Spreading Resistance Profiling Research Articles

Structure and RF performance evolution of polycrystalline silicon charge capture layer for advanced RF-SOI in in-situ annealing process

In this paper, the structures and radio-frequency (RF) properties of high resistivity silicon (HR-Si) wafers with a polycrystalline silicon (Poly-Si) charge capture layer after in-situ annealing were investigated. The thermal treatment significantly improves the wafer warpage and optimize the resistivity of Poly-Si/HR-Si interfaces. By using spreading resistance profiling (SRP), energy dispersive spectrometer (EDS) and secondary ion mass spectrometry (SIMS) technology, the rise of interfacial resistivity may be as results of the pyrolysis of the native oxide and the diffusion of hydrogen atoms. The transmission Kikuchi diffraction (TKD) analysis exhibited that the grains were recrystallized during the annealing, leading to the transformation from the small angle boundaries (SAGBs) into low-Σ coincidence lattice grain boundaries (CSL GBs). The drop of the bulk resistivity of the Poly-Si layer is attributed to the reduction of the total grain boundary (GB) density and the increase of the low electro-activity GB density. Moreover, the microwave losses and non-linearity of the RF-SOI with embedded Poly-Si layer was evaluated by on-chip testing based on CPW. It was determined that the decrease in the Poly-Si resistivity only worsens the microwave losses but not the non-linearity.

Epitaxial silicon transition zone measurements by spreading resistance profiling and Fourier transform infrared reflectometry

AbstractSilicon epitaxy is an essential building block in the manufacturing of complementary metal‐oxide semiconductor (CMOS) devices. Accurate determination of epitaxial layer thickness is indispensable for a uniform and reproducible process. In this paper, we compare thickness values of the transition zone (TZ) in silicon epitaxial wafers obtained by two of Semilab's production‐compatible electrical and optical characterization techniques: Fourier‐transform infrared (FTIR) reflectometry and spreading resistance profiling (SRP). We demonstrate a high correlation between TZ thicknesses obtained from the optical modeling of FTIR reflectance spectra and SRP profiles. The dependence of TZ thickness change on the high‐temperature annealing steps is also examined. FTIR reflectometry thus offers a quick, contactless alternative for obtaining structural parameters of an epitaxial layer, and these values can be well matched to those given by SRP.

Determination of conduction type in high-resistivity silicon by combining spreading resistance profiling (SRP) with hydrofluoric acid treatment

The effect of hydrofluoric acid (HF) treatment on the surface electrical properties was observed in high-resistivity bulk silicon by the spreading resistance profiling (SRP) technique. It is found that the near-surface resistivity decreased in n-type silicon and increased in p-type silicon after HF treatment according to the SRP measurement. The variation of surface chemical elements and energy bands of HF-treated n-type and p-type (100) silicon has been characterized by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy measurements. The results indicate that the surface Fermi level is shifted toward the conduction band minimum after HF treatment. The surface energy band bending caused by the surface electronegative groups (–F and –OH) was investigated by the method of First-principles calculation. Based on these findings, a method combining the SRP measurement with HF treatment to determine the conduction type of high-resistivity silicon was proposed, which is critical for the development of high-resistivity CZ silicon crystal growth.

Thermal activation of low-density Ga implanted in Ge

The nuclear spins of low-density implanted Ga atoms in Ge are interesting candidates for solid state-based qubits. To date, activation studies of implanted Ga in Ge have focused on high densities. Here, we extend activation studies into the low-density regime. We use spreading resistance profiling and secondary ion mass spectrometry to derive electrical activation of Ga ions implanted into Ge as a function of the rapid thermal anneal temperature and implant density. We show that for our implant conditions, the activation is best for anneal temperatures between 400 and 650 °C with a maximum activation of 69% at the highest fluence. Below 400 °C, remaining implant damage results in defects that act as superfluous carriers, and above 650 °C, surface roughening and loss of Ga ions are observed. The activation increased monotonically from 10% to 69% as the implant fluence increased from 6×1010 to 6×1012 cm−2. The results provide thermal anneal conditions to be used for initial studies of using low-density Ga atoms in Ge as nuclear spin qubits.

