Residual Donors Research Articles

Background Impurity Reduction and Iron Doping of Gallium Nitride Wafers

ABSTRACTBackground impurities and the resulting electrical characteristics were studied for GaN wafers grown using hydride vapor phase epitaxy at various growth conditions. The electron concentration was found to decrease with increasing GaN thickness, by orders of magnitude in the first few microns of growth, but continuing gradually for thousands of microns. Physical removal of the backside degenerate layer enabled improved analysis of the electrical properties. Secondary ion mass spectroscopy was used to determine that the presence of oxygen and silicon accounted for the electron concentration for unintentionally n-type doped material. The concentration of oxygen was found to vary more than that of silicon and increased with decreasing growth temperature. The resistivity was measured to be as high as 1 ohm-cm, corresponding to a carrier concentration of 1016 cm−3. Iron was demonstrated to effectively compensate the residual donors and increased the resistivity to greater than 109 ohm-cm at room temperature and greater than 3×105 ohm-cm at 250 °C. An activation energy for the iron-doped GaN was determined by variable temperature resistivity measurements to be 0.51 eV.

Magnetic resonance studies of defects in GaN with reduced dislocation densities

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 (∼200 μm-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 (<10 7 cm −2) compared to that typically observed (∼mid 10 8–10 10 cm −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 24 GHz 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 cm −3) as determined by Hall effect measurements. In addition, new deep centers are found from ODMR on the 2.4 eV “green” PL band and on the broad emission less than 1.8 eV 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.

Detection and Identification of Donors in Hydride-Vapor-Phase Epitaxial GaN Layers

Low temperature photoluminescence experiments carried out on HVPE GaN samples produced in two different reactors are consistent with a low background level of donors and an extremely low concentration of compensating shallow acceptors. Infrared absorption experiments on these samples indicated that different shallow donors are incorporated at different rates in each reactor. High sensitivity SIMS was employed to measure the concentration of background impurities of a number of samples produced in both reactors. The comparison between the infrared absorption cross-sections with the SIMS results allows the identification of Si, the shallower, and O as the two dominant residual donors in these examples of HVPE GaN.

Detection and Identification of Donors in Hydride-Vapor-Phase Epitaxial GaN Layers

Low temperature photoluminescence experiments carried out on HVPE GaN samples produced in two different reactors are consistent with a low background level of donors and an extremely low concentration of compensating shallow acceptors. Infrared absorption experiments on these samples indicated that different shallow donors are incorporated at different rates in each reactor. High sensitivity SIMS was employed to measure the concentration of background impurities of a number of samples produced in both reactors. The comparison between the infrared absorption cross-sections with the SIMS results allows the identification of Si, the shallower, and O as the two dominant residual donors in these examples of HVPE GaN.

Graft size assessment and analysis of donors for living donor liver transplantation using right lobe.

Modality of living donor liver transplantation (LDLT) has been expanded to adult cases. However, the safety of right lobectomy from living donors has not yet been proven. A total of 62 cases of LDLT, using the right lobe, were reviewed. Study 1: Discrepancy between estimated graft volume and actual graft weight was evaluated. Study 2: Postoperative liver functions were analyzed in relation to residual liver volume (RLV) or age. Residual liver volume of donors was defined using two indices, (RLV = estimated whole liver volume - estimated graft volume and %RLV = RLV/estimated whole liver volumex100). Donors were divided into two groups on the basis of either %RLV (<40%; 40%< or =) or age (<50 years old; 50 years old < or =). Study 3: Right lobe donors were compared with left lobe donors (35 cases) in terms of their postoperative liver functions. Study 1: The relationship between estimated graft volume and actual graft weight was linear (y=159.136+0.735x, R2=0.571, P<0.001). Study 2: %RLV ranged from 23.5% to 55.8% (mean +/- SD: 43.2+/-6.0). Fifteen cases showed %RLV less than 40%. Postoperative bilirubin clearance was delayed in that group (%RLV<40%). Serum total bilirubin values on postoperative day 7 in the older group (age > or =50) were significantly higher than those in the younger group (age<50). Study 3: Postoperative liver functions of right lobe donors were significantly higher than those of left-lobe donors. Eleven donors (17.7%) had surgical complications, all of which were cured with proper treatment. Right lobectomy from living donors is a safe procedure with acceptable morbidity, but some care should be taken early after the operation for donors with small residual liver and aged donors.

