that avoids having a high Al low In region adjacent to a low Al high In region.
For purposes of this invention, the content of Aluminum (Al), Indium (In), and Gallium (Ga) are Group III materials that pair with an Arsenic (As), which is a Group V material, in an AlInGaAs quantum well or quantum well barrier other system. Accordingly, the Al, In, and Ga refers to the percent of Al, In or Ga in the AlInGa fraction. When a substance, such as either Al or In is shown in parentheses (e.g., (Al) or (In) ) within an elemental formula, it indicates that the element is present in low amounts or traces. Low amounts of In can be less than 5% or less than 1% or less than 0.1%. Low amounts of Al can be less than 12%, or less than 2% or less than 1% or less than 0.1%.
In one embodiment, the quantum well can be prepared to have a trace to low amount of Al. The low amount of Al with regard to the (Al)InGa fraction in the quantum well can range from about 0% to about 5%, or about 0.0001% to about 2%, or about 0.01% to about 1%, or about 0.05%. Also, the quantum well can be prepared to have high amounts of In, especially in relation to the amount of Al. The amount of In with regard to the (Al)InGa fraction in the quantum well can range from about 1% to about 50%, or about 2% to about 40%, or about 4% to about 30%, or about 4%. The amount of Ga with regard to the (Al)InGa fraction in the quantum well can range from about 50% to about 99%, or about 60% to about 98%, or about 70% to about 96%, or about 96%. When the Al is significantly low, the quantum well can be expressed as (Al)InGaAs. When devoid of Al, the quantum well can be InGaAs.
In one embodiment, the quantum well barrier can be prepared to have a trace to low amount of In. The low amount of In with regard to the Al(In)Ga fraction in the quantum well barrier can range from about 0% to about 5%, or about 0.0001% to about 2%, or about 0.01% to about 1%, or about 0.05%. Also, the quantum well barrier can be prepared to have high amounts of Al, especially in relation to the amount of In. The amount of Al with regard to the Al(In)Ga fraction in the quantum well barrier can range from about 20% to about 60%, or about 25% to about 55%, or about 35% to about 40%, or about 35%. The amount of Ga with regard to the Al(In)Ga fraction in the quantum well barrier can range from about 40% to about 80%, or about 45% to about 75%, or about 55% to about 65%, or about 65%. When the In is significantly low, the quantum well barrier can be expressed as Al(In)GaAs. When devoid of In, the quantum well barrier can be AlGaAs.
In one embodiment, the active region can include an In1-zGazAs quantum well, where z represents the percent of Ga in a quantum well and can range from about 0.8 to about 0.99, or about 0.83 to about 0.98, or about 0.94 to about 0.98, or about 0.96.
In one embodiment, the active region can include an AlxIn1-z-xGazAs quantum well, where z represents the percent of Ga in a quantum well and can range from about 0.6 to about 0.9, or about 0.69 to about 0.88, or about 0.72 to about 0.86, or about 0.8; and x represents the percent of Al in a quantum well and can range from about 0.06 to about 1, or about 0.08 to about 0.2, or about 0.09 to about 0.11, or about 0.1. Accordingly, the amount of In (e.g., 1-z-x) can range from 0.06 to about 0.5, or about 0.7 to about 0.3, or about 0.08 to 0.2.
In one embodiment, the active region can include an Al1-kGakAs quantum well barrier, where k represents the percent of Ga in a quantum well barrier and can range from about 0.5 to about 0.85, or about 0.55 to about 0.8, or about 0.6 to about 0.75, or about 0.7. Accordingly, the amount of Al (e.g., 1-k) can range from about 0.2 to about 0.5, or from about 0.25 to about 0.4
In one embodiment, the active region can include an Al1-j-kInjGakAs quantum well barrier, where k represents the percent of Ga in a quantum well barrier and can range from about 0.5 to about 0.85, or about 0.55 to about 0.8, or about 0.6 to about 0.75, or about 0.7; and j represents the percent of In in a quantum well barrier and can range from about 0 or 0.0001 to about 1, or about 0.0001 to about 0.1, or about 0.001 to about 0.02, or about 0.005 to about 0.01, or about 0.01.
