Methods and non-transitory computer readable medium for near surface work function engineering

Abstract

A method for adjusting the work function of a structure in a substrate utilizes near-surface doping. In some embodiments, a method for adjusting the work function of a structure in a substrate may include: growing an epitaxial layer on the surface of the structure to form a homogeneous passivation region as part of the substrate material of the substrate; and performing a dopant diffusion process to further embed such dopants into the surface of the structure to adjust the work function of the structure, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius.

Description

Technical Field

[0001] The embodiments of this principle are generally related to semiconductor processing of semiconductor substrates. Prior Technology

[0002] Structures such as trenches are typically formed on a substrate as part of the construction of semiconductor devices. Extensive processing and high temperatures are often used to alter the work function of the structure. However, the inventors have discovered that during such processes, high temperatures can damage surrounding components and also lead to the formation of large charge gradient regions within the structure, thereby reducing the area available for other component layers and resulting in a significant performance degradation.

[0003] Therefore, the inventors have provided improved processes for near-surface work function engineering, which significantly improve the efficiency and quantum efficiency of the structure. Summary of the Invention

[0004] This paper provides methods and structures for improved near-surface work function engineering.

[0005] In some embodiments, a method for adjusting the work function of a structure on a substrate may include: growing an epitaxial layer on the surface of the structure to form a homogeneous passivation region with dopants as part of the substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surface of the structure to adjust the work function of the structure, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius.

[0006] In some embodiments, the method may further include: wherein the dopant diffusion process generates a charge of up to about 3e18 / cm3 to about 3e21 / cm3 positive or negative on the surface of the structure, wherein the dopant is p-type or n-type, wherein the dopant diffusion process forms a steep junction profile, wherein the method is performed in a back-end-of-line (BEOL) process, wherein the epitaxial layer is formed from a single crystal, wherein the epitaxial layer is an amorphous layer, and the epitaxial layer is grown using a low-temperature process below about 450 degrees Celsius; forming the structure using an etching process; forming an oxide layer with a controllable oxide thickness of about 1 nm to about 15 nm on the surface of the structure using a dry oxide process at a temperature below about 450 degrees Celsius; and selectively removing the oxide layer from the surface of the structure prior to growing the epitaxial layer, wherein the dry oxide process is performed in a plasma oxidation chamber; and / or etching the structure into the substrate to a high aspect ratio greater than about 75:1.

[0007] In some embodiments, a method for adjusting the work function of a structure on a substrate may include: forming an amorphous material layer on the surface of the structure using a low-temperature process to form a homogeneous passivation region with dopants as part of the substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surface of the structure, thereby adjusting the work function of the structure and forming an oxide layer from the amorphous layer, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius and forms a charge layer with a steep junction profile.

[0008] In some embodiments, the method may further include the following steps: wherein the dopant diffusion process generates a charge of up to about 3e18 / cm3 to about 3e21 / cm3 in the surface of the structure, wherein the dopant is p-type or n-type, wherein the method is performed in a back-end-of-line (BEOL) process, wherein the dopant diffusion process is a condensation-based process that embeds the dopant and forms the oxide layer, and / or wherein the amorphous material layer is formed at a temperature below 450 degrees Celsius, and the dopant diffusion process is performed at a temperature below 450 degrees Celsius.

[0009] In some embodiments, a non-transitory computer-readable medium is disclosed, the non-transitory computer-readable medium storing instructions that, when executed, cause a method for adjusting the work function of a structure in a substrate to be performed, the method including: growing an epitaxial layer on the surface of the structure to form a homogeneous passivation region as part of the substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surfaces of the structure to adjust the work function of the structure, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius.

[0010] In some embodiments, the method for the non-transitory computer-readable medium may further include: wherein the dopant diffusion process generates up to a positive or negative charge of about 3e18 / cm3 to about 3e21 / cm3 on the surface of the structure and forms a steep junction profile, and / or grows the epitaxial layer using a low-temperature process below about 450 degrees Celsius.

[0011] Other and further embodiments are disclosed below. Simple Explanation of the Diagram

[0012] The embodiments of this principle, which are briefly summarized above and discussed in more detail below, can be understood by referring to the illustrative embodiments depicted in the accompanying drawings. However, the drawings only illustrate typical embodiments of this principle and should not be considered as limiting the scope, as other equally effective embodiments are permissible with respect to this principle.