Spatial variation of luminescence in thick GaN films

The spatial variation of the optical properties of hydride vapor-phase epitaxial GaN layers of various thickness has been studied using scanning cathodoluminescence microscopy. A strong improvement of these properties with film thickness is observed in plan view. Cross-sectional studies show a strong redshift of the luminescence in the vicinity of the substrate within a typical thickness of about 2 μm, reflecting a high local impurity content. Above this initial growth region, a strong blueshift is observed up to the energy of fully relaxed high-purity GaN, indicating vertical strain relaxation as well as depletion of residual donors. This is accompanied by a sharp increase in the lateral spectral homogeneity, indicative of a significant improvement in crystalline quality.

Gas source molecular beam epitaxy of InP-based microstructures: material growth, characterization and device applications

InP epitaxial layers and InGaAs/InAlAs/InP modulation doped heterostructures were grown by gas source molecular beam epitaxy, in which arsenic and phosphorus beams were obtained by thermal cracking of AsH 3 and PH 3. High purity InP layers have been obtained by optimizing growth parameters. To identify the species of impurity and defect in high quality InP layers, low temperature photoluminescence (PL) and photothermal ionization spectroscopy measurements were carried out. The residual donors were determined to be silicon and sulfur. To obtain a deep insight to the electronic subband structure of the two-dimensional electron gas (2DEG) system, Shubnikov-de Haas, quantum Hall effect measurements and field-dependent cyclotron resonance measurements were conducted. The electron density on each subband in 2DEG was determined to be 2.84×10 12 and 3.4×10 11 cm −2, respectively. The effective masses of the two subbands are (0.061±0.001) m 0 for the zero subband and (0.049±0.001) m 0 for the first subband. An optoelectronic integrated circuit (OEIC) structure consisting of high electron mobility transistor (HEMT) and metal semiconductor metal photo detector (MSM-PD) was achieved by gas source molecular beam epitaxy with one growth run. The responsibility of MSM-PD is 0.5 A/ W. DC transconductance of an InGaAs/InAlAs HEMT with 1.5-μm gate length is 305 mS mm −1; the maximum saturation current is 350 mA mm −1 and the pinch-off voltage is −1.4 V. The gain of the HEMT amplifier is 14 dB. The integrated photoreceiver could successfully operate at a data rate of 622 Mbit s −1.

Localization of negatively charged excitons in GaAs/AlGaAs quantum wells

The localization of negatively charged excitons on isolated charged donors, located in a barrier at various fixed distances L from the heteroboundary, is investigated in isolated GaAs/AlGaAs quantum wells. The energy shift of the cyclotron replica in the emission spectra of a localized excitonic complex is studied as a function of L. It is shown that in undoped samples charged excitons localize on residual donors at distances L>350 A on account of the formation of D− complexes at shorter distances. It is established that a cyclotron replica arises with the recombination of an excited state and not the ground state, as previously thought, of an excitonic complex.

Low-temperature luminescence of exciton and defect states in heteroepitaxial GaN grown by hydride vapor phase epitaxy

Low-temperature (1.7–20 K) photoluminescence and reflectance are used to investigate the free and bound exciton and shallow impurity states in GaN. A 300-μm-thick GaN layer grown by hydride vapor phase epitaxy on sapphire(0001), with an exceptionally low dislocation density (3×106 cm−2) is used to obtain very high quality spectra. Both free and bound n=2 excitons are identified, leading to a confirmation of the A free exciton binding energy as about 26.4 meV, independent of strain. Principal neutral donor-bound exciton (D0,X) peaks involving two to three different donors are resolved, as are two-electron satellites involving up to five different residual donors with binding energies ranging from 22 to 34.5 meV.

Optical properties of residual shallow donors in GaN epitaxial layers grown by horizontal LP-MOCVD

Photoluminescence (PL) properties of the re- sidual shallow donors in GaN epitaxial layers grown by low-pressure metal organic chemical vapor deposition (LP- MOCVD) were reported. Well-resolved free exciton transi- tions and luminescence due to the residual shallow donors were observed. The localization energy and binding energy of the neutral shallow donor were found to be 7m eVand 35 meV, respectively. In addition we identified a new ion- ized donor bound exciton center. Temperature dependence of the neutral donor bound exciton PL peak was found to fol- low the same variation as the band gap with temperature and Varshni's coefficients were obtained. The linewidth of donor bound exciton transition increases linearly with temperature and their interaction with the acoustic phonons was thought to be the only contribution to the linewidth. From the PL intensi- ties the thermal activation energy of the neutral donor bound exciton was estimated as 9m eV. The ratio of the total free excitons' intensity to the total bound excitons' intensity in- creases with temperature, which shows that the donor bound excitons thermally dissociate releasing free excitons.

