PCT Application Attorney Docket: 72253-81000 PHOTOCONDUCTIVE SWITCHES BASED ON ULTRAWIDE BANDGAP SEMICONDUCTORS REFERENCE TO CROSS-RELATED APPLICATIONS This application claims priority to and benefit of U.S. Provisional Patent Application No: 63/567,043, filed on March 19, 2024, titled “PHOTOCONDUCTIVE SWITCHES BASED ON ALN and BN ULTRAWIDE BANDGAP SEMICONDUCTORS”, the content of which is incorporated herein in its entirety. TECHNICAL FIELD The present disclosure relates to ultrawide bandgap semiconductors, and more particularly to photoconductive semiconductor switches (PCSS) based on ultrawide bandgap semiconductor materials. GOVERNMENT INTEREST This invention was made with government support awarded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0001821. The government has certain rights to the invention. BACKGROUND It is widely recognized that the availability of power electronic devices based on ultrawide bandgap (UWBG) semiconductors (AlN, BN, and diamond) will create a more capable power grid and improve the future energy infrastructure, owing to their outstanding physical properties. When compared to classical power switches, photoconductive semiconductor switches (PCSS) possess the unique attributes of reliable ultrafast high voltage/current
PCT Application Attorney Docket: 72253-81000 operation with optical coupling of control circuits, compact geometry, negligible jitter time, and low inductance. GaN and SiC wide bandgap (WBG) semiconductors PCSS devices have been shown to outperform those based on Si and GaAs in terms of the operating voltage or current, but not both. This is because wide bandgap GaN and SiC PCSS devices need to employ semi-insulating materials, which were achieved by compensation doping using deep level acceptors, as illustrated in Fig.1A. This approach cannot accommodate high hold-off voltage and high photocurrent simultaneously because the total impurity concentration (both ionized) is too high to enable a high hold-off voltage, whereas the number of optically active compensated acceptors is generally too low to support a high photocurrent operation. In light of the aforementioned drawbacks, a PCSS device design is required which can overcome the current shortcomings, and have the ability to support a high hold-off voltage, high on-state current, and high frequency simultaneously. SUMMARY Disclosed herein are semiconductor materials and related PCSS devices, which utilize simple impurity doping of ultra-wide bandgap semiconductors. The UWBG materials disclosed here include aluminum nitride (AlN) and boron nitride (BN), and the impurity dopants include silicon (Si) and zirconium (Zr). Various embodiments of the PCSS devices are disclosed and employ either a lateral or vertical configuration. The PCSS devices of the present invention incorporate an optional substrate layer, an ultra-wide bandgap semiconductor active layer, at least a first electrode and second electrode. References to a first and second electrode can refer to either an anode or a cathode. The semiconductor active layer of these devices is comprised of an UWBG material doped with an impurity.
PCT Application Attorney Docket: 72253-81000 In certain embodiments, the UWBG material is AlN and the impurity dopant is Zr. The substrate layer for AlN:Zr devices can be n-type SiC, sapphire, AlN or h-BN. The thickness of the substrate layer can vary between 100 microns to 1.0 mm. In one embodiment, the PCSS devices can incorporate thin layers of highly conductive n-type Si doped AlxGa1-xN and GaN contact layers, which serve to decrease the contact resistance and eliminate device damage under the contact due to high local electric fields. In various embodiments, Si doped AlxGa1-xN contact layers are deposited on a bottom side and top side of semiconductor active layers. Additional embodiments will be described in more detail sections that follow and in the accompanying figures. Selected Definitions As used herein, the term “ultra-wide bandgap” semiconductor or material, or the term “UWBG” semiconductor or material, are used interchangeably and refer to semiconductor materials which have bandgaps larger than traditional wide-bandgap semiconductor materials, having a bandgap exceeding 3.4 eV. Examples of UWBG materials include diamond, boron nitride, gallium oxide, aluminum nitride. As used herein, the term “semiconductor layer”, “semiconductor active layer”, or “active layer” are used interchangeably throughout this disclosure and refer to UWBG layers. The term “about” is used in conjunction with numeric values to include normal variations in measurements as expected by persons skilled in the art, and is understood to have the same meaning as “approximately” and to cover a typical margin of error, such as ±15%, ±10%, ±5%, ±1%, ±0.5%, or even ±0.1% of the stated value. The term “about” also