Annealing-temperature-dependent evolution of hydrogen-related donor and its strong correlation with X-photoluminescence center in proton-irradiated silicon

We have investigated the formation and decay of hydrogen-related donors (HDs) and irradiation-induced intrinsic defects. N-type m:Cz and FZ silicon wafers, which were irradiated with 2 MeV protons and subsequently annealed at 100–600 °C, were analyzed using spreading resistance profiling and photoluminescence (PL). HDs formed at 260 °C and then disappeared in two stages at 400–440 and 500–540 °C. This decay behavior indicates the existence of two types of HDs with different thermal stabilities. PL measurements showed interstitial silicon clusters (W and X center), a carbon–oxygen complex (C center), and exciton lines bound to unknown shallow centers. The origin of the HDs was investigated based on the correlation of the formation and decay temperatures between HDs and irradiation-induced defects. The predominant defects at the early stage of annealing, such as the C and W centers, are ruled out as candidates for the core defects of HDs because annealing above 260 °C is indispensable for the HD formation. In contrast, the X center was found to be thermally generated above 200 °C and disappeared at 580 °C. The similarity of the formation and decay temperatures between the X and HD centers suggests that HDs are associated with the formation of the interstitial silicon-related defects attached to hydrogen. Our results suggest that controlling the formation of interstitial silicon-related defects is important for realizing desirable doping profiles with high accuracy and reproducibility for power devices. Annealing above 400 °C exclusively provides thermally more stable HDs, leading to the realization of more rugged power devices.

Physical mechanism of field modulation effects in ion implanted edge termination of vertical GaN Schottky barrier diodes

In this study, the physical properties of F ion-implanted GaN were thoroughly studied, and the related electric-field modulation mechanisms in ion-implanted edge termination were revealed. Transmission electron microscopy results indicate that the ion-implanted region maintains a single-crystal structure even with the implantation of high-energy F ions, indicating that the high resistivity of the edge termination region is not induced by amorphization. Alternately, ion implantation-induced deep levels could compensate the electrons and lead to a highly resistive layer. In addition to the bulk effect, the direct bombardment of high-energy F ions resulted in a rough and nitrogen-deficient surface, which was confirmed via atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. The implanted surface with a large density of nitrogen vacancies can accommodate electrons, and it is more conductive than the bulk in the implanted region, which is validated via spreading resistance profiling and conductive AFM measurements. Under reverse bias, the implanted surface can spread the potential in the lateral direction, whereas the acceptor traps capture electrons acting as space charges, shifting the peak electric field into the bulk region in the vertical direction. As a result, the Schottky barrier diode terminated with high-energy F ion-implanted regions exhibits a breakdown voltage of over 1.2 kV.

Effect of carbon, oxygen, and intrinsic defects on hydrogen-related donor concentration in proton irradiated n-type silicon

We investigated the effect of the concentration of carbon, oxygen, and irradiation-induced intrinsic defects on hydrogen-related donor (HD) concentration. Several n-type silicon wafers having different carbon and oxygen concentrations were irradiated with 2 MeV protons, subsequently annealed at 300–400 °C, and analyzed by spreading resistance profiling. The HD concentration had no correlation with carbon and oxygen concentration. Additionally, the HD concentration showed a strong increasing linear dependence with proton-irradiation dose at 350 and 400 °C and a square root dependence at 300 °C. In the decay process of HD concentration at 400 °C, fast- and slow-decay components were observed regardless of wafer type. Our results show that the HD formation is based on the interactive process of irradiation-induced intrinsic defects and hydrogen, rather than hydrogen-catalyzed thermal double donor formation. Magnetic-field-applied Czochralski (m:Cz) wafers with 300 mm diameter, which are critical for the production scaling of power devices, have a relatively higher oxygen concentration than conventional floating-zone wafers. Our results further suggest that controlling the intrinsic defect formation, rather than oxygen impurity concentration, is more important in realizing designed doping profiles with high accuracy and reproducibility for next-generation power devices using large-diameter m:Cz wafers as a standard starting material.

