N-type Epilayers Research Articles

The Effect of Light Ion Irradiation on 4H-SiC MPS Power Diode Characteristics: Experiment and Simulation

In this article, the effect of local radiation damage on the electrical characteristics of 1700 V 4H-SiC Merged-Pin Schottky (MPS) diode have been investigated. Radiation defects introduced by irradiation with 670 keV protons were placed into the low-doped n-type epi-layer and their influence on diode characteristics were characterized by capacitance DLTS, C-V profiling and I-V measurements. Simulation model accounting for the effect of proton irradiation was developed, calibrated and used for analysis of underlying effects and temperature dependencies. Results show that the forward voltage drop and breakdown voltage is insensitive to the position of the damage region when the defect peak is placed far away from the Schottky metal contact of the MPS diode. However, when the damage region approaches to the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> regions, forward voltage drop degrades significantly. For fluences higher than 3.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">- 2</sup> , the acceptor concentration in the peak region achieves donor doping level of the epi-layer and a sharp increase in the diode forward voltage drop is observed. Acceptor centers introduced by proton irradiation also slightly increase the breakdown voltage while decreasing the leakage current at voltages close to the MPS diode breakdown ( > 2000 V).

Features of free carrier and exciton recombination, diffusion, and photoluminescence in undoped and phosphorus-doped diamond layers

We investigated carrier dynamics and photoluminescence in undoped and n-type phosphorus-doped diamond epilayers under interband picosecond-pulse photoexcitation at 213nm. Carrier/exciton lifetime and diffusion coefficient D were determined in 4×1017 to 4×1019cm−3 carrier density range. The low D=7÷11cm2/s value in the undoped epilayer was ascribed to exciton formation and contribution of many-body effects. At low excitations, photoluminescence spectra in the undoped layer exhibited exciton emission, while at the higher ones (above 2×1018cm−3) free carrier emission became dominant. The calculated radiative lifetime of excitons (860ns) was found much higher than the experimentally measured nonradiative lifetime of 11ns, governed by excess carrier diffusive flow to the surface, with S=(3±2)×105cm/s surface recombination velocity. In the n-type layer, donor-related emission dominated and the phosphorus-related defect complexes diminished the carrier lifetime to 30–50ps. Optically recharged phosphorus states resulted in millisecond long decay tails of trapped carriers.

Fabrication of current confinement aperture structure by transforming a conductive GaN:Si epitaxial layer into an insulating GaOx layer.

We report here a simple and robust process to convert embedded conductive GaN epilayers into insulating GaOx and demonstrate its efficacy in vertical current blocking and lateral current steering in a working LED device. The fabrication processes consist of laser scribing, electrochemical (EC) wet-etching, photoelectrochemical (PEC) oxidation, and thermal oxidization of a sacrificial n(+)-GaN:Si layer. The conversion of GaN is made possible through an intermediate stage of porosification where the standard n-type GaN epilayers can be laterally and selectively anodized into a nanoporous (NP) texture while keeping the rest of the layers intact. The fibrous texture of NP GaN with an average wall thickness of less than 100 nm dramatically increases the surface-to-volume ratio and facilitates a rapid oxidation process of GaN into GaOX. The GaOX aperture was formed on the n-side of the LED between the active region and the n-type GaN layer. The wavelength blueshift phenomena of electroluminescence spectra is observed in the treated aperture-emission LED structure (441.5 nm) when compared to nontreated LED structure (443.7 nm) at 0.1 mA. The observation of aperture-confined electroluminescence from an InGaN LED structure suggests that the NP GaN based oxidation will play an enabling role in the design and fabrication of III-nitride photonic devices.

Decay curve analyses in carrier lifetime measurements of p- and n-type 4H-SiC epilayers

The impacts of the ambipolar diffusion constant and surface recombination in carrier lifetime measurements of p- and n-type 4H-SiC epilayers are investigated in detail by comparing a numerical simulation based on a diffusion equation and the measurement of microwave photoconductance decay (µ-PCD) curves measured from 4H-SiC epilayers. The simulation reveals that the shapes of decay curves of excess carrier concentration in epilayers, which defines the effective carrier lifetime, are different between p- and n-type 4H-SiC under a low-level injection condition, even when the bulk lifetime and the surface recombination velocity are fixed to the same values for p- and n-type epilayers. In experiments, the shapes of the microwave photoconductance decay curves measured from p- and n-type 4H-SiC epilayers show a similar tendency to the simulation results under a low-level injection condition. This is attributed to the difference in the dependence of the ambipolar diffusion constant on the excess carrier concentration for p- and n-type 4H-SiC. The comparison of µ-PCD decay curves obtained from 50-µm-thick epilayers with different surface passivation indicates that the surface recombination velocity on the epilayer passivated with deposited SiO2 followed by NO annealing is about one order of magnitude lower than that of the epilayer passivated with the dry oxide.

