Residual Donors Research Articles

Breakdown voltage enhancement of p-GaN/AlGaN/GaN diode by controlling Mg acceptors for compensating residual Si donors

Breakdown voltage enhancement was studied for p-GaN/AlGaN/GaN heterostructure diodes, where residual Si donors during growth were compensated with Mg acceptors doped in the p-GaN layer. As the thicknesses of the p-GaN layer (T p-GaN) decreased from 140 nm, breakdown voltages were increased and maximized at 20 nm, then decreased at 0 nm. Moreover, breakdown voltages of the 20 nm T p-GaN diodes improved with the increase of the drift region lengths. This is because a uniform electric field was obtained by compensation of the residual Si donors with the Mg acceptors. In addition, for a thicker p-GaN layer, the effects of the surface states were suppressed, and injection of a large number of holes under forward bias was observed. Consequently, the thick p-GaN layer is expected to enhance forward current. Adopting the results, the potential of low on-resistance and high current for p-GaN gated anode diodes was demonstrated.

Influence of residual dopants to net doping concentration in N-type 4H-SiC films grown using high-speed wafer rotation vertical CVD method

In this study, the influence of residual dopants on the net doping concentration in n-type 4H-SiC epitaxial films grown at different N2 flow rates and C/Si ratios were investigated. By reducing the N2 flow rate, the influence of the residual donors on the net doping concentration was observed to become dominant for the films grown at low C/Si ratios and that of the residual acceptors on the net doping concentration becomes dominant for the films grown at high C/Si ratios. For the films grown at the middle C/Si ratio, an apparent proportional relation due to the compensation balance between the residual and intentional donors and the residual acceptors was observed in the N2 flow rate dependence of the net doping concentration. Furthermore, the decay curve of the net doping concentration observed in the C/Si ratio dependence is affected by the compensation balance between the intentional dopant concentration and the residual dopant concentration.

Deep and shallow electronic states associated to doping, contamination and intrinsic defects in ε-Ga2O3 epilayers

Deep and shallow electronic states in undoped and Si-doped ε-Ga2O3 epilayers grown by MOVPE on c-oriented Al2O3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varying in the range 107- 1013 Ωcm depending on the carrier gas used during growth. Films grown with He carrier were generally more resistive than those grown with H2 carrier and exhibited a Fermi level located at about 0.8 eV below the conduction band edge, which tends to shift deeper with temperature. This can tentatively be attributed to the combined action of deep donors (probably carbon impurities and oxygen vacancies) and deep acceptors (Ga vacancies and related complexes), which compensate residual shallow donors. There are strong experimental hints that nitrogen also behaves as deep acceptor.Room temperature resistivity as low as 0.42 Ωcm and electron concentrations around 1018 cm−3 were obtained by silicon doping. Si was confirmed to act as shallow donor with sufficiently high solubility. A variable range hopping conduction was observed in a wide temperature interval in the n-type layers, and compensation by native acceptors also plays a major role on conduction mechanisms. Previous evaluations of curvature and anisotropy of the conduction band are confirmed, which allows for the estimation of the electron effective mass.The present experimental data are discussed considering the theoretical predictions for point defect formation in the ε-polymorph as well as literature data on extrinsic and intrinsic defects in β-Ga2O3.

Tunable tunnel barriers in a semiconductor via ionization of individual atoms

We report scanning tunneling microscopy (STM) studies of individual adatoms deposited on an InSb(110) surface. The adatoms can be reproducibly dropped off from the STM tip by voltage pulses, and impact tunneling into the surface by up to ∼100×. The spatial extent and magnitude of the tunneling effect are widely tunable by imaging conditions such as bias voltage, set current and photoillumination. We attribute the effect to occupation of a (+/0) charge transition level, and switching of the associated adatom-induced band bending. The effect in STM topographic images is well reproduced by transport modeling of filling and emptying rates as a function of the tip position. STM atomic contrast and tunneling spectra are in good agreement with density functional theory calculations for In adatoms. The adatom ionization effect can extend to distances greater than 50 nm away, which we attribute to the low concentration and low binding energy of the residual donors in the undoped InSb crystal. These studies demonstrate how individual atoms can be used to sensitively control current flow in nanoscale devices.

