W.E. Carlos

Paramagnetic resonance in GaN‐based light emitting diodes

Significant advances in GaN‐based materials and devices have prompted intense interest in the group III nitrides. In this letter, we report electroluminescence‐detected magnetic resonance (ELDMR) and electrically detected magnetic resonance (EDMR) results on InGaN/AlGaN double heterostructures which have an intense blue emission. The dominant feature detected by either technique is a broad resonance (ΔB∼21 mT) at g≊2.00. Our ELDMR measurements show that this is associated with the blue emission and we ascribe this resonance signal to a deep Zn‐related acceptor. A second resonance, resolved in EDMR, is tentatively identified as a deep donor trap. Based on our results we propose a model for the blue emission from these diodes.

Characterization of Nitrides by Electron Paramagnetic Resonance (EPR) and Optically Detected Magnetic Resonance (ODMR)

Abstract We will highlight our recent work on the properties of residual defects and dopants in GaN heteroepitaxial layers and on the nature of recombination from InGaN single quantum well (SQW) light emitting diodes (LEDs) through magnetic resonance techniques. Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) were performed on undoped (highly resistive and n-type) and intentionally doped (Si, Mg, or Be) GaN films grown by a variety of techniques (MOCVD, MBE, and HVPE) in order to obtain general trends and behavior. Through the spin-Hamiltonian parameters, these methods can reveal symmetry information, the character of the wave function and (ideally) the chemical identity of the defect. In addition, low temperature EPR intensities can be used to determine the neutral acceptor or donor concentrations without the need for contacts or the high temperatures required for Hall effect measurements. The ODMR was performed on both bandedge (mainly shallow donor–shallow acceptor recombination) and deep (visible and near-IR) PL bands. In spite of the radically different (non-equilibrium) growth techniques, many of the same defects were found in the various samples. Finally, earlier ODMR studies of recombination from Nichia InGaN ‘green’ and ‘blue’ LEDs were extended to include shorter (‘violet’) and longer (‘amber’) wavelength LEDs and an undoped 30 A In0.3Ga0.7N/GaN heterostructure. The results provide evidence for spatially separated electrons and holes in the optically-active 30 A InGaN layers under low photoexcitation conditions, likely due to localization at different potential minima in the x–y planes and/or the large strain-induced piezoelectric fields parallel to the growth direction.

Optical and magnetic resonance characterization of undoped and doped wurtzite GaN films deposited on sapphire substrates

Abstract We have used a combination of defect-sensitive techniques to study native defects and impurities in GaN films deposited on the basal plane of sapphire. The optical and electronic properties of undoped samples are dependent on the degree of compensation between shallow donor and acceptors levels. The paramagnetic studies of undoped films reveal resonances from effective-mass and deep-donor states. Photoluminescence studies of films doped with Mg-acceptors indicate a possible dependence of the localization of the Mg-related center with the increasing concentration of activated acceptors. The optically detected magnetic resonance measurements of Mg-doped samples show that the Mg-acceptor is perturbed from its effective-mass state.

Magnetic resonance studies of defects in GaN with reduced dislocation densities

Abstract Magnetic resonance experiments, including optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR), have been performed on Si-doped homoepitaxial GaN layers grown by MOCVD and on high quality, free-standing (∼200xa0μm-thick) GaN grown by HVPE. This allowed us to obtain information on the properties of native defects and dopants in GaN with a significantly reduced density of dislocations ( 7 xa0cm −2 ) compared to that typically observed (∼mid 10 8 –10 10 xa0cm −2 ) in conventional heteroepitaxial GaN layers. The high structural and optical quality of the layers was revealed by cross-sectional TEM and detailed low-temperature photoluminescence (PL) studies, respectively. ODMR at 24xa0GHz on strong shallow donor–shallow acceptor recombination from the Si-doped homoepitaxial layer reveals evidence for Si or C shallow acceptors on the N sites. EPR of the new free-standing HVPE GaN confirms the low concentration of residual donors (∼10 16 xa0cm −3 ) as determined by Hall effect measurements. In addition, new deep centers are found from ODMR on the 2.4xa0eV “green” PL band and on the broad emission less than 1.8xa0eV from the HVPE GaN template. However, contrary to expectations, the reduction of random strain fields (associated with dislocations) has not led to significant changes in the character of the magnetic resonance (such as resolved electron-nuclear hyperfine structure) compared to that typically found for heteroepitaxial GaN layers.