The effects of high-energy uranium ion irradiation on Au/n-GaN Schottky diodes

The I–V and C–V characteristics of Au/n-GaN Schottky diodes irradiated with 290-MeV 238U32+ ions are presented. The U ions can penetrate the n-type GaN epi-layer with a thickness about 3μm grown on the c-plane of a sapphire substrate using the MOCVD technique, leaving a purely electronic energy deposition. The Au/n-GaN Schottky diodes were irradiated to successively increasing fluences from 1×109 to 5×1011ionscm−2. The measured I–V curves show that the height of the Schottky barrier decreases after irradiation and that the Schottky barrier almost disappears when the ion fluence reaches 5×1010ionscm−2. Meanwhile, the irradiation increases the series resistance. The C–V curves show that the capacitance drops sharply when the ion fluence reaches 5×1010ionscm−2. The dielectric constant also decreases following the irradiation. The changes of the electrical properties are ascribed to the neutralization of the donor-like surface state and the acceptor-like surface state due to the migration of Au atoms at the interface of Au/n-GaN under energetic U ions irradiations.

Structural, Raman scattering and magnetic characteristics of Er+-implanted GaN thin films

Er+ ions were implanted into the n-type, unintentionally-doped (u-type) and p-type GaN epilayers, respectively. Subsequent rapid thermal annealing process was carried out at 800°C in N2 ambience for 5min. It is found that the micro-structure of GaN:Er films have a close relationship with the implantation and annealing process. Four new Raman peaks were introduced by Er+ ions implantation at 300, 362, 670 and 855cm−1, respectively. All the samples exhibit room-temperature ferromagnetism, which can be well explained by the bound magnetic polaron theory. Our experiments also revealed that the magnetic properties of GaN:Er samples are closely related to their conduction type and initial carrier concentration, and the magnetic coupling between the magnetic moments of Er3+ ions and electrons is stronger than that with holes.

Multi-Cycle High Temperature Rapid Thermal Annealing for Dislocation Elimination in 4H-SiC Epitaxy