PCT Application Attorney Docket: 72253-81000 encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial composition. Whether or not modified by the term “about,” the claims include equivalents to the quantities. It should be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound” includes having two or more compounds that are either the same or different from each other. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. As used herein, “and/or” refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative (“or”). In the interest of brevity and conciseness, any ranges of values set forth in this specification contemplate all values within the range and are to be construed as support for claims reciting any sub-ranges having endpoints which are real number values within the specified range in question. By way of a hypothetical illustrative example, a disclosure in this specification of a range of from 1 to 5 shall be considered to support claims to any of the following ranges: 1-5; 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5. The term “substantially” is utilized herein to represent the inherent degree of uncertainty that can be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation can vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. The term “comprise,” “comprises,” and “comprising” as used herein, specify the presence of the stated features, integers, steps, operations, elements, and/or components,
PCT Application Attorney Docket: 72253-81000 but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the transitional phrase “consisting essentially of” means that the scope of a claim is to be interpreted to encompass the specified materials or steps recited in the claim and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. Thus, the term “consisting essentially of” when used in a claim of this invention is not intended to be interpreted to be equivalent to “comprising.” The terms “preferred” and “preferably” refer to embodiments that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the present disclosure. The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one component or material with respect to other components or materials where such physical relationships are noteworthy. For example in the context of materials, one material or material disposed over or under another may be directly in contact or may have one or more intervening materials. Moreover, one material disposed between two materials or materials may be directly in contact with the two layers or may have one or more intervening layers. In contrast, a first material or material “on” a second material or material is in direct contact with that second material/material. Similar distinctions are to be made in the context of component assemblies. As used throughout this description, and in the claims, a list of items joined by the term “at least one of” or “one or more of” can mean any combination of the listed terms.
PCT Application Attorney Docket: 72253-81000 For example, the phrase “at least one of X, Y or Z” can mean X; Y; Z; X and Y; X and Z; Y and Z; or X, Y and Z. BRIEF DESCRIPTION OF THE DRAWINGS Other advantages of the present invention will be apparent from the following detailed description of the invention in conjunction with embodiments as illustrated in the accompanying drawings, in which: FIG. 1A generally depicts an energy diagram illustrating excitation mechanisms of PCSS operation in Iron (Fe) compensation doped GaN or Vanadium (V) compensation doped 4H-SiC. FIG.1B generally depicts an energy diagram illustrating excitation mechanisms of PCSS operation in Silicon (Si) doped h-BN and Zirconium (Zr) doped AlN. FIG. 1C generally depicts an energy diagram of PCSS operation with impurities in h-BN and AlN, which can be acceptors (A) that provide photo-excited holes in the valence band under optical excitation. FIGS. 2A-2D generally illustrates aluminum nitride (AlN) PCSS layer structures in accordance with various embodiments described herein. Fig. 2A illustrates a zirconium (Zr) doped AlN (AlN:Zr) wafer deposited on highly conductive n-type SiC substrate; Fig. 2 B shows a AlN:Zr wafer deposited on highly n-type SiC substrate by incorporating highly conductive n-type AlxGa1-xN:Si and GaN:Si contact layers; Fig. 2C illustrates an AlN:Zr wafer deposited on highly insulating substrate including sapphire, AlN, and h-BN; and Fig. 2D shows an AlN:Zr wafer incorporating thin contact layers of highly conductive n-type AlxGa1-xN:Si and GaN:Si on both sides.