Optical and EPR Signatures of Intrinsic Defects in Ultra High Purity 4H-SiC

The defect(s) responsible for the semi-insulating behavior of undoped 4H-SiC grown by physical vapor transport are investigated by correlated EPR and optical spectroscopy. We establish that the UD2 PL is due to the VC-CSi pair defect by comparing changes with annealing and comparing individual lines in the four line spectra with specific lines of the EPR center. Annealing studies also suggest that the Si-vacancy is more stable than in irradiated SiC and preliminary results indicate that it begins to evolve into the pair defect for T~1000° C. The various simple lattice defects and their possible role in the SI behavior are discussed. Introduction The 4H polytype of SiC can be produced in reasonable quantities of good crystal quality and has high electron mobility, a large bandgap and high thermal conductivity, making it an important material for high power microwave electronics. Native defect-based semi-insulating (SI) SiC has recently become commercially available while the defect or defects responsible for the SI behavior remain unidentified. Although the carbon vacancy is a ubiquitous intrinsic defect observed in EPR studies of these substrates, its concentration appears to be significantly lower (≤ 1 0 15 cm -3 ) than that of the residual background impurities, commonly B and N. In this work, we discuss the Si vacancy and the carbon vacancy-carbon antisite pair (VC-CSi) and show that the latter is responsible for the UD2 IR photoluminescence (IRPL). Experimental Results The samples were grown by physical vapor transport (PVT) with B and N levels of ~10 16 cm -3 . [1] One sample was electron irradiated and annealed at 1000° C to give an increased defect concentration where needed. EPR spectra were obtained using a conventional 9.5 GHz spectrometer equipped with a liquid helium flow system for temperature control. The only signals typically observed in the EPR spectra acquired in the absorption mode were the carbon vacancy and Si antisite in the dark with concentrations ≤ 10 15 cm -3 and, with optical excitation, shallow N donors and B acceptors. Our EPR studies required rapid passage conditions [2] in dispersion mode and optical excitation (by either a tungsten lamp with a small monochrometer and/or a series of long pass filters or a Ti-sapphire laser) of the centers from a diamagnetic ground sate into an observable paramagnetic state. The PL (at 1.6 K) was excited by the 351-nm line of an Ar + laser, analyzed by a 1⁄4-m double-grating spectrometer, and detected by a liquid nitrogen-cooled Ge photodiode. In SI and irradiated SiC, several sets of sharp IRPL lines labeled UD1, UD2 and UD3 are commonly observed near 1 eV in addition to residual impurities such and V and Cr. We observe EPR signals due to the +2 charge state of the carbon vacancy-carbon antisite pair (VCCSi) in the photoexcited S=1 state, and in some samples the neutral Si vacancy (also photoexcited into an S=1 state) as shown in Fig. 1 for B||c. We can clearly resolve the Si vacancy on the cubic site, while the vacancy on the hexagonal site has a very small crystal field and is difficult to resolve in our measurements. There are two variants of the pair defect, the P6 center with the pair aligned along the c-axis and the P7 center which has been identified as the (VC-CSi) ++ pair aligned along one of the basal bond directions. [3]. For both P6 and P7 we use the subscripts a and b to refer to the inner and outer pairs of lines (for B||c), respectively – these are due to the cubic and hexagonal sites Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 461-464 doi:10.4028/www.scientific.net/MSF.457-460.461