Electrical properties and deep trap spectra in Ga2O3 films grown by halide vapor phase epitaxy on p-type diamond substrates

Films of Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al2O3 deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga2O3/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al2O3 interlayer was mainly composed of (−201) β-Ga2O3 with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 1015 cm−3, with deep electron traps spectra consisting of the well documented centers for β-Ga2O3 near Ec 0.27, Ec 0.7, and Ec 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga2O3 films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).

Analysis and Modeling of Vertical Current Conduction and Breakdown Mechanisms in Semi-Insulating GaN Grown on GaN: Role of Deep Levels

Vertical current conduction in a 1.3- μm-thick semi-insulating (SI) C-doped GaN grown on a GaN substrate is analyzed. During the growth, pressure was varied from 100 to 20 mbar in order to increase C concentration from ~ 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to ~ 6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ; SI GaN is sandwiched between two n-GaN layers. Optical transitions suggest two acceptor levels: ~0.9 eV above the valence band and ~0.6 eV below the conduction band. Current-voltage characterizations reveal a space-charge-limited-current conduction with an impact-ionization-assisted filling of traps in a moderately C-doped sample. On the other hand, highly compensated SI GaN forms a ~0.5-eV potential barrier at the interface with n-GaN, whereas the breakdown voltage exceeds 350 V. As the model explains, deep acceptors above the valence band compensate residual donors and lead to an electron mobility collapse. On the other hand, depending on C concentration, acceptors below the conduction band play a different role. In the moderately doped SI GaN, they act as electron traps and define the breakdown voltage. On the other hand, acceptors below the conduction band in highly compensated SI GaN are responsible for the barrier-controlled conduction and reaching of the avalanche.

Nationwide Seroprevalence of SARS-CoV-2 in Saudi Arabia

BackgroundEstimated seroprevalence of Coronavirus Infectious Disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) is a critical evidence for a better evaluation of the virus spread and monitoring the progress of COVID-19 pandemic in a population. In the Kingdom of Saudi Arabia (KSA), SARS-CoV-2 seroprevalence has been reported in specific regions, but an extensive nationwide study has not been reported. Here, we report a nationwide study to determine the prevalence of SARS-CoV-2 in the population of KSA during the pandemic, using serum samples from healthy blood donors, non-COVID patients and healthcare workers (HCWs) in six different regions of the kingdom, with addition samples from COVID-19 patients. MethodsA total of 11,703 serum samples were collected from different regions of the KSA including; 5395 samples from residual healthy blood donors (D); 5877 samples from non-COVID patients collected through residual sera at clinical biochemistry labs from non-COVID patients (P); and 400 samples from consented HCWs. To determine the seroprevalence of SARS-CoV-2, all serum samples, in addition to positive control sera from RT-PCR confirmed COVID-19 patients, were subjected to in-house ELISA with a sample pooling strategy, which was further validated by testing individual samples that make up some of the pools, with a statistical estimation method to report seroprevalence estimates. ResultsOverall (combining D and P groups) seroprevalence estimate was around 11% in Saudi Arabia; and was 5.1% (Riyadh), 1.5% (Jazan), 18.4% (Qassim), 20.8% (Hail), 14.7% (ER; Alahsa), and 18.8% in Makkah. Makkah samples were only D group and had a rate of 24.4% and 12.8% in the cities of Makkah and Jeddah, respectively. The seroprevalence in Saudi Arabia across the sampled areas would be 12 times the reported COVID-19 infection rate. Among HCWs, 7.5% (4.95–10.16 CI 95%) had reactive antibodies to SARS-CoV-2 without reporting any previously confirmed infection. This was higher in HCWs with hypertension. The study also presents the demographics and prevalence of co-morbidities in HCWs and subset of non-COVID-19 population. InterpretationOur study estimates the overall national serological prevalence of COVID-19 in Saudi Arabia to be 11%, with an apparent disparity between regions. This indicates the prevalence of asymptomatic or mild unreported COVID-19 cases.