[0013] Figure 1 illustrates a method for adjusting the work function of the surface of a trench structure in a substrate according to some embodiments of this principle.

[0014] Figure 2A depicts a cross-sectional view of a trench structure after an etching process, according to some embodiments of this principle.

[0015] Figure 2B depicts a cross-sectional view of a trench structure following a dry oxide process according to some embodiments of this principle.

[0016] Figure 2C illustrates a cross-sectional view of a trench structure following a selective oxide removal process, according to some embodiments of this principle.

[0017] Figure 2D depicts a cross-sectional view of a trench structure after the formation of the passivation region, according to some embodiments of this principle.

[0018] Figure 2E depicts a cross-sectional view of a trench structure after optional gas doping in the passivation region, according to some embodiments of this principle.

[0019] Figure 2F depicts a cross-sectional view of a trench structure after adjusting the work function according to some embodiments of this principle.

[0020] Figure 2G depicts a cross-sectional view of a trench structure following an alternative method of dopant diffusion process according to some embodiments of this principle.

[0021] Figure 3 depicts an integration tool based on some embodiments of this principle.

[0022] Figure 4A depicts the first process of a second method according to some embodiments of this principle.

[0023] Figure 4B depicts a second process of a second method according to some embodiments of this principle.

[0024] To facilitate understanding, the same reference numerals are used to denote common elements in the figures where possible. The figures are not drawn to scale and may be simplified for clarity. Elements and features of one embodiment may be advantageously incorporated into other embodiments without further description. Implementation

[0025] These methods provide a high-performance near-surface work function engineering solution that significantly improves charge manipulation capabilities on surfaces such as trenches. They offer innovative dopant and junction formation with precise profile control, without the thermal budget constraints present in conventional processes. These techniques enable the formation of charge-laden junctions near surfaces with ultra-high activation doping without crystal damage. Furthermore, these techniques allow for use in back-to-office (BEOL) processes without concerns about thermal damage to existing structures on the substrate. These methods are also compatible with surfaces having high aspect ratios greater than 100:1.

[0026] Although trenches are used in the following examples for simplicity, other structures can also benefit from the methods of this principle, and therefore the use of trenches in the examples is not intended to be limiting. For example, the techniques of this principle can also be used in planar structures. Figure 1 is a method 100 for adjusting the work function of a trench structure 204 in a substrate 202 according to some embodiments. In block 102, in some embodiments, an etching process forms the trench structure 204 into the substrate 202, as depicted in view 200A of Figure 2A. The etching process typically uses a hard masking layer 212 that protects the area from the effects of the etching process. In some embodiments, the aspect ratio of the trench structure 204 is at least about 50:1. In some embodiments, the aspect ratio of the trench structure 204 is at least about 75:1. In some embodiments, the aspect ratio of the trench structure 204 is at least about 100:1. As a side effect of the etching process, damage 208 occurs on the surface 206 of the trench structure 204. Damage 208 may include crystal damage to the substrate material, contaminants or residues from the etching process, and / or dangling bonds in the substrate material. After the etching process is completed, the hard mask layer 212 is removed.

[0027] To further fabricate the trench structure 204, in some embodiments, an oxide layer 216 is formed on the substrate 202 using a dry oxidation process. As depicted in view 200B of Figure 2B, the field 214 or top surface of the substrate 202 and the surface 206 of the trench structure 204 undergo a dry oxidation process to form the oxide layer 216, which partially consumes material from the substrate 202, including the damaged portions. The dry oxidation process can be performed at temperatures below 450 degrees Celsius and produces less contamination and residue compared to wet oxidation processes. Furthermore, the dry oxidation process can be used in structures with substantially higher aspect ratios (e.g., greater than 100:1) than wet oxidation (e.g., aspect ratios less than 50:1). In some embodiments, the dry oxidation process is performed using a plasma oxidation chamber with or without a remote plasma source. The dry oxidation process facilitates the embedding of oxygen into the surface 206 of the trench structure 204 to repair damage to the surface 206 and reduce stress-induced leakage current (SILC) and interface trap density (Dit).