Optical spectroscopy of Si-related donor and acceptor levels in Si-doped GaN grown by hydride vapor phase epitaxy

The optical properties of n-type GaN grown by hydride vapor phase epitaxy, with intentional Si doping levels ranging from nominally undoped to ND−NA=4×1017 cm−3, are investigated using low temperature photoluminescence. We identify free and neutral donor-bound exciton transitions and two-electron satellites (TES) at 1.7 K. The energy difference between the principal neutral donor-bound exciton peak and its TES yields a Si donor binding energy of 22 meV. The intensity of the Si-related TES increases with increasing Si concentration. The Si donor is much shallower than the two residual donors, which have binding energies of 28 and 34 meV. This result suggests that the main residual donors in this material (and possibly in many layers grown by metal organic chemical vapor deposition and metal organic molecular beam epitaxy as well) are not Si. Silicon doping also introduces an acceptor level with a binding energy of about 224 meV.

Residual Donors in GaN Epitaxial Films - A Correlation of HALL Effect, SIMS and Photoluminescence Data

Residual Donors in GaN Epitaxial Films - A Correlation of HALL Effect, SIMS and Photoluminescence Data

Luminescence of Be-doped GaN layers grown by molecular beam epitaxy on Si (111).

Low temperature photoluminescence spectra of Be-doped layers grown on Si (111) by molecular beam epitaxy have been analyzed. Emissions at 3.466 eV and 3.384 eV, and a broad band centered at 2.4-2.5 eV are observed. Their evolution with temperature and excitation power, and time resolved PL measurements ascribe an excitonic character for the luminescence at 3.466 eV, whereas the emission at 3.384 eV is associated with a donor-acceptor pair transition. This recombination involves residual donors and Be-related acceptors, which are located around 90meV above the valence band, confirming Be as the shallowest acceptor reported in GaN. The intensity of the band at 2.4-2.5 eV increases with the Be content. This emission involves a band of deep acceptors generated by Be complex defects, as suggested by the parameter g = 2.008 ± 0.003 obtained by photoluminescence-detected electron paramagnetic resonance.

Optical and magneto-optical characterization of heteroepitaxial gallium nitride

We describe some recent work on the characterization of strain effects on excitons and band structure in heteroepitaxial GaN, as well as magnetoluminescence studies of excitons and donors in this material. Epilayer strains were measured directly from wafer curvature, which avoids the need to know the lattice constants of unstrained GaN in advance. The change in strain between room temperature (where it is usually measured) down to low temperature, where detailed optical spectroscopy is usually performed, was estimated and found to be significant. Low temperature photoluminescence and reflectance were used to determine the exciton energies as a function of measured strain, yielding some of the deformation potentials and band structure parameters of the material. Magnetospectroscopy and reflectance were used to identify excited states of the excitons, yielding an exciton binding energy around 26.4 meV. Residual donors and their binding energies were studied from two-electron transitions, which have been identified for the first time by their Zeeman splittings in high magnetic fields. Resonant electronic Raman scattering was also used to study excited states of shallow donors, and yielded improved resolution compared to photoluminescence.

Carbon doping in InAlAs grown by metalorganic chemical vapor deposition

Carbon doping in InAlAs by metalorganic chemical vapor deposition (MOCVD) is reported for the first time. The growth rate is found to decrease with increasing CBr4 flow due to the etching effect. Annealing in N2 at 450°C for 5 min. is confirmed to be sufficient for reactivating the hydrogenated C acceptors in InAlAs. Although the existence of residual donors and the hydrogenation have considerable influences on the hole concentration, the acceptor incorporation rate in InAlAs is revealed to be proportional to the CBr4 flow rate. The highest hole concentration ever achieved in MOCVD-grown InAlAs of 7 × 1018/cm3 demonstrates the promise of C as a p-type dopant in InAlAs.