Demonstration of polarization-induced hole conduction in composition-graded AlInN layers grown by metalorganic chemical vapor deposition

This Letter reports the polarization induced hole conduction in composition-graded AlInN epitaxial layers grown by metalorganic chemical vapor deposition. First, the composition-graded AlInN layer with an InN mole fraction from 0.12 to 0.20 was formed on c-plane GaN on sapphire, and they were confirmed to show the p-type hole conduction with a less temperature dependence, which is a feature of polarization-induced carriers. Then, blue light-emitting diodes (LEDs) with the composition-graded AlInN layers inserted in the p-type side were fabricated and their vertical current injection was investigated. The electroluminescence (EL) spectra confirmed that the fabricated LEDs exhibited a single-peak blue-light emission with the help of the impurity Mg doping. The LED simulation indicated that the impurity and polarization co-doping effectively compensated and overcame the residual oxygen donors in the AlInN layer and promoted the carrier recombination at the light-emitting layers. Finally, LEDs with the Mg-doped and composition-graded AlInN insertion layer exhibited good current–voltage characteristics with a low forward voltage drop of approximately 3 V in addition to the good EL spectra.

Nationwide Seroprevalence of SARS-CoV-2 in Saudi Arabia

Background: Estimated seroprevalence of Coronavirus Infectious Disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) is a critical evidence for a better evaluation of the virus spread and monitoring the progress of the COVID-19 pandemic in a population. In the Kingdom of Saudi Arabia (KSA), SARS-CoV-2 seroprevalence has been reported in specific regions, but an extensive nationwide study has not been reported. Here, we report a nationwide study to determine the prevalence of SARS-CoV-2 in the population of KSA during the pandemic, using serum samples from healthy blood donors, non-COVID patients and healthcare workers (HCWs) in six different regions of the kingdom, with addition samples from COVID-19 patients.Methods: A total of 11703 serum samples were collected from different regions of the KSA including; 5395 samples from residual healthy blood donors (D); 5877 samples from non-COVID patients collected through residual sera at clinical biochemistry labs from non-COVID patients (P); and 400 samples from consented HCWs. To determine the seroprevalence of SARS-CoV-2, all serum samples, in addition to positive control sera from RT-PCR confirmed COVID-19 patients, were subjected to in-house ELISA with a sample pooling strategy, which was further validated by testing individual samples that make up some of the pools, with a statistical estimation method to report seroprevalence estimates.Results: Overall (combining D and P groups) seroprevalence estimate was around 11% in Saudi Arabia; and was 5.1% (Riyadh), 1.5% (Jazan), 18.4% (Qassim), 20.8% (Hail), 14.7% (ER; Alahsa), and 18.8% in Makkah. Makkah samples were only D group and had a rate of 24.4% and 12.8% in the cities of Makkah and Jeddah, respectively. The seroprevalence in Saudi Arabia across the sampled areas would be 12 times the COVID-19 infection rate. Among HCWs, 7.5% (4.95-10.16 CI 95%) had reactive antibodies to SARS-CoV-2 without reporting any previously confirmed infection. This was higher in HCWs with hypertension. The study also presents the demographics and prevalence of co-morbidities in HCWs and subset of non-COVID-19 population.Conclusion: Our study estimates the overall national serological prevalence of COVID-19 in Saudi Arabia to be 11%, with an apparent disparity between regions.Funding Statement: This study was funded by KAIMRC, Grant: RC20/180; PI: Naif K. Alharbi.Declaration of Interests: The authors declare no conflict of interest.Ethics Approval Statement: The study was approved by the IRB in KAIMRC (Ministry of National Guard Health Affairs) for project number RC20-180. Residual samples from blood banks and clinical laboratories were obtained after an IRB approval. COVID-19 patients and HCWs signed informed consents to give blood samples.

Clinicopathological study of recipients’ corneal tissue excised during penetrating keratoplasty. Best out of waste!

Introduction: Corneal buttons removed during penetrating keratoplasties (PK) from recipients’ which were routinely discarded were studied with aim to analyze the common indications of PK and changing pattern of corneal diseases in developing countries like India. Materials and Methods: This is an observational case series carried out in tertiary care hospital of central India from October 2012 to October 2014. Histopathology of 156 corneal tissues was done of which 75 were recipients’ corneal tissues removed during PK and 81 were residual adjacent donors’ corneal tissues left out after corneal button was removed for transplant. Results: The most common indication of PK in our study was corneal ulcer (n=37, 49.33%), followed by infectious keratitis (n=22, 29.33%) which included ulcers in various stages of healing and complications associated with ulcer like adherent leucoma. Amongst infectious keratitis, 17 were (77.27%) of bacterial keratitis and 5 (22.73%) of fungal keratitis. Followed by bullous keratopathy 8(10.66%), 3(4%) of graft rejection and one each (1.33%) of Fuchs’ dystrophy and chronic actinic keratopathy. Of 81 donor’s corneal tissues, 75 were the residual corneal tissues from corneo-conjunctival junction which were transplanted within stipulated time. Only one case showed changes of chronic actinic keratopathy, rest showed normal histology. 6 corneas which were not transplanted within stipulated time showed epithelial denudation and stromal degeneration with loss of keratocytes. Conclusion: Histopathology gives additional insight to clinical diagnosis. Specific diagnosis can be commented in corneal opacities and infectious keratitis. Keywords: Penetrating Keratoplasty, PK, Histopathology, Corneal buttons, Recipient.