[0028] The dry oxidation process can also be controlled to provide oxide layers 216 of varying thicknesses. Parameters such as exposure time, plasma density, and temperature facilitate the determination of the oxidation rate. The thickness is then controlled by the duration of the dry oxidation process. In conventional methods such as wet oxidation, the oxidation process is self-limiting (wet oxidation is self-terminating), and the oxide layer thickness cannot be adjusted. Wet oxidation typically stops at a thickness of 1 nm to 2 nm at a saturation point. Dry oxidation does not have a saturation point and is not self-limiting, thus allowing thicknesses at any level to be obtained. In some embodiments, for trenches with an aspect ratio greater than 100:1, the dry oxidation process can achieve greater than 95% conformality in the trench structure 204, enabling the scaling of trench isolation structures using this principle. In some embodiments, as depicted in view 200C of Figure 2C, the oxide layer 216 is selectively removed from the surface 206 of the trench structure 204 and the field 214 of the substrate 202. In some embodiments, a plasma-based chamber can be used to selectively remove oxide layer 216 (e.g., oxide over Si or SiGe) at a selectivity ratio greater than, for example, greater than 50:1. Selective removal of oxide layer 216 removes all of oxide layer 216 without damaging any underlying material of substrate 202 or generating contaminants / residues, thereby leaving an undamaged surface of trench structure 204.

[0029] In block 104, passivation regions 210 are formed on the surface 206 of trench structure 204 and the field 214 of substrate 202, as depicted in view 200D of Figure 2D. In some embodiments, passivation regions 210 are formed of a homogeneous material similar to that of substrate 202. Passivation regions 210 are formed by incorporating a single crystal of a material having a dopant 222 (a P-type material is illustrated, but not intended to be limiting) to form positive or negative charges on substrate 202 (e.g., silicon doped with boron, gallium, phosphorus, arsenic, etc.), and can be manufactured by two methods. The first method is a low-temperature epitaxial growth process using a temperature below approximately 450 degrees Celsius. The second method involves a two-part process. The first part involves forming an amorphous doped material layer 402 on field 214, as depicted in view 400A of Figure 4A. The second part includes a materials engineering process 404, such as oxidation or thermal treatment, to drive dopant from the amorphous doped material layer 402 into the surface of substrate 202, as depicted in view 400B of Figure 4B. The amorphous doped material layer 402 becomes an oxide layer 402A. Therefore, the passivation region 210 can be formed as a single-crystal material under field 214 or an amorphous material layer under field 214.

[0030] Essentially, the passivation region 210 becomes part of or extends the substrate material and does not form an interface between the passivation region 210 and the substrate 202, thereby eliminating the interface commonly found in conventional processes for forming heterogeneous passivation regions. When photons pass through the passivation region 210, there is no optical loss because the photons pass through the material of the substrate 202 without any path degradation or alteration (refractive). The growth of the charge layer in the passivation region 210 also serves to repair dangling bonds on the surface 206 of the trench structure 204 caused during the trench etching process. In some embodiments, a portion of the passivation region 210 on the field 214 of the substrate 202 can be removed, leaving only the portion of the passivation region 210 within the trench structure 204. In some embodiments, the passivation region 210 can be doped with substances for engineering material properties (such as band structure, photosensitivity, etc.) and can have a gradient composition transition from the substrate 202 to the passivation region 210 to avoid interface formation.

[0031] Depending on the circumstances, in some embodiments, the passivation region 210 may be formed without dopant and then dopant may be introduced via gas 220, as depicted in view 200E of Figure 2E. In a further alternative embodiment, the passivation region 210 may be formed with dopant and then enhanced with additional dopant by exposure to a gas. The substrate 202 is exposed to gas 220 containing dopant 222B, which is embedded in the passivation region 210 on the field 214 and the surface 206 of the trench structure 204 of the substrate 202. For simplicity, the dopant 222B in Figure 2E (as shown) has a positive charge and is not intended to be limiting. The dopant 222B may also have a negative charge (not shown). For materials engineering purposes regarding composition, band structure, photosensitivity, etc., gas 220 may also be doped with substances such as Ge or carbon. The density and / or type of dopant 222B can be adjusted to provide the given positive or negative charge level required for the trench structure 204.