PCT Application Attorney Docket: 72253-81000 FIGS.3A-3B illustrates boron nitride (BN) PCSS layer structures in accordance with embodiments described herein. Fig. 3A depicts an Si doped h-BN (h-BN:Si) wafer deposited on sapphire or h-BN substrate. Fig 3B illustrates a h-BN:Si wafer incorporating thin contact layers of highly conductive graphene or h-(BN)1-x(C2)x on a top side and bottom side of the h-BN:Si wafer. FIGS. 4A-4C generally depict methods of fabrication of freestanding AlN:Zr and h-BN:Si wafers. Fig.4A shows a sapphire substrate removal from AlN:Zr by laser lift-off; Fig. 4B shows an h-BN substrate removal from AlN:Zr by mechanical force; and Fig. 4C illustrates a h-BN:Si wafer self-separating from sapphire substrate during cooling down after growth. FIGS.5A-5B illustrates lateral geometry of AlN PCSS devices in accordance with various embodiments disclosed herein. Fig. 5A shows a lateral AlN PCSS devices fabricated from AlN:Zr wafers deposited on highly insulating sapphire or AlN substrate by employing circular contacts; Fig. 5B shows a lateral AlN PCSS devices fabricated from freestanding AlN:Zr wafers lift-off from substrates by employing circular contacts. FIGS. 6A-6D depicts AlN PCSS devices having vertical geometry. Fig. 6A shows a vertical AlN PCSS device fabricated from AlN:Zr wafers deposited on highly conductive n-type SiC substrate by employing circular contact for the anode; Fig. 6B shows top view (left) and bottom view (right) of a vertical AlN PCSS device fabricated from freestanding AlN:Zr wafers lift-off from substrates by employing circular contacts; Fig. 6C depicts a top view of a vertical AlN PCSS device fabricated from freestanding AlN:Zr wafers lift-off from substrates by employing circular contacts and incorporating thin layers of highly conductive n-type Si doped AlxGa1-xN:Si and GaN:Si as contact layers to reduce the contact resistance; Fig.6D is a bottom view of the device shown in Fig.6C.
PCT Application Attorney Docket: 72253-81000 FIGS.7A-7D illustrates lateral PCSS devices in accordance with embodiments disclosed herein. Fig.7A shows a lateral h-BN PCSS device fabricated from h-BN:Si wafers deposited on h-BN substrates; Fig. 7B shows a lateral h-BN PCSS device fabricated from freestanding h-BN:Si wafers lift-off from substrate; Fig.7C shows a lateral h-BN PCSS device fabricated from freestanding h-BN:Si wafers with highly conductive contact layer of graphene or h-(BN)1-x(C2)x to reduce contact resistance; Fig. 7D shows a lateral h-BN PCSS device fabricated from h-BN:Si strips with a width of d by depositing the two electrodes on the strip edges. FIGS. 8A and 8B illustrates vertical PCSS devices in accordance with embodiments disclosed herein. Fig 8A shows a vertical h-BN PCSS device fabricated from freestanding h-BN:Si wafers lift-off from substrate; Fig. 8B shows a vertical h-BN PCSS device fabricated from freestanding h-BN:Si wafers with conductive contact layers of graphene or h-(BN)1-x(C2)x deposited on the opposite sides of the wafer to reduce contact resistance. FIG.9A shows a PCSS device with a C-plane sapphire substrate, an AlN buffer layer and an AlN:Zr wafer. FIG. 9B illustrates a plot of the Zr concentration profile measured by SIMS, revealing a uniform Zr concentration of 1020 cm-3 along the c-axis in the entire AlN:Zr epilayer. FIG. 10A shows an XPS survey spectrum performed on an AlN:Zr epilayer with a measured Zr concentration by SIMS of 1020 cm-3. FIG 10 B shows an XPS tight scan of Zr (3d) peaks in an c epilayer with a measured Zr concentration by SIMS of 1020 cm-3. FIG. 11 shows an embodiment of a PCSS device in accordance with embodiments disclosed herein.