(Invited) Evaluation of 2" Power-Grade GaN Substrates Produced By the Near Equilibrium Ammonothermal (NEAT) Method

This paper provides evaluation of 2" GaN substrates produced by the near equilibrium ammonothermal (NEAT) method. Ammonothermal growth is a proven method of producing low-dislocation GaN crystals with practical productivity. [1, 2] By choosing a near-equilibrium growth condition, consistent growth with maintained crystal quality has been achieved. [3] Several tens of bulk GaN crystals were grown simultaneously, yielding many 2" GaN substrates in one growth batch. Figure 1 is an example of bulk GaN crystals after shaping and Figure 2 is an example of 2" GaN wafer.We have developed an X-ray mapping technique which presents the distribution of full width half maximums (FWHMs) of rocking curves from 002 and 201 diffractions over a 2" wafer. The X-ray mapping revealed superior microstructure quality of NEAT GaN substrates. NEAT GaN substrates typically show an average full width half maximum (FWHM) of the 002 X-ray rocking curve of 30 arcsec or better, as shown in Figure 3.The X-ray mapping data were compared with other characterization techniques, including X-ray topography. X-ray topography confirmed that a dislocation density of typical NEAT GaN substrate was about 2 x 105 cm-2. Wide area X-ray topography also revealed flatter lattice of a NEAT GaN substrate compared with a GaN substrate fabricated by hydride vapor phase epitaxy (HVPE).Additionally, recent progress in oxygen reduction yielded NEAT GaN substrates with reduced residual donors, coloration and optical absorption. The oxygen concentration was about 2 x 1018 cm-2. An absorption coefficient of 1.3 cm-1 was obtained at 450 nm.The usability of NEAT GaN substrates was also confirmed through device fabrication. Multiple collaborators successfully fabricated high-power diodes with breakdown voltage over 1000V.

Luminescence spatial characteristics of ZnSe:Fe

Two types of thermal-diffusion-based technological routes have been realized to obtain ZnSe:Fe plates demanded for laser application in the mid-IR spectral range. For the technological routes used, the iron doping process is shown to lead to the formation of auxiliary luminescence bands including (1) luminescence of residual tetrahedral donors and acceptors; (2) unknown bound exciton line located at 459 nm; (3) broad bands with maxima at 490, 520, 670, and 820 nm; and (4) IR band at 960 nm previously assigned to centers related to transition metals. Using the measurements of photoluminescence under conditions of two-photon excitation and measurements of low-temperature microphotoluminescence, the intensity profiles of the bands have been shown to be uncorrelated with the concentration profile of the optically active (Fe2+) iron. This trend does not depend on the technological routes used. Thus, heavily Fe doped areas, as well as adjacent regions in ZnSe:Fe plates produced by thermal-diffusion-based techniques, contain auxiliary centers governing luminescence in visible and near IR spectral ranges. These centers are not directly related to the optically active (Fe2+) iron.

Electrical properties of bulk semi-insulating β-Ga2O3 (Fe)

The Fermi level in bulk semi-insulating β-Ga2O3 doped with Fe (∼5 × 1018 cm−3) is found to be pinned near Ec − 0.85 eV. At temperatures ≥400 K, Ni Schottky diodes showed good rectification and measurable low frequency capacitance, allowing the measurement of capacitance-frequency (C-f), capacitance-voltage (C-V), and capacitance-temperature (C-T) characteristics. The activation energy and the electron capture cross section obtained were (0.75–0.82) eV and (2–5) × 10−15 cm2, in good agreement with the reported signature of the E2 electron trap assigned to Fe. The concentration of the filled centers determined from C-V was close to the concentration of residual shallow donors in undoped materials. Photoinduced current transient spectroscopy measurements showed that Fe doping does not promote the generation of high densities of deep traps other than those related to Fe.