[0032] In block 106, the work function of the passivation region 210 can be adjusted as depicted in view 200F of Figure 2F. In some embodiments, a charge layer 226 is formed in situ during the formation of the passivation region 210. The work function of the trench structure 204 is also adjusted by changing the dopant density, dopant type, and dopant depth. The density and / or type of dopant 222 can be adjusted to provide a given positive or negative charge level required for the trench structure 204 and a given work function. The work function of the trench structure 204 is further adjusted by embedding the dopant 222 and / or a substance different from the substrate 202 into the material of the substrate 202 through a dopant diffusion process 224. A higher work function near the surface can promote or increase carrier mobility within adjacent structures such as pixel structures and reduce sensitivity to surface recombination. The aforementioned techniques allow substantial flexibility in engineering work functions by adjusting the depth, density, and type of dopants, while simultaneously increasing the area of ​​adjacent structures by using steeply tapered profiles.

[0033] In some embodiments, the dopant diffusion process 224 may be a millisecond annealing process that uses a high-power, high-temperature laser (e.g., 700°C to 900°C) pulsed in the millisecond range to anneal the substrate 202 without heating the substrate to a high temperature. The dopant diffusion process 224 embeds the charge layer 226 into the material of the substrate 202 adjacent to the passivation region. In some embodiments, a plasma oxidation process may be performed on the substrate 202 as the dopant diffusion process 224 to drive the dopant 222 and the charge layer 226 deeper into the substrate 202 than can be obtained using a millisecond annealing process. In some embodiments, a combination of a millisecond annealing process and a plasma oxidation process may be used as the dopant diffusion process 224.

[0034] In contrast to conventional techniques that form gradient charge regions (which require more area adjacent to the trench structure, thus reducing the efficiency of adjacent structures), the dopant diffusion process 224 creates steep charge boundaries, steep junction profiles, or steep compositional transitions in the material of the substrate 202, thereby increasing the effective area of ​​adjacent structures (e.g., pixel structure area). In some embodiments, depending on the permissible temperature and pressure, the charge layer can have a charge formation of up to approximately 3e18 / cm³ to approximately 3e21 / cm³ (higher temperatures and / or higher pressures result in higher charge densities). The charge layer can be formed homogeneously or heterogeneously. The charge layer of this principle has near 100% dopant activation during processing without requiring any post-activation treatment. In some embodiments, the above process can be performed without air interruption to prevent the generation of surface impurities, contaminants, and / or particles. In some embodiments, the growth of the charge layer in the passivation region 210 can be achieved by oxidation through condensation to drive the dopant deeper into the material of the substrate 202 and form an oxide layer 230 as depicted in view 200G of Figure 2G. In effect, the passivation region 210 produces a pad layer 218 and an embedded charge layer without requiring an annealing process, such as the millisecond annealing process discussed above. Furthermore, the charge layer can be amorphous rather than single-crystal grown.

[0035] The methods described herein can be performed in a separate processing chamber, which can be configured independently or as part of a clustering tool (e.g., the integration tool 300 described below with respect to Figure 3). An advantage of using the integration tool 300 is the absence of vacuum disruption, and therefore no degassing and pre-cleaning of the substrate is required before processing in the chamber. For example, in some embodiments, the inventive methods discussed above can be advantageously performed in the integration tool such that there is limited or no vacuum interruption between processes, thereby limiting or preventing the substrate from being disrupted. The integrated tool 300 includes a vacuum sealing processing platform 301, a factory interface 304, and a system controller 302. The processing platform 301 includes multiple processing chambers, such as 314A, 313B, 314C, 314D, 314E, and 314F, which are operatively coupled to vacuum substrate transfer chambers (transfer chambers 303A and 303B). The factory interface 304 is operatively coupled to transfer chamber 303A via one or more loading gate chambers (two loading gate chambers, such as 306A and 306B shown in Figure 3).