In one embodiment, the active region can include a GaP transitional layer between each quantum well and quantum well barrier. In one aspect, the GaP transition layer is devoid of In. In another aspect, the transitional layer can be devoid of Al. In another aspect, the transitional layer can be pure GaP.
In one embodiment, the active region can include a transitional layer with an intermediate material that has GaP and is devoid of one or In or Al, or devoid of In, or devoid of Al, or devoid of In and Al. For example, the transitional layer can be GaP, GaAsP or GaAs, or GaP, GaAsP and GaAs. Also, the material of the transitional layer can be selected from the group consisting of GaP, GaAsP, or GaAs. For the transitional layer having GaAs1-wPw can have w range from about 0.0 to about 1, or from about 0.4 to about 1, or from about 0.5 to about 1.
In one embodiment, the transitional layer can include GaAs1-w-vPwSbv, where x range from about 0.3 to about 1, or from about 0.4 to about 1, or from about 0.5 to about 1, and v can range from about 0 to about 0.1, or from about 0.001 to about 0.05, or about 0.01 to about 0.2.
In one embodiment, the material of the transition layer can be a suitable material that is devoid of both Al and In. In one embodiment, the transition layer can have a monolayer on the quantum well side that is devoid of Al. In one embodiment, the transition layer can have a monolayer on the quantum well barrier side that is devoid of In.
In one embodiment, the transition layer can have two or more monolayers that are of different materials, such as a range of monolayers having a step in composition from the quantum well side to the quantum well barrier side. When having the stepped compositions, the monolayer on the quantum well side can be at least substantially devoid of Al and a monolayer on the quantum well barrier side that is at least substantially devoid of In. Of course, trace amounts of Al and In may be suitable so long as the traps are avoided.
In one embodiment, the transitional layer can include a plurality of monolayers that are intermediate compositions between (Al)InGaAs and Al(In)GaAs. That is, the monolayers can have the same elements of as the quantum well and quantum well barrier with different amounts of Al and In. Here, the monolayer on the quantum well side can be at least substantially devoid of or have a low amount of Al and a monolayer on the quantum well barrier side that is at least substantially devoid of or have a low amount of In. As such, the monolayers can have an increasing amount of Al from the quantum well side to the quantum well barrier side of the transitional layer, and the monolayers can have an increasing amount of In from the quantum well barrier side to the quantum well side.
The transitional layer between the quantum well and quantum well barrier can overcome problems associated with the formation of traps at interfaces of quantum wells and quantum well barriers. The transitional layer can be formed at the interface between high Al and low In quantum well barriers and high indium and low aluminum quantum wells so that traps are reduced or do not substantially form. As such, the transitional layer can be useful to inhibit reliability problems in lasers that are caused by the formation of traps between quantum wells and quantum well barriers.
Without a transitional layer as described herein, a quantum well barrier having Al(In)GaAs (e.g., high Al of at least 0.35) adjacent to a quantum well having (Al)InGaAs (e.g., low Al and high indium) have interface traps formed between the quantum well and quantum well barrier due to the extreme difference in chemical nature of the Al and In. Accordingly, a transitional layer between the quantum well and quantum well barrier that is devoid of both indium and Al, such as Gap, GaAs, or GaAsP as well as others described herein, can be used at the interface to prevent the formation of traps.
The elements In and Al are both group III, but they have large size differences and large chemical differences between them, and thereby have different properties when in a laser semiconductor composition. The differences between In and Al can cause the traps to form when laser semiconductor compositions, such as AlGaAs are adjacent to InGaAs. These traps can form when the laser is operated under normal forward bias device operation, and can coagulate into micro-dislocation loops which grow into larger loops and cause non radiative recombination, which causes the laser device to fail. It is thought that the traps formed when AlGaAs is adjacent to InGaAs, which are EL2 traps. To overcome the formation of traps, a transitional layer having one or more monolayers can be positioned between the quantum well and quantum well barrier. Preferably, a material devoid of both Al and In is used in the transitional layer, such as GaAs or GaAsP or GaP; however, it is also possible to reduce the traps with steps of intermediate AlInGaAs compositions from the composition of the quantum well to the composition of the quantum well barrier as described herein. It can be beneficial to inhibit direct transition from high Al and low In quantum well barriers to low Al and high In quantum wells. Multiple material types can also be used at the interface as long as the high Al and low In layers are not directly in contact with the high In and low Al layers.