High power devices fabricated in 4H-silicon carbide (SiC) epilayers are known to suffer from forward voltage degradation caused by basal plane dislocation (BPD) induced stacking faults (SF) [[1], [2]]. Over the past decade, several efforts have been successful in removal of more than 90% of the BPDs in the substrate by converting them to benign threading edge dislocations (TED) during epigrowth [[3], [4]]. However, BPDs in the many areas of the epilayers are still > 100 cm-2, which adversely affects the device yield and reliability. We have developed a technique to remove the existing BPDs from the epilayers by using a novel multi-cycle rapid thermal annealing (MRTA) process [[5]], where the BPDs in the epilayers were eliminated, while preserving the epitaxial surface.We used 4 offcut, 15 mm, n-type 6E15 4H-SiC epilayers, commercially obtained. The as-grown samples were imaged using ultraviolet photoluminescence (UVPL) imaging. Carriers were excited with the 334 nm line from an Ar-ion laser, and images were collected in the emission range of 600-1000 nm. The samples were graphite capped and annealed at 1750 oC-1875 oC for a total time of ~5 minutes in 20 cycles. The annealing was performed in a custom designed chamber with inductive sample heating and ultrapure N2 overpressure at 0.41MPa. The base temperature was maintained near 1500 oC, from where the temperature was ramped up and down to the desired temperature in rapid cycles to get an accumulated time of 5 mins at the desired temperature. The combination of high N2 overpressure and rapid cycles enabled us to perform the anneals at a higher temperature than conventional methods, while preserving the graphite cap, and thus maintaining a pristine epitaxial surface. Post-annealed samples were also imaged to observe the BPDs in the epilayers. The UVPL images of the as grown samples showed an average BPD density ~1000 /cm2. We wanted to achieve two goals with our annealing efforts, which were to eliminate BPDs in the epitaxial layers and to preserve the epitaxial surface morphology. We conducted several previous attempts to anneal similar samples using high temperature microwave annealing as well as annealing under high N2 pressure. From our work with microwave annealing, we were able to eliminate BPDs by annealing at above 1750 oC. However, the surface morphology was poor. The graphite capping technique was tweaked but the microwave annealing performed at ambient pressure conditions was not able to achieve a good sample surface. Annealing under high pressure preserved the surface, however, could not go to temperatures higher than 1750 oC without damaging the cap. For complete BPD elimination much higher temperature annealing was required. Thus, the novel MRTA process was adopted, which went to higher temperatures and preserved the graphite cap. Results from the post annealed samples using the different techniques would also be presented.The UVPL images of the samples after the MRTA process indicated a surface without any significant pits or generation of any new BPDs during the annealing process, which has been shown to happen using conventional annealing at these high temperatures. Almost all the BPDs that were previously present disappeared in the epilayer. It is proposed that during annealing BPDs that had not yet converted to threading edge dislocations (TED) during epigrowth forms a TED near the surface. Then the TEDs at the end of all the BPDs glide in a prismatic plane causing the BPD to shorten. Since this glide doesn't occur in the lower energy basal plane, the required energy for this glide is provided by the very high annealing temperature. This way a BPD segment converts to a TED with the TED glide and thus shortens the BPD. The UVPL images of the annealed samples also show a dot where the original BPD had propagated from the substrate, which represents a TED.[[1]] J. P. Bergman, Mater. Sci. Forum 353, 299 (2001)[[2]] R. E. Stahlbush, Mater. Sci. Forum 389, 427 (2002)[[3]] S. Ha, Phys. Rev. Lett. 92, 175504 (2004)[[4]] R. L. Myers-Ward, Mat. Sci. Forum 615-617, 105 (2009)[[5]] T. J. Anderson, Elect. Lett. 50, 197 (2014)Figure: Typical UVPL image of as grown sample with BPD density ~1000 /cm2. The short horizontal white lines are BPDs. Lower image is sample after MRTA at 1875 oC for 5mins, with no BPDs. The long scratch line is scribe mark on backside.

Development of 10 kV 4H-SiC JBS diode with FGR termination

The design, fabrication, and electrical characteristics of the 4H-SiC JBS diode with a breakdown voltage higher than 10 kV are presented. 60 floating guard rings have been used in the fabrication. Numerical simulations have been performed to select the doping level and thickness of the drift layer and the effectiveness of the edge termination technique. The n-type epilayer is 100 μm in thickness with a doping of 6 × 1014 cm−3. The on-state voltage was 2.7 V at JF = 13 A/cm2.

Low resistance palladium/molybdenum based ohmic contacts to n-GaSb grown on GaAs

Low resistance ohmic contacts were fabricated on n-type GaSb grown by molecular beam epitaxy. N-type GaSb epilayers with different doping concentrations and thicknesses were fabricated and studied in order to investigate the current transport mechanism between the metal contacts and the semiconductor. Different metallization schemes were implemented to achieve the lowest possible contact resistance. Rapid thermal annealing was performed at various temperatures to achieve the optimal gold penetration into the GaSb epilayers for low resistance. Ohmic contact fabrication and electrical characterization are discussed in detail. The microstructure analysis of the semiconductor and metal contact interfaces was performed using cross-section transmission electron microscopy and energy dispersive spectroscopy. Specific contact resistances as low as 3 × 10−6 Ω cm2 were obtained.

Zinc oxide thin films on silicon carbide substrates (ZnO/SiC): electro-optical properties and electrically active defects

The electrical and optical properties of heterojunctions formed by thermally deposited ZnO thin films on n-type 4H-SiC substrates have been investigated. Current–voltage characteristics of the fabricated light emitting devices revealed excellent rectifying behaviors with a typical leakage current lower than 1 nA at a reverse bias of −3 V, and with a forward current at 3 V in the range of 2 mA. A study of the electroluminescent characteristics of ZnO/SiC heterojunctions over the temperature range of 50–450 K showed an emission peak around 410 nm and a broad defect-related electroluminescence at room temperature in the visible range for a forward current of 300 mA. Electrically active deep level centers in ZnO and n-type 4H-SiC epilayers have been investigated by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS). Additionally, LDLTS has successfully been employed to resolve the closely spaced hole trap energy levels.