[0036] In some embodiments, the fab interface 304 includes at least one docking station 307 and at least one fab interface robot 338 to facilitate the transfer of semiconductor substrates. The docking station 307 is configured to receive one or more front-opening unified pods (FOUPs). Four FOUPs, such as 305A, 305B, 305C, and 305D, are illustrated in the embodiment of Figure 3. The fab interface robot 338 is configured to transfer substrates from the fab interface 304 to the processing platform 301 via negative loading gate chambers (such as 306A and 306B). Each loading gate chamber 306A and 306B has a first port coupled to the fab interface 304 and a second port coupled to the transfer chamber 303A. Loading gate chambers 306A and 306B are coupled to a pressure control system (not shown) that evacuates and degasses the loading gate chambers 306A and 306B to facilitate the transfer of the substrate between the vacuum environment of the transfer chamber 303 and the substantially surrounding (e.g., atmospheric) environment of the plant interface 304. Transfer chambers 303A and 303B have vacuum robots 342A and 342B disposed in the respective transfer chambers 303A and 303B. Vacuum robot 342A is capable of transferring substrate 321 between loading gate chambers 306A and 306B, processing chambers 314A and 314F, and cooling station 340 or pre-cleaning station 342. Vacuum robot 342B is capable of transferring substrate 321 between cooling station 340 or pre-cleaning station 342 and processing chambers 314B, 314C, 314D, and 314E.

[0037] In some embodiments, processing chambers 314A, 314B, 314C, 314D, 314E, and 314F are coupled to transfer chambers 303A and 303B. Processing chambers 314A, 314B, 314C, 314D, 314E, and 314F may include, for example, atomic layer deposition (ALD) processing chambers, physical vapor deposition (PVD) processing chambers, chemical vapor deposition (CVD) chambers, annealing chambers, etc. As discussed above, these chambers may include any chamber suitable for performing all or part of the methods described herein, such as dry oxide removal chambers or pre-cleaning chambers and epitaxial growth chambers, as well as etching and deposition chambers. In some embodiments, one or more optional repair chambers (illustrated as 316A and 316B) may be coupled to transfer chamber 303A. The maintenance chambers 316A and 316B are configured to perform other substrate processing, such as degassing, orientation, substrate metering, cooling, etc.

[0038] System controller 302 controls the operation of tool 300 using direct control of processing chambers 314A, 314B, 314C, 314D, 314E, and 314F, or alternatively by controlling a computer (or controller) associated with processing chambers 314A, 314B, 314C, 314D, 314E, and 314F and tool 300. During operation, system controller 302 enables data collection and feedback from the respective chambers and systems to optimize the performance of tool 300. System controller 302 typically includes a central processing unit (CPU) 330, memory 334, and support circuitry 332. CPU 330 can be any type of general-purpose computer processor suitable for use in an industrial environment. Support circuitry 332 is typically coupled to CPU 330 and may include cache, clock circuitry, input / output subsystems, power supply, etc. Software routines such as those described above can be stored in memory 334, and when executed by CPU 330, CPU 330 is converted into a dedicated computer (system controller) 302. Software routines can also be stored and / or executed by a second controller (not shown) located remotely from tool 300.

[0039] Embodiments based on this principle can be implemented in hardware, apparatus, software, or any combination thereof. Embodiments can also be implemented using instructions stored on one or more computer-readable media, which can be read and executed by one or more processors. Computer-readable media can include any means for storing or transmitting information in a machine-readable form (e.g., a computing platform or a "virtual machine" running on one or more computing platforms). For example, computer-readable media can include any suitable form of volatile or non-volatile memory. In some embodiments, computer-readable media can include non-transitory computer-readable media.

[0040] Although the foregoing are embodiments of this principle, other and further embodiments of this principle can be designed without departing from the basic scope of this principle.

[0041] 100: Method 102: Square 104: Square 106: Square 200A: View 200B: View 200C: View 200D: View 200E: View 202:Substrate 204: Trench Structure 206: Surface 208: Damage 210: Passivation region 212: Hard mask layer 214: Field 216: Oxide layer 222: Dopant 222B: Dopant 224: Dopant diffusion process 226: Charge layer 300: Integration Tools 301: Vacuum Sealing Processing Platform 302: System Controller 303A: Transfer Chamber 303B: Transfer Chamber 304: Factory Interface 305A: Front-Opening Wafer Container (FOUP) 305B: Front-Opening Wafer Container (FOUP) 305C: Front-Opening Wafer Container (FOUP) 305D: Front-Opening Wafer Container (FOUP) 306A: Loading gate chamber 306B: Loading gate chamber 314A: Processing Chamber 314B: Processing Chamber 314C: Processing Chamber 314D: Processing Chamber 314E: Processing Chamber 314F: Processing Chamber 316A: Maintenance Chamber 316B: Maintenance Chamber 321:Substrate 330: Central Processing Unit (CPU) 332: Support Circuit 334: Memory 338: Factory Interface Robot 340: Cooling Station 342: Pre-cleaning station 342A: Vacuum Robot 342B: Vacuum Robot 400A: View 400B: View 402: Amorphous doped material layer 402A: Oxide layer 404: Materials Engineering Process