PCT Application Attorney Docket: 72253-81000 FIG. 12A shows a vertical PCSS with a AlN:Zr active layer and n-AlGaN contact layer, in accordance with an embodiment disclosed herein. FIG. 12B shows a different embodiment of a vertical PCSS with a AlN:Zr active layer and n-AlN, in accordance with embodiments disclosed herein. DETAILED DESCRIPTION [0022] Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. [0023] Disclosed herein are semiconductor materials and related PCSS devices, which utilize simple impurity doping of ultra-wide bandgap semiconductors. The UWBG materials disclosed here include aluminum nitride (AlN) and boron nitride (BN), and the impurity dopants include silicon (Si) and zirconium (Zr). [0024] Existing PCSS devices cannot satisfy the requirements of high voltage/high current/high frequency/compactness operation simultaneously. The use of GaN and SiC wide
PCT Application Attorney Docket: 72253-81000 bandgap (WBG) semiconductors has improved the operating voltage or current, but not both. This is because WBG PCSS devices need to employ semi-insulating materials, which were achieved by doping of deep acceptors to compensate donors, as illustrated in Fig.1A. The deep acceptors trap electrons from shallow donors. The electrons are then optically excited from the deep acceptors into the conduction band to switch GaN or SiC materials from an insulating (Off) to a conducting (On) state. This approach cannot accommodate high hold-off voltage and high photocurrent simultaneously because the total impurity concentration (shallow donors + deep acceptors) is generally too high to enable high voltage operation, whereas the number of optically active compensated acceptors is generally too low to support a high photocurrent operation. [0025] As shown in Fig.1B, Silicon (Si) and Zirconium (Zr) impurities in hexagonal BN (h-BN) and AlN ultrawide bandgap (UWBG) semiconductors are simple donors and sufficiently deep to provide very low dark current in the Off-state but are shallow enough to be excited by low energy photons using widely available compact and fast pulsed infrared laser diodes to turn-on the device into the On-state. Moreover, the carrier scattering mechanism in Fig.1B is dominated by the neutral impurity scattering which is known to have less detrimental impact on carrier mobility. Most importantly, UWBG semiconductors have much higher breakdown fields. These properties allow the realization of extrinsic PCSSs with abilities to support optical trigger, high hold-off voltage, high on-state current, high frequency, and compactness operation simultaneously. Likewise, as shown in Fig. 1C, the high electrically resistive state (Off-state) and low electrical resistive state (On-state) can also be accomplished by incorporating simple acceptor impurities which are sufficiently deep to provide very low dark current in the Off-state but are shallow enough to be excited by low energy photons using widely available compact and fast pulsed infrared laser diodes (LDs) to turn-on the device into the On-state.
PCT Application Attorney Docket: 72253-81000 Most simple impurities doped UWBG semiconductors are sufficiently deep (e.g., the substitutional donors of Si in h-BN and Zr in AlN are located at 1.2 eV and 1.37 eV below the conduction band edges of h-BN and AlN, respectively). These energy levels are very favorable for PCSS applications. First, these levels are sufficiently deep to yield very low free electron concentration values in the Off state (ndark). For example, Si donors in h-BN have an ionization energy of 1.2 eV, which yields an extremely low electron concentration in the Off- state of ndark 1.4 x 10-21 ND
≈ 1 cm-3. (1) Secondly, as shown in Fig. 1, the mobility of photoexcited electrons in h-BN and AlN PCSS devices are affected primarily by neutral impurity scattering, as opposed to ionized impurity scattering in SiC and GaN PCSSs. Third, the energy levels, 1.2 eV and 1.37 eV match very well with compact laser diodes (LDs) operating at 980 nm and 905 nm. These LDs are available at low cost and capable of providing high optical power and nanosecond trigger pulses and can easily be integrated with PCSS chips to form compact packages. Lastly, the intrinsic breakdown fields (EC) of AlN and h-BN are several times larger than SiC and GaN. These attributes together allow the realization of extrinsic PCSSs with abilities to support optical trigger, high hold-off voltage, high on-state current, high frequency, and compactness operation simultaneously without invoking compensation doping. Compared to intrinsic PCSS devices which uses band-edge photons as a trigger source, extrinsic PCSS devices support a large optical absorption length, which can effectively utilize the entire wafer thickness of the UWBG material,