SiC Epi-Channel Lateral MOSFETs

SiC lateral MOSFETs with multi-layers epi-channels were studied in this work. The epi-channel with a high concentration n-type epilayer sandwiched by two lightly doped p-type layers showed a maximum field effect mobility of 17 cm2/V.s, improved from 1.53 cm2/V.s of devices without epi-channels. These devices are normally-off with an average threshold voltage of 1.34V.

Deep traps in n-type GaN epilayers grown by plasma assisted molecular beam epitaxy

In this study, we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77 K–350 K. Undoped GaN layers were grown using the plasma-assisted molecular beam epitaxy technique on commercial GaN/sapphire templates. The quality of the epilayers was studied by micro-Raman spectroscopy (μ-RS) which proved the hexagonal phase and good crystallinity of GaN epilayers as well as a slight strain. The photoluminescence spectrum confirmed a high crystal quality by intense excitonic emission but it also exhibited a blue emission band of low intensity. DLTS signal spectra revealed the presence of four majority traps: two high-temperature and two low-temperature peaks. Using the Laplace DLTS method and Arrhenius plots, the apparent activation energy and capture cross sections were obtained. For two high-temperature majority traps, they were equal to E1 = 0.65 eV, σ1 = 8.2 × 10−16 cm2 and E2 = 0.58 eV, σ2 = 2.6 × 10−15 cm2 whereas for the two low-temperature majority traps they were equal to E3 = 0.18 eV, σ3 = 9.7 × 10−18 cm2 and E4 = 0.13 eV, σ4 = 9.2 × 10−18 cm2. The possible origin of the traps is discussed and the results are compared with data reported elsewhere.

Influence of charge imbalance on breakdown voltage of 4H-SiC semi-superjunction VDMOSFET

SiC semi-superjunction vertical double diffused MOS (VDMOSFET) has higher breakdown voltage than conventional SiC VDMOSFET with the same on-resistance. The ion implantation to form p pillar region on N-type epilayer is a key process to form semi-superjunction stucture. The influences of charge imbalance induced by ion implantation on breakdown voltages of 4H-SiC superjunction and semi-superjunction VDMOSFET are investigated through two-dimensional numerical simulation, and the largest breakdown voltage is obtained when charge imbalance is 30%. With the same structure parameters of devices, when breakdown voltage decreases by 15% due to the deviation of doping concentration in P pillars, the tolerance of doping concentration for the semi-superjunction VDMOSFET is 69.5% higher than for superjunction VDMOSFET which means that less precise process control of ion implantation for semi-superjunction VDMOSFET, will be required with less difficulty in the manufacture of pillars.

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors based on metalorganic vapor-phase epitaxy-grown epilayers

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors, fabricated by growing p-and n-type CdTe epilayers on (211) n+-Si substrates using metalorganic vapor-phase epitaxy (MOVPE), were studied by analyzing current-voltage characteristics measured at various temperatures. The diode fabricated shows good rectification properties, however, both forward and reverse biased currents deviate from their ideal behavior. The forward current exhibits typical feature of multi-step tunneling at lower biases; however, becomes space charge limited type when the bias is increased. On the other hand, the reverse current exhibits thermally activated tunneling-type current. It was found that trapping centers at the p-CdTe/n-CdTe junction, which were formed due to the growth induced defects, determine the currents of this diode, and hence limit the performance of the nuclear radiation detectors developed.

Effect of periodic Si-delta-doping on the evolution of yellow luminescence and stress in n-type GaN epilayers

We report the effect of periodic Si-delta-doping on the yellow luminescence (YL) and stress evolution in n-GaN epilayers. Photoluminescence measurements indicated that the YL component could be effectively suppressed with increasing dopant flow rate or decreasing period length. X-ray diffraction, transmission electron microscopy and secondary ion mass spectroscopy measurements revealed that the variations of dislocation density and carbon concentration are not the responsible mechanisms for the of YL emissions. The reduction of Ga vacancy related defects seems to be the probable reason for the YL suppression by Si-delta-doping. Moreover, Raman spectroscopy showed no obvious stress variation among these samples with different equivalent electron concentration. These observed features are entirely different from those reported for uniformly Si-doped GaN.