[0042] Domestic storage information (please note in order of storage institution, date, and number) none Overseas storage information (please note in the order of storage country, institution, date, and number) none

Claims

1. A method for adjusting the work function of a structure on a substrate, comprising the steps of: forming the structure using an etching process; forming an oxide layer with a controllable oxide thickness of about 1 nm to about 15 nm on the surface of the structure using a dry oxide process at a temperature below about 450 degrees Celsius; selectively removing the oxide layer from the surface of the structure; growing an epitaxial layer on the surface of the structure to form a homogeneous passivation region having dopants as part of a substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surface of the structure to adjust the work function of the structure, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius.

2. The method as described in claim 1, wherein the dopant diffusion process generates a charge of up to about 3e18 / cm3 to about 3e21 / cm3 in the surface of the structure, either positive or negative.

3. The method as described in claim 1, wherein the dopants are P-type or N-type.

4. The method as described in claim 1, wherein the dopant diffusion process forms a steeply abutted surface profile.

5. The method as described in claim 1, wherein the method is performed in a back-end (BEOL) process.

6. The method as described in claim 1, wherein the epitaxial layer is formed from a single crystal.

7. The method as described in claim 1 further includes the step of growing the epitaxial layer using a low-temperature process at a temperature below approximately 450 degrees Celsius.

8. The method as described in claim 1, wherein the dry oxide process is performed in a plasma oxidation chamber.

9. The method as described in claim 1 further includes the step of: etching the structure into the substrate to a high aspect ratio greater than about 75:

1.

10. A method for adjusting the work function of a structure on a substrate, comprising the steps of: forming an amorphous material layer on the surface of the structure using a low-temperature process to form a homogeneous passivation region having dopants as part of a substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surface of the structure, thereby adjusting the work function of the structure and forming an oxide layer from the amorphous material layer, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius and forms a charge layer having a steeply protruding surface profile.

11. The method as described in claim 10, wherein the dopant diffusion process generates a charge of up to about 3e18 / cm3 to about 3e21 / cm3 in the surface of the structure, either positive or negative.

12. The method as described in claim 10, wherein the dopants are P-type or N-type.

13. The method as described in claim 10, wherein the method is performed in a back-end (BEOL) process.

14. The method as described in claim 10, wherein the dopant diffusion process is a condensation-based process.

15. The method as described in claim 10, wherein the amorphous material layer is formed at a temperature below 450 degrees Celsius.

16. A non-transitory computer-readable medium storing instructions that, when executed, cause a method for adjusting the work function of a structure in a substrate to be performed, the method comprising the steps of: forming the structure using an etching process; forming an oxide layer with a controllable oxide thickness of about 1 nm to about 15 nm on the surface of the structure using a dry oxide process at a temperature below about 450 degrees Celsius; selectively removing the oxide layer from the surface of the structure; growing an epitaxial layer on the surface of the structure to form a homogeneous passivation region as part of a substrate material of the substrate; and performing a dopant diffusion process to further embed the dopants into the surfaces of the structure to adjust the work function of the structure, wherein the dopant diffusion process is performed at a temperature below about 450 degrees Celsius.

17. The non-transitory computer-readable medium as described in claim 16, wherein the dopant diffusion process generates a charge of up to about 3e18 / cm3 to about 3e21 / cm3 positive or negative on the surface of the structure and forms a steeply abutted surface profile.

18. The non-transitory computer-readable medium as described in claim 16, further comprising: The epitaxial layer was grown using a low-temperature process at a temperature below approximately 450 degrees Celsius.

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