PCT Application Attorney Docket: 72253-81000 In analogies to traditional power electronic devices, we can also define “figures- of-merit” for PCSS devices. For PCSS devices designed to support a desired operating voltage V, the minimum device thickness (d) is related to the critical field (EC) of the PCSS material by V =d⋅Ec. (2) For simplicity, neglecting the surface reflectivity and assuming 100% optical pump efficiency, the On-state switch resistance, which describes energy loss in the On-state can be written as,
On the other hand, PoT ∆N=device volume⋅(∆n) = (dA)(∆n) ∝ (4) ℎν where µ=carrier mobility, ∆n=photogenerated carrier density, ∆N=total number of photogenerated carriers. Po= optical power of the excitation pumping laser, T = pulse duration of the pumping laser, PoT = optical energy of the pumping laser, ℎν = photon energy of the pumping laser. These together provide, the minimum
PCT Application Attorney Docket: 72253-81000
A previous extensive examination of numerous references reveals that the dependence of the critical field (Ec) on the bandgap (Eg) follows Ec ∝ ^^^^^ 1 ^^^ .83 [5], we can also express minimum Ron as a function the bandgap as
Equations (5) and (6) show that the minimum On-state resistance of PCSS devices is proportional to 1/^^^^^ ^ ^^ ^^ ^^ or to 1/^^^^^ ^ ^^ ^^ ^^.^^^^. Similarly, the maximum operating current capability in the On-state can be written as,
o Therefore, the operating current capability scales with ^^^^^ ^ ^^ ^^ ^^ or to ^^^^^ ^ ^^ ^^ ^^.^^^^. Physical parameters of various semiconductors are summarized in Table 1. It can be seen that PCSS devices based on ultrawide bandgap (UWBG) semiconductors (AlN, BN, diamond) which all have bandgaps around Eg ≈ 6 eV and large critical field values are expected to dramatically outperform those based traditional semiconductors such as Si with a bandgap of Eg (Si) ∼1.1 eV by a factor of [Eg/Eg(Si)]3.7 ≈760 and even those based on wide bandgap semiconductors such as SiC and GaN with bandgaps of about Eg(GaN) ∼ 3.4 eV by a factor of approximately of [Eg/Eg(GaN)]3.7≈8.
PCT Application Attorney Docket: 72253-81000 Table 1.
The realization of UWBG PCSS switches will also open new applications in simulation of nuclear weapon effects, for the active interrogation of special nuclear materials, in aircraft radars, vehicle collision avoidance systems, as well as in UWBG sensor systems by providing enhanced performance, efficiency, and reliability, in concert with reductions in size, weight, and operational costs. PCSS device layer structures Various embodiments of the PCSS devices disclosed herein are shown in Figs. 2-9A and Fig.11. The devices incorporate a substrate layer 100 (this layer is optional), an ultra- wide bandgap semiconductor active layer 200, a first electrode 700 and second electrode 800 (shown in Figs.5-10). References to a first and second electrode can refer to either an anode or a cathode. The semiconductor active layer 200 is comprised of an UWBG material doped with an impurity.
PCT Application Attorney Docket: 72253-81000 In certain embodiments, the UWBG material is AlN and the impurity dopant is Zr. PCSS device layer structures for AlN:Zr embodiments can be seen in Figs.2A-2D. The substrate layer 100 for AlN:Zr devices can be n-type SiC, sapphire, AlN or h-BN, as shown in Figs.2A through 2D. The thickness of the substrate layer can vary between 100 microns to 1.0 mm. In one embodiment, the PCSS devices can incorporate thin layers of highly conductive n-type Si doped AlxGa1-xN and GaN as contact layers (300 and 400, respectively in Figs. 5A and 5B) to decrease the contact resistance and eliminate device damage under the contact due to high local electric fields. Fig.2D additionally shows an embodiment wherein Si doped AlxGa1-xN contact layers are deposited on a bottom side and top side of semiconductor layer 200. In other embodiments, the UWBG layer comprises hexagonal boron nitride (h- BN), doped with silicon (Si). These embodiments can be seen in Figs. 3A and 3B. The embodiment shown in Fig.3B shows a