Doping profiling of beveled Si wafers via UV-micro Raman spectroscopy

Doping profiling methods are required to provide doping monitoring of heavy doped Silicon wafers, widely used in electronic devices, with high concentration sensitivity and spatial resolution. Herein, we demonstrate that ultraviolet (UV) micro-Raman spectroscopy implemented on small-angle beveled surfaces is able to produce a Raman-based doping profile without any further measurements. The reliability of the approach is verified on ion-implanted p-type B-doped single-crystalline Si (100) wafers with shallow (100 nm) doping profile, directly comparing the Raman-based doping profiling results with independent secondary ion mass spectrometry (SIMS) and spreading resistance profiling (SRP) measurements. The Raman-based technique is capable to fully reproduce the doping profile down to 100 nm with excellent doping sensitivity (10 ppm) in the 1018–1020 cm−3 doping concentration range providing high lateral (vertical) resolution of 1μm (10 nm). The technique is comparable to SIMS in terms of resolution and sensitivity down to 1018 cm−3 concentration but requires the beveling process. Instead, it is a better alternative to SRP on bevel due to its higher spatial resolution and avoiding probe contact. The Raman-based doping profiling technique is, therefore, suitable for heavy doped Si with typical shallow doping profiles of state-of-the-art Si-based nanoelectronics.

Investigation and reduction of RF loss induced by Al diffusion at the AlN/Si(111) interface in GaN-based HEMT buffer stacks

GaN-based high electron mobility transistors (HEMT) on Si (111) substrates have large potential for applications in the 5G telecommunication field. However, for this potential to be fully realized, all loss mechanisms need to be minimized. It is known that typical metal-organic chemical vapor deposition (MOCVD) processes used to grow the GaN epitaxial layers can cause considerable parasitic conductivity at the interface of the AlN nucleation layer to the high-resistivity Si substrate, leading to reduced gain and power added efficiency in amplifiers. Reducing this parasitic conductivity is hence of utmost importance to render GaN-on-Si a significant contributor to next-generation 5G power amplifier technology. In this work, we employ secondary ion mass spectroscopy, spreading resistance profiling and insertion loss measurements up to 28 GHz using coplanar waveguides fabricated on the epitaxial layer stacks to study the origin and characterize the parasitic conductivity. While a single heat-up process in an AIXTRON G5+ reactor chamber cleaned using Cl2 does not introduce any extra dopants in the Si substrate, the epitaxial growth of (Al,Ga)N-based HEMT buffer layer stacks leads to the diffusion of Al and, to a lower extent, Ga acceptors into the Si substrate. Optimization of the MOCVD process towards lower growth temperatures leads to a strong reduction of density of diffused acceptors. This reduction goes in line with a significant decrease of the insertion loss from 0.45 dB mm−1 to only 0.20 dB mm−1 at a frequency of 28 GHz.

Defect Distribution in Doped Silicon Nanostructures Characterized By Means of Scanning Spreading Resistance Microscopy

Scanning Spreading Resistance Microscopy (SSRM) is a well-known technique for electrical characterization of dopant distributions in semiconductors. The resistance between a conductive atomic force microscope (AFM) tip and a large area contact on a semiconductor sample gives direct hint of sample resistivity underneath the tip. The sample surface has to be polished to achieve a smooth surface which is crucial for a low ohmic electrical contact between tip and sample. In theory, the formula of spreading resistance Rspr = ρ/4a with tip radius a connects SSRM raw data to resistivity data which can be transformed to concentration data of corresponding dopants. In experiment, additional series resistances and electrical defects have an impact on the SSRM analysis so that the formula does not hold any more. A SSRM scan on a calibration sample with regions of different well-known dopant concentrations enables an empirical relationship between R and ρ. This relationship is only valid for the specific defect distribution and tip-sample contact of the calibration sample. Both the defect distribution and the tip sample contact, respectively, depend strongly on sample preparation. Reliable calibration of unknown dopant distributions in semiconductor samples is only possible in the case of a perfectly reproducible sample preparation which is however hard to attain. Therefore, an accurate quantification of SSRM data remains difficult.One first step towards SSRM quantification requires an improved understanding and control of electrical defects on the sample surface which affect the SSRM measurement. In this work, we apply SSRM on well characterized n- and p-type dopant profiles in silicon bulk- and nanostructures. The deviation of the experimental SSRM data from simulated spreading resistance profiles provides direct information on type, quantity and origin of electrical defects in the sample.Our analysis of bulk samples reveals two type of preparation induced defects: n-type surface defects and hydrogen traps which deactivate boron atoms. The number of surface defects is proportional to the surface roughness. An improved sample preparation with additional polishing steps reduces the impact of this defect on the SSRM analysis. Hydrogen traps are driven into the sample during water based chemical mechanical polishing steps. An annealing of the sample at 250°C for 20 minutes reactivates most of the boron atoms.A third type of defect was noticed in nanostructured samples. Silicon nano pillars with diameters between 500 and 600 nm are manufactured in a top-down approach out of a silicon wafer with a 600 nm deep boron diffusion profile. Figure 1 shows the SSRM data of the bulk sample (a), the cross-sectional prepared nano pillar (b), and the depth profiles of SSRM analysis (c). The SSRM signal at the top of the pillar is dominated by the boron profile (b). The side walls are covered with gold as second contact for SSRM analysis. Obviously, the SSRM depth profile at the shell region (#2, circles) of the pillar is deeper than at the core (#3, triangles) and in the bulk sample (#1, squares) (c). The dopant distribution itself cannot vary across the pillar as the nanofabrication is done by dry-etching at cryogenic temperatures. We explain the variance in SSRM signal by acceptor defect states at the pillar shell which increase the number of p-type charge carriers in the shell region.COMSOL simulations of the SSRM analysis, including the impact of defects on the resistance profile, is used to quantify the defect distributions. The calculated resistance profiles accurately describe the experimental data (black dotted lines in (c)) and support our assumption on the distribution of electrical defects in silicon nanostructures. Figure 1