Simulation, Fabrication and Characterization of 4500V 4H-SiC Normally-Off VJFET

Simulation, Fabrication and characteristics of high voltage, normally-off JFETs in 4H-SiC are presented. The devices were built on ND= 1.01015 cm-3 doped 50μm thick n-type epilayer grown on a n+ 4H-SiC. Parameters of edge termination have been optimized by simulations. Its blocking voltage exceeds 4500V at gate bias VG = -6V and forward drain current is in excess of 3A at gate bias VG = 3V and drain bias VD = 5V corresponding a current density of 80A/cm2.

A comparative study of the structural and electrical properties of n-type InGaN epilayer grown by MBE and commercially

Abstract This work reports the growth of n-In 0.27 Ga 0.73 N/GaN/AlN epitaxial layer on Si(1 1 1) substrate by using plasma-assisted molecular beam epitaxy (MBE) and commercially obtained n-In 0.08 Ga 0.92 N/AlN. As-grown and commercial thin films were characterized by using field emission scanning electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. A high work function metal (Pt) was deposited as metal contact on the thin films, and the electrical characteristics of the films pre- and post-annealed at 500 °C were studied under 3 V. Results show that the electrical characteristics of post-annealed thin films are better than those of pre-annealed thin films.

Investigation on origin of Z1/2 center in SiC by deep level transient spectroscopy and electron paramagnetic resonance

The Z1/2 center in n-type 4H-SiC epilayers—a dominant deep level limiting the carrier lifetime—has been investigated. Using capacitance versus voltage (C-V) measurements and deep level transient spectroscopy (DLTS), we show that the Z1/2 center is responsible for the carrier compensation in n-type 4H-SiC epilayers irradiated by low-energy (250 keV) electrons. The concentration of the Z1/2 defect obtained by C-V and DLTS correlates well with that of the carbon vacancy (VC) determined by electron paramagnetic resonance, suggesting that the Z1/2 deep level originates from VC.

Point defect reduction and carrier lifetime improvement of Si- and C-face 4H-SiC epilayers

The impact of two post-growth processes, namely, C+-implantation/annealing process and thermal oxidation/annealing process, on trap concentrations in thick n-type 4H-SiC epilayers was studied for both Si- and C-face. Conditions such as the implantation dose and annealing temperature of the C+-implantation/annealing processes were optimized for Si-face epilayers, and consequently the Z1/2 center was eliminated up to 100 μm or more, and the minority carrier lifetime reached 13 μs while maintaining a good surface morphology. The effect of the process conditions on the creation of new traps, including ON1 center, was also studied in both Si- and C-face epilayers. The ON1 center was introduced in both Si- and C-face by two post-growth processes, although the concentration was found to vary according to the polar face and the post-growth processes. The mechanism of the different impacts on Z1/2 center reduction and ON1 center creation by the two post-growth processes on Si- and C-face is discussed.

16 kV, 1 cm&lt;sup&gt;2&lt;/sup&gt;, 4H-SiC PiN Diodes for Advanced High-Power and High-Temperature Applications

In this work, we report our recently developed 16 kV, 1 cm2, 4H-SiC PiN diode results. The SiC PiN diode was built on a 120 µm, 2×1014/cm3 doped n-type SiC drift layer with a device active area of 0.5175 cm2. Forward conduction of the PiN diode was characterized at temperatures from 20°C to 200°C. At high injection-current density (JF) of 350 ~ 400 A/cm2, the differential on-resistance (RON,diff) of the SiC PiN diode decreased from 6.08 mΩ·cm2 at 20°C to 5.12 mΩ·cm2 at 200°C, resulting in a very small average temperature coefficient of –5.33 µΩ·cm2/°C, while the forward voltage drop (VF) at 100 A/cm2 reduced from 4.77 V at 20°C to 4.17 V at 200°C. This is due to an increasing high-level carrier lifetime with an increase in temperature, resulting in reduced forward voltage drop. We also observed lower RON,diff at higher injection-current densities, suggesting that a higher carrier lifetime is needed in this lightly doped n-type SiC thick epi-layer in order to achieve full conductivity modulation. The anode to cathode reverse blocking leakage current was measured as 0.9 µA at 16 kV at room temperature.