h-BN:Si semiconductor active layer 200 incorporating thin contact layers 300A and 300B of highly conductive graphene or h-(BN)1-x(C2)x on a top side and bottom side of active layer 200. Lateral and vertical PCSS devices The placement and location of the anode and cathodes will now be described in specificity. PCSS devices generally employ two device geometries: (1) Lateral geometry - in which the anode and cathode contacts are fabricated on the same side of the wafer and (2) vertical geometry – in which the anode and cathode contacts are fabricated on the opposite side of the wafer. Vertical devices have the advantages of minimizing electric field enhancements
PCT Application Attorney Docket: 72253-81000 and distributing current throughout the bulk of the material, whereas lateral devices are more easily scaled. The embodiment shown in Fig. 5A discloses a lateral AlN PCSS device fabricated from AlN:Zr active layer 200 deposited on highly insulating sapphire or AlN substrate 100 by employing circular contacts (anode 700 and cathode 800). Figure 5B shows a lateral AlN PCSS device fabricated from freestanding AlN:Zr wafers lift-off from substrates and employing circular contacts (anode 700 and cathode 800). Both devices have the option to incorporate thin layers of highly conductive n-type Si doped AlxGa1-xN:Si 300 and GaN:Si 400 contact layers to decrease the contact resistance and eliminate device damage under the contact due to high local electric fields. When highly conductive contact layers are incorporated to reduce the contact resistance, a gap between the anode 700 (first electrode) and cathode 800 (second electrode) contact needs to be created by etching to ensure that the field is applied to AlN:Zr active layer 200. The gap spacing between anode and cathode contacts (d) then defines the operating voltage (V) according to Eq.2. The embodiment illustrated in Fig. 6A shows vertical AlN PCSS devices fabricated from AlN:Zr active layer 200 deposited on highly conductive n-type SiC substrates 100, in which n-SiC serves as contact layer for cathode 800 contact. The devices have the option to incorporate thin layers of highly conductive n-type Si doped AlxGa1-xN:Si and GaN:Si as contact layers to reduce the contact resistance of metal contacts. Figure 6B shows a top view and bottom view of vertical AlN PCSS devices fabricated from freestanding AlN:Zr wafers lift-off from insulating substrates. As can be seen a first electrode is on a top side of the device, and a second electrode is on an opposite bottom side of the device, in this vertical configuration. Figure 6C shows vertical AlN PCSS devices fabricated from freestanding AlN:Zr active layer 200 which incorporate highly conductive alternating n-type AlxGa1-xN:Si
PCT Application Attorney Docket: 72253-81000 and GaN:Si contact layers to reduce contact resistance. In these vertical devices, the device thickness (d) is the thickness of the AlN:Zr layer and defines the operating voltage (V) according to Eq.2. Figure 7A shows lateral h-BN PCSS devices fabricated from h-BN:Si wafers deposited on highly insulating and undoped h-BN substrate by employing circular contacts. Figure 7B shows lateral h-BN PCSS devices fabricated from freestanding h-BN:Si wafers lift- off from substrates by employing circular contacts. Both devices have the option to incorporate thin layers of highly conductive graphene or n-type h-(BN)1-x(C2)x as contact layer to decrease the contact resistance and eliminate device damage under the contact due to high local electric fields. When highly conductive contact layers are incorporated to reduce the contact resistance, a gap between the anode contact and cathode contact needs to be created by etching to ensure that the field is applied to h-BN:Si wafer. The gap spacing between anode and cathode contacts (d) then defines the operating voltage (V) according to Eq.2. Figure 7C shows another design of lateral PCSS devices which are fabricated from freestanding h-BN:Si wafers, in which the circular shaped anode contact is deposited on the center, while the cathode contact is a circular shaped ring deposited outside the anode. The graphene or n-type h-(BN)1-x(C2)x in the spacing (d) between the anode and cathode is removed by etching all the way down to active h-BN:Si layer’s surface. The PCSS spacing (d) then defines the operating voltage (V) according to Eq. 2. Figure 7D shows another design of lateral PCSS devices which are fabricated by dicing h- BN:Si wafers into strips with a width d, in which the anode and cathode contacts are deposited on the two edges of the strip. The PCSS strip width (d) then defines the operating voltage (V) according to Eq.2. Figure 8A shows a top view (left) and bottom view (right) of vertical h-BN PCSS devices fabricated from freestanding h-BN:Si active layer by employing circular