Influence of oxygen on trap-limited diffusion of hydrogen in proton-irradiated n-type silicon for power devices

The growing demand for power devices has led to the use of magnetic field-applied Czochralski (m:Cz) wafers owing to the limited production capacity and available diameters of the traditionally used floating zone (FZ) wafers. Consequently, the influence of oxygen impurities in the wafers on the electrical properties of devices, regardless of the growth method, needs to be investigated to achieve a stable fabrication process for power devices. Using the proton irradiation doping process and spreading resistance profiling technique, we evaluated the effective diffusion coefficient (Deff) related to trap-limited diffusion of hydrogen and the effects of impurities on diffusivity. We irradiated n-type silicon wafers, which have different carbon, oxygen, and phosphorus concentrations, with 2 MeV protons and annealed them at 300–400 °C. By analyzing the width of the n-type region, where hydrogen-related shallow donors (HDs) are induced, we estimated Deff to be five to six orders of magnitude lower than the intrinsic diffusion coefficient, indicating that hydrogen motion is highly trap-limited. Deff was significantly dependent on the oxygen concentration, and the activation energy of hydrogen diffusion varied from 0.57 ± 0.15 eV (pure epitaxial wafer) to 2.19 ± 0.15 eV (m:Cz wafer). This trend suggests that oxygen-related defects preferentially trap the mobile hydrogen released from thermally dissociated HDs. This study also reveals that the diffusion coefficients of different materials when annealed at 400 °C are comparable. This information is essential to realize the cost-effective production of power devices because we can treat m:Cz and FZ wafers equivalently during the doping process.

Effects of boron doping on non-linear properties of SOI with embedded polycrystalline silicon layer for RF applications

The doped polycrystalline silicon (poly-Si) is used to investigate the effects of boron on non-linear properties of trap-rich silicon-on-insulator (TR-SOI) substrates. A dip at poly-Si/Si interface in spreading resistance profiles (SRP) is found and attributed to certain combinations of the poly-Si layer and the substrate dopant levels. A multilayered SRP model is employed to explain the SRP characteristics of the poly-Si films. The non-linear performance of TR-SOI is also studied using 50 Ω coplanar waveguide (CPW) lines. The results show little impact to the second harmonic distortion (HD2) up to 1 × 1015 cm−3 boron concentration in the poly-Si layer. At boron doping levels of above 5 × 1015 cm−3 measurable deteriorations of HD2 are detected and significantly worse when boron doping level in poly-Si reaches 1 × 1017 cm−3, presumably the traps in grain boundaries of poly-Si layer are filled by doping charges.

Growth and Characterization of Undoped Polysilicon Thick Layers: Revisiting an Old System

Thick layer of polycrystalline silicon (poly-Si) grown by Atmospheric Pressure Chemical Vapor Deposition (APCVD) is still a reference material in a number of applications, despite the high thermal budget of this technique. This work presents a material study of undoped poly-Si layers of different thicknesses, using different characterization techniques such as secondary electron microscope in backscattered detection configuration, electron backscattering diffraction imaging, secondary ion mass spectrometry and spreading resistance profiling. The poly-Si layers, grown at 1000 °C by APCVD on thermal oxide, were found to have a columnar microstructure with [110] main orientation. By correlating layer purity, grain size and electrical resistivity, no straightforward relation between grain size and resistivity could be found. The layers resistivity is found almost independent on thickness and thus grain size. The possible reasons for such difference with previous other works are discussed taking into account the grain size determination uncertainty and the electrical characterization limitations.