The number of monolayers of the transitional layer can vary; however, from 2-3 monolayers can be adequate. For example, the transitional layer can include 1, 2, 3, 4, 5, or up to 9 or 10 monolayers of the materials described in order to prevent traps from being formed between the quantum well and quantum well barriers. The monolayers can each be about 2.5 A thick.
In one embodiment, the barrier layer, transitional layer, and quantum well sequence of semiconductor material can be prepared by molecular beam epitaxy (MBE). Also, the active region or whole semiconductor layers of a VCSEL can be produced with molecular beam epitaxy (MBE). Lower growth temperatures during the MBE can now be used to prepare the VCSEL semiconductor layers with the transitional layer between the quantum well and the quantum well barrier. The growth of these structures by MBE can be performed at <(less than) 500° C. Comparatively, the temperatures for MOCVD can be >(greater than) 600° C. Additionally, the VCSELs can be prepared by methods that are similar to MBE, such as GSMBE (gas source MBE) and MOMBE (metalorganic MBE) or the like that can produce the transitional layers between the quantum well and quantum well barrier as described.
Various aspects of the present invention will now be illustrated in the context of a VCSEL. However, those skilled in the art will recognize that the features of the present invention can be incorporated into other light emitting semiconductor devices that have an active region.
An isolation region 128 restricts the area of the current flow 130 through the active region 122. Region 128 can be formed by an ion implantation and/or oxidation. Other isolation regions may be used as is known or developed for VCSEL devices.
Mirror stacks 116 (bottom) and 124 (top) can be distributed Bragg reflector (DBR) stacks, and include periodic layers (e.g., 132 and 134). Periodic layers 132 and 134 are typically AlGaAs and AlAs, respectively, but can be made from other III-V semiconductor materials. Mirror stacks 116 and 124 can be doped or undoped and the doping can be n-type or p-type depending on the particular VCSEL design. However, other types of VCSEL mirrors may be used.
Metal contact layers 112 and 126 can be ohmic contacts that allow appropriate electrical biasing of VCSEL 100. When VCSEL 100 is forward biased with a voltage on contact 126 different than the one on contact 112, active region 122 emits light 136, which passes through top mirror stack 124. Those skilled in the art will recognize that other configurations of contacts can be used to generate a voltage across active region 122 and generate light 136, such as illustrated in
Confining region 120 can include a ramp region 146 that is positioned between active region 122 and high aluminum content region 144. As discussed below, the combination of high aluminum content region 144 and the ramp region 146 provide an injection structure with good carrier confinement and good electron injection.
Depending on the design of the VCSEL device and the thickness of high aluminum content regions 142 and 144, the confining regions 118 and 120 can optionally include spacer layers 148 and 150, respectively. The thickness of spacer layers 148 and 150 can be dependent upon the kind of VCSEL device being fabricated. In a vertical cavity resonant device such as a VCSEL, or VECSEL the spacer layers provide resonant spacing between mirrors and provide that the quantum wells of the active region are centered on a peak of the optical field if desired.
The confining layers 118 and 120 and active region 122 can be formed from one or more types of semiconductor materials, such as GaAs, AlAs, InP, AlGaAs, InGaAs, InAlAs, InGaP, AlGaAsP, AlGaInP, InGaAsP, InAlGaAs, SiGe, or the like.
In one example, the lower electrical confining layer is AlInP. In another example, the upper electrical confining layer can be AlInGaP.
The following description of the VSEL 400 can be used as an example; however, variations known in the art can be applied. The crystalline substrate 420 can be GaAs or other. The first mirror region 416 located on the GaAs substrate can have a plurality of first mirror layers having one or more indices of refraction. The first conduction region 414 can be operably coupled to the active region 412. The contact 428 can be associated with the first conduction region 414 so as to provide a path for electrons when the active region 412 is charged with electrical current. As described in more detail herein, the active region 412 can include one or more quantum wells bounded by one or more quantum well barrier layers, with a transitional layer between each quantum well and quantum well barrier. The oxide layer 422 can be any protective oxide such as silicon dioxide; however, protective nitrides or carbides may also be used. The second conduction region 410 can be operably coupled with the active region 412. The second mirror region 418 can be located on the second conduction layer and opposite of the active region, the second mirror region having a plurality of second mirror layers having one or more indices of refraction. The contact 424 can be any type of electrical contact for the conduction of electricity for operation of the active region. The laser output aperture 426 can be arranged in an operable VCSEL format.