Contactless studies of semi-insulating 4H–SiC

Abstract Semi-insulating (SI) silicon carbide is important for applications in high-power, high-frequency electronics, such as SiC MESFETs and GaN FETs. In this work, we discuss the use of low-temperature electron paramagnetic resonance (EPR), room- and low-temperature FTIR and photoluminescence as potential screening probes. In addition, the improved materials quality enhances the resolution of such spectroscopic measurements to better understand the material. The EPR spectra reveal the expected V4+ as well as the shallow boron center, suggesting inhomogeneities in the B and/or V distribution. We observe significant reduction in the free-carrier absorption compared with n-type material and the intra-3d-shell E2–2T2 IR absorption. In addition, we observe an N-related gap mode which may serve as a quantitative probe of the N content. The photoluminescence from the V-doped sample exhibits weak donor-bound excitons and shows a broad structured band near 3.0xa0eV related to recombination between photoneutralized N-donors and photoneutralized acceptors.

Magnetic resonance studies of GaN based light emitting diodes

We report the application of electrical detection of magnetic resonance (EDMR) and electroluminescence detection of magnetic resonance (ELDMR) to study the recombination processes in InGaN/AlGaN double heterostructure p-n junctions. These techniques are especially well suited to the problems of defects in device structures in that they are much more sensitive than conventional paramagnetic resonance and are responsive to only those defects involved in the electrooptical properties of the structure. One resonance is observed at g≈2.00 and is identified as a Zn-related acceptor trap in the InGaN layer. A second resonance with g≈1.99 is identified as a deep donor.

Infrared PL Signatures of n-Type Bulk SiC Substrates with Nitrogen Impurity Concentration between 1016 and 1017 cm-3

Low temperature infrared photoluminescence (PL) performed on a large set of bulk SiC substrates has revealed distinct series of lines between 0.8 and 1.5 eV for samples with nitrogen levels between ~ 1016 and 1017 cm-3. Semi-insulating and intentionally N-doped wafers grown by PVT and HTCVD were investigated. Two groups of PL lines clustered near 1.0 and 1.35 eV, respectively, were observed in n-type 4H-SiC. Not surprisingly, a multiplicity of features at slightly different energy positions was found for this emission from the 6H- and 15R-SiC polytypes. Both sets of lines were not observed for substrates with N doping concentrations greater than 3x1017cm-3. Thus, it appears this IR emission can serve as optical “fingerprints” of bulk n-type substrate with moderate levels of N impurities. Models for the possible origins of these lines will also be discussed.

Behavior of Native Defects in Semi-Insulating 4H-SiC after High Temperature Anneals and Different Cool-Down Rates

We have recently explored the nature and stability of native defects in high-purity semi-insulating 4H-SiC bulk substrates grown by PVT and HTCVD methods after post-growth anneal treatments up to 2400oC using electron paramagnetic resonance (EPR) and low-temperature photoluminescence (PL) experiments. In the present study we have extended these investigations to SI 4H-SiC subjected to the same post-growth high-temperature anneal treatments, where significantly enhanced carrier lifetimes have been reported for such conditions, but cooled at different rates ranging from ~2-25oC/min. Previously, the intensities of the native defects decreased monotonically with anneals from 1200–1800oC; however, it was recently observed that several of these defects reappear after annealing at 2100oC and above. Our results illustrate the effects of the post-growth anneal treatments and cool-down rates on the concentrations of native defects.

Thermal Evolution of Defects in Semi-Insulating 4H SiC

High temperature anneals were used to study the evolution of native defects in semiinsulating (SI), ultrahigh purity SiC using electron paramagnetic resonance (EPR), infrared and visible photoluminescence (PL) and COREMA (Contactless Resistivity Mapping) measurements. In EPR we observe a defect that we tentatively identify as VC-CSi-VC. The EPR intensities of this defect and the UD1 IRPL increase significantly with annealing in all samples.