Modeling and interpretation of UV and blue luminescence intensity in β-Ga2O3 by silicon and nitrogen doping

Temperature-dependent cathodoluminescence spectra of (2¯01) Si-doped β-Ga2O3 single crystals and (010) N-doped epitaxial films were comprehensively shown to investigate their electronic structure and defect states. The decrease in the self-trapped exciton (STE) emission at low temperatures in heavily Si-doped crystals implied the Debye-Hückel screening of STEs with the critical charge density larger than 2 × 1018 cm−3. The analysis based on the rate equation model suggested a significant influence of the donor-acceptor-pair recombination involving a SiGa donor on the UV luminescence band in the Si-doped crystals. The blue luminescence band was suppressed for the heavily N-doped epitaxial films with a N concentration of 1 × 1018 cm−3, implying the decrease in the oxygen vacancy (VO) concentration by N doping. The increase in the NO acceptor concentration as well as the decrease in the VO concentration were found to contribute to the compensation of residual Si donors resulting in high resistivity of N-doped epitaxial films.

High electron mobility transistors with Fe-doped semi-insulating GaN buffers on (1 1 0) Si substrates grown by ammonia molecular beam epitaxy

The electrical properties of AlGaN/GaN high electron mobility transistor structures with composite Fe-doped GaN/undoped GaN buffers grown on (1 1 0) Si substrates by ammonia molecular beam epitaxy were reported. Fe concentration in the range of 1017 cm−3 is sufficient to compensate the residual donors (<1016 cm−3) of undoped GaN and demonstrated a highly resistive GaN on (1 1 0) Si substrates. For buffers with Fe concentration of 1 × 1017 ∼ 1 × 1018 cm−3, the buffer was semi-insulating, with the Fermi level pinned near 0.5–0.7 eV which is determined by temperature-dependent I-T measurements. From the PICTS measurements, the Fe-related peaks of 0.69 eV (B) and 0.86 eV (C) are observed. The peak B is most likely due to the center pinning the Fermi level which is close to the activation energy observed in the temperature-dependent current measurement of Fe-doped sample, tentatively attributed to complexes between substitutional Fe ions and native defects such as nitrogen vacancies. The peak C is similar to the hole trap of 0.85 eV previously reported. In spite of similar transfer device characteristics, the Fe-doped buffer shows a noticeable difference in the off-state leakage behaviors. The sample grown with the optimized Fe concentration of 1 × 1017 cm−3 had a buffer resistivity of approximately 3.84 × 1011 Ω cm, increased by almost four orders of magnitude with respect to that (4.5 × 107 Ω cm) of the reference sample with undoped GaN buffer.

The trap states in lightly Mg-doped GaN grown by MOVPE on a freestanding GaN substrate

We investigated traps in lightly Mg-doped (2 × 1017 cm−3) p-GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on a freestanding GaN substrate and the subsequent post-growth annealing, using deep level transient spectroscopy. We identified four hole traps with energy levels of EV + 0.46, 0.88, 1.0, and 1.3 eV and one electron trap at EC − 0.57 eV in a p-type GaN layer uniformly doped with magnesium (Mg). The Arrhenius plot of hole traps with the highest concentration (∼3 × 1016 cm−3) located at EV + 0.88 eV corresponded to those of hole traps ascribed to carbon on nitrogen sites in n-type GaN samples grown by MOVPE. In fact, the range of the hole trap concentrations at EV + 0.88 eV was close to the carbon concentration detected by secondary ion mass spectroscopy. Moreover, the electron trap at EC − 0.57 eV was also identical to the dominant electron traps commonly observed in n-type GaN. Together, these results suggest that the trap states in the lightly Mg-doped GaN grown by MOVPE show a strong similarity to those in n-type GaN, which can be explained by the Fermi level close to the conduction band minimum in pristine MOVPE grown samples due to existing residual donors and Mg-hydrogen complexes.