In one embodiment, the VCSEL can be configured by modulating the quantum wells and quantum well barriers such that the spatial extent of the fundamental wavefunction for either of the electron or hole carriers is 85% or less than that of the other carrier, which enhances the matrix element. This can be done by making the physical size on one well less than that of the other well. For example using Sb at the boundary between common quantum wells and quantum well barriers widens the hole well relative to the electron well. Using Sb as a supplement in the well makes the electron well shallower, decreasing its confinement relative to the holes allowing some more spreading of the wavefunction relative to the holes. The carrier wavefunction relative spatial extent percentage can be less than or about 70, or less than or about 55%.
Such modulation can be obtained by use of the transitional layers at boundaries between quantum wells and quantum well barriers or by judicious choice of quantum well and barrier materials and dimensions. The enhancement of the matrix element is contemplated to benefit all semiconductor lasers and all devices which use quantum wells for optical interaction such as electroabsorption modulators.
In one embodiment, the quantum well barriers on either side of a quantum well can have a thickness of from about 40 A to about 100 A or about 45 A to about 75 A, or about 50 A to about 60 A, or about 55 A. The relative thinness improves gain saturation by decreasing diffusion lengths and increasing the minority carrier population over the quantum well. The relative thinness can also enhance carrier transport through the quantum wells with tunneling. In one embodiment, the quantum well can have a thickness of from about 40 A to about 100 A or about 45 A to about 75 A, or about 50 A to about 60 A, or about 50 A. Outer quantum well barriers that bound the active region can be thicker at 100 A to about 140 A, or about 110 A to about 130 A, or about 120 A to about 125 A, or about 130 A.
In one embodiment, the GaAs transitional layer thickness can be from about 5 A to about 20 A, or from about 5 A to about 15 A. The GaAsP transitional layer thickness can be from about 2.5 A to about 10 A, or from about 2.5 A to about 5 A. The GaAsPSb transitional layer thickness can range from about 2.5 A to about 10 A, or from about 2.5 A to about 5 A.
In one embodiment, an active region can include an InGaAs quantum well and both adjacent transitional layers can have a thickness of about 30 A to about 60 A, or from about 40 A to about 60 A. The AlInGaAs quantum well and both adjacent transitional layers can have a thickness of about 30 A to about 70 A, or about 40 A to about 70 A.
In one embodiment, the VCSEL can be grown without Be. Such a VCSEL can include about 85-100% AlGaAs as the p-type injection layer behind an AlInGaP upper electrical confining layer (See
In one embodiment, the VCSEL can include double oxide layers to reduce capacitance with flare on both, but primarily on the first oxide layer. That is, the oxide layer 422 of
In one embodiment, the VCSEL includes substantial periodic doping with heavy doping at nulls in high mobility materials. Also, the mirror can be configured to be not quite ¼ ¼. In some instances, the mirrors can begin adjacent the quantum wells or quantum well barriers with the first oxide at the first null.
To enhance speed and adjust wavelength, various quantities of In and P are added to the quantum wells. In can be added to the quantum wells to enhance speed. This results in a depression of the energy level to longer wavelengths so the wells are narrowed. The resulting wavefunction can penetrate into the quantum well barrier with its high density of states. To compensate for this, the barrier is grown to provide a maximum conduction band offset and provide the most carrier confinement. If the band offset is insufficient, then narrower wells can be used to allow the maximum In without significant penetration into the barriers.
An example of quantum wells designed using these principles is shown in the Simulase images of the wavefunctions in
The greater quantum well depth gives what is the greatest performance advantage to the InGaP—InGaAs quantum wells. If the gain spectra of the AlGaAs—InGaAs well is adjusted to roughly match that of the InGaP—InGaAs well then about 7% In is used (compared versus 12% In), and the valence band density of states becomes as shown in the density of states of
The gain is also reduced and transparency increased as shown in the gain of
Relaxation into the wells can be enhanced by having a narrow active region. The number of wells can be reduced to one well (
Table 1 shows various compositions that can be used in the quantum wells, barrier layers, and transitional layers as well as the thicknesses for an InGaAs quantum well. Here, any of the quantum well barriers and/or any of the transitional layers can be used with the InGaAs quantum well. The active regions having InGaAs quantum wells can be used in 14 Gbps lasers with enhanced reliability, and can be used in 28 Gbps lasers.