Resistivity profile of epitaxial layer for the new ALICE ITS sensor

Wafers with different epitaxial layer thicknesses of 12, 18, 20, 25, 30 and 40 μm and high resistivities ranging from 0.03 to 8.0 kΩ⋅cm have been investigated in this study. To verify their properties, surface resistivity measurement, scanning electron microscopy inspection and spreading resistance profiling have been performed. The results indicate that wafers with a 25-μm epitaxial thickness are well-suited to our requirements for use as a starting material for ALPIDE chip production in the ALICE ITS upgrade project.

Advanced Organic Molecular Doping Applied to Si: Influence of the Processing Conditions on the Electrical Properties

The molecular doping (MD) process consists of depositing a molecular precursor on a silicon substrate, covering the precursor with a cap layer and annealing the sample to decompose the precursor and diffuse dopant atoms within Si. In the literature, preliminary results have shown that dopant diffusion inside a substrate correlates with the presence or absence of the cap layer. The purpose of this work is to study how the cap coating changes the doping process and efficiency. The authors investigate and compare the electrical properties of silicon samples after MD doping with three different cap layers and without a cap layer. The authors examined a 500‐nm‐thick layer of SiO2 deposited by spin‐on‐glass (SOG), a 500‐nm‐thick layer of SiO2 deposited in a chemical vapor deposition (CVD) chamber at room temperature and a 100‐nm‐thick layer of oxidized silicon placed over and in contact with the samples to be doped. Spreading resistance profiling (SRP) measurements are then performed on these samples to monitor important doping features, such as carrier dose, carrier concentration, sheet resistance and junction depth, obtained with different capping conditions.

High level active n+ doping of strained germanium through co-implantation and nanosecond pulsed laser melting

Obtaining high level active n+ carrier concentrations in germanium (Ge) has been a significant challenge for further development of Ge devices. By ion implanting phosphorus (P) and fluorine (F) into Ge and restoring crystallinity using Nd:YAG nanosecond pulsed laser melting (PLM), we demonstrate 1020 cm−3 n+ carrier concentration in tensile-strained epitaxial germanium-on-silicon. Scanning electron microscopy shows that after laser treatment, samples implanted with P have an ablated surface, whereas P + F co-implanted samples have good crystallinity and a smooth surface topography. We characterize P and F concentration depth profiles using secondary ion mass spectrometry and spreading resistance profiling. The peak carrier concentration, 1020 cm−3 at 80 nm below the surface, coincides with the peak F concentration, illustrating the key role of F in increasing donor activation. Cross-sectional transmission electron microscopy of the co-implanted sample shows that the Ge epilayer region damaged during implantation is a single crystal after PLM. High-resolution X-ray diffraction and Raman spectroscopy measurements both indicate that the as-grown epitaxial layer strain is preserved after PLM. These results demonstrate that co-implantation and PLM can achieve the combination of n+ carrier concentration and strain in Ge epilayers necessary for next-generation, high-performance Ge-on-Si devices.

Effects of Low Boron Concentration on Electrical Properties of Commercial Trap-Rich High Resistivity SOI Substrate

Electrical properties of boron lightly doped trap-rich layers of trap-rich high resistivity silicon-on-insulator (trap-rich HR-SOI) substrates are investigated. Secondary Ion Mass Spectroscopy (SIMS) is used to measure boron distribution. Resistivity profiles are studied by means of Spreading-resistance profiling (SRP). Moreover, radio frequency (RF) performance of the trap-rich HR-SOI substrates is evaluated using coplanar waveguide (CPW) lines. It is found that RF losses and harmonic distortions keep unchanged when boron concentration of the trap-rich layer is lower than 5 × 1014 cm−3. It is suggested that the trap-rich layer can still effectively restrain the parasitic surface conductance (PSC) effect.