PCT Application Attorney Docket: 72253-81000 contacts. Figure 8B shows top view (left) and bottom view (right) of vertical AlN PCSS devices fabricated from freestanding h-BN:Si active layer by employing circular contacts and incorporating thin layers of highly conductive graphene or n-type h-(BN)1-x(C2)x as contact layers to decrease the contact resistance and eliminate device damage under the contact due to high local electric fields. In this embodiment a first contact layer and second contact layer is shown on a top side/surface and a bottom side/surface of the active layer 200. The thickness (d) of h-BN:Si then defines the operating voltage (V) according to Eq.2. A further embodiment shown in Fig. 11 discloses a PCSS device 1200. This embodiment is another design of a vertical AlN:Zr PCSS device structure deposited on AlN bulk substrate 1210 to utilize the excellent lattice match between AlN substrate 1210 and active layer AlN:Zr 1220. By incorporating a Si doped n-type AlGaN contact layer 1230a on top surface of active layer 1220, and a second Si doped n-type AlGaN contact layer 1230B on bottom surface of active layer 1220, the contact resistance and the overall resistance in the on- state will be greatly reduced. The structure is etched all the way to the surface of the layer 1230B to allow the deposition of metal cathode contact 1240. The top circular shaped metal contact serves as an anode 1250. The thickness of the AlN substrate can vary between 100 microns to 1 millimeter. In another embodiment, as shown in Figs.12A and 12B an AlN:Zr active layer 200 is deposited on conductive n-type SiC (n-SiC) substrate 100. Electrically conductive n-SiC substrate 100 naturally serves as the bottom contact/current spreading layer (cathode 800). The device structure can incorporate conductive n-type (n-AlN) 300 (Fig. 12B) or n-type Al-rich AlGaN (n-AlGaN) (Fig.12A) to serve as a current spreading contact layers 300 to reduce the contact resistance. This device structure does not require etching process.
PCT Application Attorney Docket: 72253-81000 The growth methods for producing AlN and BN epitaxial materials are known in the art and include chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), and plasma-enhanced chemical vapor deposition (PECVD). Simple donor impurities, including Si in BN and Zr in AlN, will be incorporated into these crystals during growth of the epitaxial layers (in-situ doping) or by ion-implantation after crystal growth is complete. Several layer structures of AlN and h-BN PCSS devices disclosed here are illustrated in Figs.2-3. Methods to form freestanding AlN:Zr and h-BN:Si wafers In certain instances, it is advantageous to remove the substrate from the active semiconductor layer before fabricating PCSS devices. As shown in Fig.4A, for AlN:Zr wafers deposited on sapphire, freestanding AlN:Zr wafers can be obtained by laser lift-off method, in which the AlN/sapphire interface will melt by absorbing laser energy and sapphire substrate can then be lift-off. As shown in Fig. 4B, for AlN:Zr wafers deposited in h-BN substrate, freestanding AlN:Zr wafers can be lift-off from h-BN by mechanical force because h-BN has a layered crystalline structure. Additionally, as shown in Fig.4C, h-BN:Si wafer deposited on sapphire substrate will self-separate from sapphire during colling down after growth because h-BN has a layered crystalline structure. Examples Epitaxial growth of AlN:Zr epilayers by MOCVD is disclosed herein. To produce AlN:Zr, Trimethylaluminum (TMAl) and NH3 were used as Al and N precursors, respectively and hydrogen was used as carrier gas. The metal-organic (MO) Zr doping source, Tetrakis(dimethylamino)zirconium (TDMAZr, Zr[N(CH3)2]4) was carried into the reactor using hydrogen gas. AlN:Zr epilayers of 1.8 microns were deposited directly on c-plane