Fe-doped semi-insulating GaN with solid Fe source grown on (110) Si substrates by NH3 molecular beam epitaxy

Iron doped GaN layers were grown on (110) Si substrates by ammonia molecular beam epitaxy (MBE) using solid elemental iron as a source. Specular films with concentrations up to 1×1020cm−3, as determined by secondary ion mass spectroscopy, were grown, unlike a limited incorporation of Fe into GaN by metal-rich rf plasma MBE. The Fe concentration in the film showed an exponential dependence on the inverse of source temperature with an activation energy of 3.4eV, which agrees well to the reported value for the sublimation of Fe. A 1.5µm thick GaN film with a sheet resistance of 1GΩ/sq. was obtained by compensating unintentional residual donors with a small Fe concentration of 1×1017cm−3. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that the Fe incorporation required to obtain the semi-insulating film properties did not affect the structural properties of the film. The low-temperature PL spectra of highly resistive and semi-insulating Fe:GaN in the range of 1017~1018cm−3 show dominant exciton emissions and enhanced donor-acceptor-pair (DAP) emissions, implying that Fe ions contribute to the DAP transition between donor levels and Fe-related acceptor levels, possibly compensating the residual donors to achieve the semi-insulating electrical properties.

Effects of Mg/Ga and V/III source ratios on hole concentration of N-polar p-type GaN grown by metalorganic vapor phase epitaxy

The effects of growth conditions such as Mg/Ga and V/III ratios on the properties of N-polar p-type GaN grown by metalorganic vapor phase epitaxy were studied. Photoluminescence spectra from Mg-doped GaN depended on Mg/Ga and V/III ratios. For the lightly doped samples, the band-to-acceptor emission was observed at 3.3 eV and its relative intensity decreased with increasing V/III ratio. For the heavily doped samples, the donor–acceptor pair emission was observed at 2.8 eV and its peak intensity monotonically decreased with V/III ratio. The hole concentration was maximum for the Mg/Ga ratio. This is the same tendency as in group-III polar (0001) growth. The V/III ratio also reduced the hole concentration. The higher V/III ratio reduced the concentration of residual donors such as oxygen by substituting nitrogen atoms. The surface became rougher with increasing V/III ratio and the hillock density increased.

Growth of High Crystalline Quality HVPE-GaN Crystals with Controlled Electrical Properties

Thick free-standing hydride vapor phase epitaxy (HVPE) GaN wafers were grown with close to perfect crystalline quality and with more than two orders of magnitude reduced free carrier concentration relative to ammonothermal GaN without resorting to deep level compensation. These new homoepitaxially grown crystals exceed the quality of any commercially available GaN substrates. High crystalline quality is confirmed by XRD rocking curve measurements having full width at half-maximum of only 16 arc s. Detailed Raman scattering spectroscopy and imaging show that the wafers are biaxially stress-free and have a uniform and low background doping. The stress-free, flat nature of these substrates is essential for high-yield device fabrication. High-resolution photoluminescence studies in combination with SIMS analysis identify Si and O as residual shallow donors with concentration levels orders of magnitude lower than those present in ammonothermal GaN substrates.

DEEP TRAPS SPECTRA IN UNDOPED GAN FILMS GROWN BY HYDRIDE VAPOR PHASE EPITAXY UNDER VARIOUS CONDITIONS

Decreasing the residual donors density and deep tra ps spectra densities in undoped GaN films grown by Hydride Vapor Phase Epitaxy (HVPE) is very important for promoting the use of such material in highvoltage/high-power rectifiers, radiation detectors. In this study we studied the effects of changing t he growth temperature of undoped HVPE GaN films on these properties. The two groups of undoped GaN HVPE samples analyzed in this study were grown at growth temperature being either 850oC or 950oC. Measurements by means of Capacitance-Voltage (C-V) profiling, deep levels transient spectroscopy, Micr o Cathode Luminescence (MCL) spectroscopy and imaging and by Electron Beam Induced Current (EBIC) showed a much lower density of residual donors (by almost two orders of magnitude), of deep electron t raps and hole traps (by about an order of magnitude) and considerably (about 1.5 times) longer diffusion le ngth of charge carriers in the films grown at 850oC comp ared to samples prepared at 950oC. The data obtaine d indicate that there is an optimal reduced growth te mperature (close to 850oC) resulting in lower concentration of shallow donors and deep traps whil e still preserving the high crystalline quality of the layer. This is of paramount importance for device a pplications of HVPE grown undoped GaN films.