Table 2 shows various compositions that can be used in the quantum wells, barrier layers, and transitional layers as well as the thicknesses for an AlInGaAs quantum well. Here, any of the quantum well barriers and/or any of the transitional layers can be used with the AlInGaAs quantum well. The active region having the AlInGaAs quantum well can be used in 28Gbps lasers.
In one embodiment, the quantum well can have InGaAs and/or AlInGaAs and the barrier can include AlGaAs. However, the interface between InGaAs and AlGaAs an arsenic antisite defect trap level (EL2) commonly exists because of the strong difference in electronegativity of aluminum and indium. By placing an interfacial transition layer of GaAs between the AlGaAs and InGaAs so that In and Al compounds are not adjacent the antisite defect can be reduced dramatically. This interfacial transition layer can be the same dimensions as the GaP and GaAsP interfacial transition layers described herein, such as 1 monolayers, 2 monolayers, 3 monolayers, 4 monolayers, 5 monolayers, or less than 10 monolayers.
An embodiment of an GaAs transition layers separating Al containing quantum well barriers from In containing quantum wells is shown in
For the case of AlInGaAs quantum wells with AlGaAs quantum well barriers, a transition layer without indium, and with the Aluminum composition similar to that of the indium containing layer can used.
In addition, wide energy band gap barrier layers can be used to reduce the carrier population in the barrier layers, thus reducing recombination in the barrier layer on trap levels at or near the interface. This causes the trap levels to not be important for reliability.
In one embodiment, a VCSEL can be prepared to include an improved active region designed to reduce or eliminate reliability problems inherent in InGaAs—AlGaAs 850 nm VCSEL active regions for operation at, for example 14+ Gbps. The active region can have a transitional layer between quantum wells and barriers, the transitional layer having a thickness of 5-30 A. Such a transitional layer may reduce or eliminate recombination centers at the InGaAs—AlGaAs interface. Such recombination is believed to result in poor laser reliability in 850 nm VCSELS, therefore the present invention can reduce recombination in order to improve reliability. The higher bandgap of the transitional layer also allows for thicker InGaAs quantum wells, which can further improve reliability. It can be better for the quantum well thickness to be greater than 40 A.
An example of an improved active region includes: four In0.04Ga0.96As quantum well layers that are about 55-60 A thick; a Gap or GaAs, or GaAs0.35P0.65 transitional layer adjacent to each surface of each quantum well that is about 1-10 monolayers thick; a Al0.35Ga0.65As barrier layer that is about 50 A is adjacent to each transitional layer such that the transitional layers separate each of the quantum wells and barrier layer. In one aspect, the barrier layers that bound the active region can be larger than the barrier layers that are inside the active region, where the larger barrier layers can range from 50 to 130 A. Variations in amounts of elements in different layers can vary the laser between 850-900 nm.
In one embodiment, a VCSEL can include: a GaAs substrate; a first mirror region located on the GaAs substrate and having a plurality of first mirror layers having one or more indices of refraction; a first conduction region located on the first mirror region; and an active region located on the first conduction region opposite of the first mirror region. The active region can include: one or more quantum well layers; one or more quantum well barriers; and one or more transitional (e.g., interfacial) monolayers as described herein can be deposited between each of the quantum well layers and quantum well barriers. A second conduction region can be located on the active region opposite of the first conduction region. A second mirror region can be located on the second conduction layer and opposite of the active region, the second mirror region can have a plurality of second mirror layers having one or more indices of refraction.
In one embodiment, each quantum well and quantum well barrier can be separated by one or more transitional monolayers having GaP, GaAs, and/or GaAsP.
In one embodiment, the active region includes one or more transitional monolayers between the quantum well barrier and quantum well, which one or more transitional monolayers being formed from a third material selected such that group III interdiffusion and/or group V interdiffusion with the quantum well barrier and/or quantum well results in one or more transitional monolayers having a wider band gap compared to a low band gap interface that results from group III interdiffusion and/or group V interdiffusion between the quantum well barrier and quantum well without the one or more transitional monolayers.