PCT Application Attorney Docket: 72253-81000 sapphire via a 20 nm thick low temperature undoped AlN buffer layer grown at 7500 C, as schematically illustrated in Fig. 9A. With no intermediate undoped AlN epilayer inserted in between the substrate and doped layer, this simple structure allows examination of AlN:Zr epilayer properties without complications and rapid feedback to improve growth conditions. The growth temperature for AlN:Zr epilayer was 12500 C providing a growth rate of around 0.9 microns/h. Secondary ion mass spectrometry (SIMS) measurements were performed to determine the Zr doping concentration in AlN:Zr epilayers. X-ray photoelectron spectroscopy (XPS) technique (Physical Electronics PHI 5000 Versa Probe II Hybrid) was used in tandem to SIMS to characterize the elemental compositions of the epilayers and the measurement was done in a high vacuum (10-7 torr) chamber. An argon (Ar) ion gun was used to etch the surface to remove any dust and surface adsorbents prior to survey scans. The epilayer crystalline quality was monitored by X-ray diffraction (XRD). XRD in both ω -2θ scan and ω -scan (or rocking curves) were measured to assess the effects of Zr doping to AlN:Zr crystalline quality. To the best of our knowledge, this represents the first MOCVD growth study for producing AlN:Zr epilayers. Figure 9B plots the Zr concentration profile measured by SIMS for a representative AlN:Zr sample, revealing a uniform Zr concentration of 1020 cm-3 along the c- axis in the entire AlN:Zr epilayer. The results demonstrated that very high Zr concentrations can be attained in AlN:Zr. Based on theoretical insights, Zr dopants substitute on the Al sites (ZrAl) with an energy level at ~1.4 eV below the conduction minimum. Thus, in absence of compensating defects or impurities, ZrAl donors are neutral during growth, which contrasts to the case of Si doping in AlN, in which mostly SiAl donors are ionized during growth owing to a much lower ionization energy of SiAl donors in AlN.
PCT Application Attorney Docket: 72253-81000 Figure 10A is a plot of XPS survey spectrum and Fig. 10B of narrow energy scan of Zr (3d) core level of an AlN:Zr epilayer. The Zr concentration estimated from the survey spectrum shown in Fig. 2 (a) is around 0.3%. As XPS is a surface analysis technique (depth ~10 nm), the estimated Zr concentration from XPS is not necessarily the precise value inside AlN:Zr, but we find a strong correlation between the Zr concentrations estimated from XPS and SIMS results. Figure 10A shows an XPS tight scan of the Zr (3d5/2) core level of an AlN:Zr sample, revealing a clear peak near 180.9 eV, corresponding to Zr bonding with N (Zr- N) , i.e., Zr 3d with nearest neighbors of N. An additional peak near 183.3 eV is due to Zr 3d5/2-3d3/2 orbital splitting energy of ∆~2.4 eV. XPS results thus clearly demonstrated that Zr substitutes on the Al site and forms donor (ZrAl) in AlN, in agreement with theoretical prediction. The narrow line widths of these two peaks indicate high crystalline quality of Zr doped AlN epilayers. While not shown, tight scans of Al 2p and N 1s revealing binding energies of 73.5 eV and 396.6 eV, corresponding to Al bonding with nearest neighbors of N and N bonding with nearest neighbors of Al, respectively, are in good agreement with previous reported results. In summary, MOCVD growth of Zr doped AlN epilayers (AlN:Zr) on c-plane sapphire using MOCVD has been conducted. By first establishing the capability for producing exceptionally high crystalline quality undoped AlN epilayers exhibiting a FWHM of (002) XRD rocking curve as low as 20 arcsec, AlN:Zr with a Zr concentration up to [NZr]=1020 cm-3 have been produced. The incorporation of Zr was confirmed by SIMS, XPS, XRD, optical absorption and PL measurement results. High crystalline quality of AlN:Zr was confirmed via the observation of a very narrow FWHM of (002) XRD rocking curve of 216 arcsec at [NZr]=1020 cm-3, in comparison to a FWHM greater than degrees reported previously for AlN:Zr materials produced by other techniques. It was observed that the c-lattice constant of AlN:Zr (4.992 Ǻ at NZr=1020 cm-3) is slightly enlarged over that of undoped AlN epilayer of
PCT Application Attorney Docket: 72253-81000 the same thickness (4.980 Ǻ) due to the fact that Zr substitutes on the Al site and the atomic size of Zr is larger than that of Al. The formation of (ZrAl-VN) complexes, which were predicted to possess all the desired properties of quantum qubits, was confirmed by optical absorption spectroscopy. While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.