Electrical and Structural Characterization of Self-Aligned InGaZnO Thin-film Transistors Fabricated by Excimer Laser Irradiation

We conducted electrical and structural characterization of a self-aligned InGaZnO (IGZO) thin-film transistor (TFT) fabricated by selectively reducing the resistance of the source and drain (S/D) regions by backside excimer laser irradiation. We present the influence of the resistance of the S/D regions on the transfer characteristics and discuss the applicability of our resistance reduction method to the self-aligned oxide TFT fabrication process. In our proposed method, since the low-resistance regions are formed directly by using the gate electrode as a mask, the positions of the S/D regions can be accurately set. We demonstrate this by analyzing the spreading resistance and height profiles for the IGZO film.

Very High-Resolution Mobility, Resistaivity and Activation for High-Mobility Materials

Advanced high-mobility material based devices require knowledge of carrier concentration, mobility and resistivity profiles as well as the effects of strain. Active Layer Parametrics, Inc.’s ALPro system represents a technique to directly measure these parameters as well as the effects of strain from high dopant concentrations in high mobility materials like SiGe, Ge, and III-V high-mobility materials. Current standard electrical metrology techniques such as Scannning Spreading Resistance Microscopy (SSRM) cannot provide complete mobility and activation data on these high-mobility materials. Furthermore resolutions currently achieved by the ALPro System (>0.5nm in Si, SiGe and Ge) cannot be achieved by SSRM or Spreading Resistance Profiling (SRP). Sample preparation of relatively fragile high-mobilty materials like Ge and GaAs is difficult to standardize, is known to have resulted in widely varying resistivity data values. The continual making and breaking of contacts in both SSRM and SRP, though reasonlably well-understood for Silicon, has not been adequately understood in the case of Ge requiring modeling to account for bandgap changes due to applied pressure that vary with probe dimensions and properties. The ALPro System overcomes all the above deficiencies while providing a high-resolution, high-throughput system that may be safely deployed in a fabrication environment. This paper discusses some of the data extracted using the ALPro System for high and low-doped Gematerials.

(Invited) GaN/Si Buffer Development for RF and Power Applications

Gallium nitride transistors grown on Si(111) are becoming commercially available for applications such as microwave communication at 2-6 GHz and power conversion in the 600V range. Three-terminal off-state breakdown voltage of 1.35 kV and a low specific on-resistance of 1.3 mohm-cm2 have been achieved on 200 mm diameter GaN/Si wafers with 4.5-µm-thick device epilayer thickness.[1] However additional improvements to materials and processes are desirable in order to drive further market penetration. For power applications, breakdown voltage can be increased by tuning the resistivity of the buffer layers.Transistor structures incorporating GaN or Al(0.08)Ga(0.92)N buffer layers were grown and fabricated into isolation test structures for leakage characterization. For GaN buffer layers, the leakage current was significantly decreased and the isolation blocking voltage was increased by increasing the carbon doping concentration. AlGaN buffer layers exhibited smaller increase in leakage current at elevated temperature relative to GaN buffer layers. Room-temperature two-terminal breakdown voltage (defined at 1 µA/mm off-state leakage for 5 µm isolation spacing) was 100 V higher for the AlGaN-based buffer relative to the GaN buffer (1000V and 900V, respectively). At 150°C, the two-terminal breakdown voltage was 150V higher for the AlGaN buffer compared to the GaN, indicating a lower temperature coefficient for the AlGaN-based buffer. The effect of buffer composition on leakage current at high temperature was magnified at higher voltage, where the AlGaN-based buffer sustained over 1200 V at 100 µA/mm compared to 1000V for GaN-based buffer. RF devices can suffer from parasitic capacitance resulting from mobile charge at the substrate interface. Crystalline rare-earth oxides (cREO) can be deposited on Si(111) to form a single-crystal nucleation layer for AlN or GaN [2] and avoid charge accumulation at the substrate interface. GaN transistor structures were grown on cREO/Si(111) and characterized by transmission electron microscopy, X-ray diffraction, and spreading resistance profile measurements.The film morphology and growth modes were comparable to a baseline GaN/Si process, with an offset in the density of extended defects which indicated the need for additional optimization. In contrast to a baseline process which resulted in p-type doping at the substrate interface, spreading resistance measurements indicate that the cREO / substrate interface was depleted, consistent with the cREO layer acting as a barrier for the group-III elements at the onset of growth. The benefits of this reduced charge density for device performance improvements and material optimization are discussed.