In one embodiment, a VCSEL can include: one or more quantum wells having (Al)InGaAs; two or more quantum well barriers having Al(In)GaAs bounding the one or more quantum well layers; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier, wherein the quantum wells, barriers and transitional monolayers are substantially devoid of traps. In one aspect, the one or more transitional monolayers include GaP, GaAs, and/or GaAsP. In another aspect, the two or more transitional monolayers includes AlInGaAs with a barrier-side monolayer having lower In and higher Al compared to a quantum well side monolayer that has higher In and lower Al. In another aspect, the one or more transitional monolayers is devoid of Al and In.
In one embodiment, the VCSEL can include: one or more quantum wells having AlxIn1-z-xGazAs; and one or more quantum well barriers having Al1-j-kInjGakAs. Here, j ranges from 0 to 0.1; k ranges from 0.55 to 0.65; x ranges from 0 to 0.1; and z ranges from 0.7 to 0.96. Also, the one or more transitional monolayers can have GaAs1-wPw, wherein w ranges from 0.5 to 1.
In one embodiment, the VCSEL can include 3 transitional monolayers in the active region between each quantum well and quantum well barrier. The 3 transitional monolayers can have GaP, GaAs, and/or GaAsP.
In one embodiment, the active region can include: one or more quantum wells having In0.04Ga0.96As; one or more quantum well barriers having Al0.35Ga0.65As; and one or more transitional monolayers having GaAs0.35P0.65.
In one embodiment, the one or more transitional monolayers is sufficient to inhibit formation of a low gap interfacial layer between the quantum walls and quantum well barriers. Also, the one or more transitional monolayers can be configured to increase differential gain of the active region, wherein the increase of differential gain is compared to the VCSEL without the one or more transitional monolayers.
In one embodiment, the VCSEL can include an oxide layer between the one or more quantum well barrier layers and at least one of a first conduction region and a second conduction region bounding the quantum well barrier layers. The oxide layer can be a double oxide configured to reduce capacitance. Alternatively, the oxide layer can be at a first null with respect to the one or more quantum wells and an associated mirror region.
In one embodiment, a method for preparing a VCSEL can include using molecular beam epitaxy (MBE) for growing a crystalline structure having: one or more quantum wells; two or more quantum well barriers bounding each of the one or more quantum wells; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier.
In one embodiment, a method for preparing an active region of a VCSEL can include: (a) growing a quantum well barrier having Al(In)GaAs; (b) growing an transitional layer having one or more of GaP, GaAsP, or GaAs; (c) growing a quantum well layer having (Al)InGaAs; (d) growing another transitional layer having one or more of GaP, GaAsP, or GaAs; (e) repeating processes (a) through (d) over a plurality of cycles; and (f) growing a quantum well barrier having Al(In)GaAs.
In one embodiment, a VCSEL can include: one or more quantum wells having high In and low Al; two or more quantum well barriers having high Al and low In bounding the one or more quantum well layers; and one or more transitional monolayers deposited between each quantum well layer and quantum well barrier, wherein the quantum wells, barriers and transitional monolayers are substantially devoid of traps. For example, the one or more transitional monolayers can include GaP, GaAs, and/or GaAsP. Alternatively, the two or more transitional monolayers includes AlInGaAs with a barrier-side monolayer having lower In and higher Al compared to a quantum well side monolayer that has higher In and lower Al.
One skilled in the art will appreciate that, for this and other processes and methods disclosed herein, the functions performed in the processes and methods may be implemented in differing order. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations may be optional, combined into fewer steps and operations, or expanded into additional steps and operations without detracting from the essence of the disclosed embodiments.
The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.
The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected”, or “operably coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “operably couplable”, to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components.
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.
As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells, and so forth.
The present application claims the benefit of U.S. Provisional Application 61/453,851, filed on Mar. 17, 2011, and U.S. Provisional Application 61/453,635, filed on Mar. 17, 2011, which provisional applications are incorporated herein by specific reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61453851 | Mar 2011 | US | |
| 61453635 | Mar 2011 | US |