Temperature dependence of excitonic photoluminescence and residual shallow donors in high-purity GaN/Al 2O 3

The temperature dependence of free and bound exciton lines in high-purity, undoped wurtzite GaN layers has been studied by photoluminescence (PL) between 2 and 300 K. Below 30 K the neutral donor bound exciton D °X produces the strongest near band gap PL signal whereas free A and B excitons dominate the spectrum at room temperature. A deconvolution of the asymmetric D °X line shape provides strong evidence for two residual shallow donors differing in ionisation energies by a factor of 1.5. The origin of a PL line occurring at E g = 116 meV is discussed in two alternative models.

Magnetoluminescence and Resonant Electronic Raman Scattering Investigation of Donors and Excitons in Hydride Vpe and Mocvd GaN

AbstractThe donor and exciton states in ultra high-quality heteroepitaxial GaN grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire substrates are investigated using low temperature photoluminescence (PL), reflectance, magnetospectroscopy in fields up to 12 T, and resonant electronic Raman scattering (RERS). The A free exciton is confirmed to have a binding energy of about 26.4 meV, independent of strain in the material. Bound n=2 exciton peaks are distinguished in the PL spectrum by their thermalization and sample dependence. The Si donor is shown to have a binding energy of about 21 meV using Si-doped HVPE samples grown at Epitronics. Up to five additional residual donor species are observed when comparing various HVPE and MOCVD samples. Pronounced temperaturedependence of the two-electron satellites is observed, suggesting the existence of unresolved excited rotator states of the neutral donor-bound exciton. Highly resolved magnetic splitting patterns are observed in the two-electron satellites. A nonperturbative theory of these donor splittings is developed, including anisotropy. Resonant electronic Raman scattering from residual donors is reported, and yields improved linewidths compared to PL.

High Purity Liquid Phase Epitaxial GaAs For Radiation Detectors

AbstractWe report on the growth of high purity n-GaAs using Liquid Phase Epitaxy (LPE) and the fabrication of room temperature p-i-n radiation detectors. Our epilayers are grown from a Ga solvent in a graphite boat in a pure hydrogen atmosphere. Growth is started at a temperature of approximately 800 °C. Our best epilayers show a net-residual-donor concentration of 2×1013 cm−3, confirmed by Hall effect measurements. The residual donors have been analyzed by far infrared spectroscopy and found to be sulfur and silicon. Epilayers with thicknesses of up to 120 µm have been deposited on 650 µm thick semi-insulating GaAs substrates and on 500 µm thick n+-type GaAs substrates. We report the results obtained with Schottky barrier diodes fabricated from these high purity n-type GaAs epilayers and operated as X-ray detectors. The Schottky barrier contacts consisted of evaporated circular gold contacts on epilayers on n+ substrates. The ohmic contacts were formed by evaporated and alloyed Ni-Ge-Au films on the back of the substrate. Several of our diodes exhibit currents of the order of 1 to 10 nA at reverse biases depleting approximately 50 µm of the epilayer. This very encouraging result, demonstrating the possibility for fabricating GaAs p-i-n diodes with depletion layers in high purity GaAs instead of semi-insulating GaAs, is supported by similar results obtained by several other groups. The consequences of using high purity instead of semi-insulating GaAs will be much reduced charge carrier trapping. Diode electrical characteristics and detector performance results using 55Fe and 241Am radiation will be discussed.

Location Of Residual Donors In GaN Epitaxial Layers

AbstractOptically-detected electron-nuclear double resonance (ODENDOR) studies at 24 GHz on high-resistivity GaN films grown on Al2O3 have been combined with x-ray diffraction measurements to obtain information on the location of the residual shallow donors. Strong ODENDOR assigned to 69,71Ga lattice nuclei was detected on the g=1.951 effective-mass donor resonance found on the 2.2 eV emission bands. The x-ray studies reveal that the layers are under biaxial compression with high values of strain (∼ 2–3 × 10−3). The quadrupole splittings for 69Ga are smaller than those reported for strain-free samples by 15–25 %. The dominant sources of the local electric field gradient (EFG) responsible for the splittings are attributed to the wurtzite crystal structure and the strain fields that arise from the lattice constant mismatch and the difference in thermal expansion coefficients. An EFG/strain relationship of 3 × 1022 Vm−2 per unit strain at the 69,71Ga nuclei is deduced. The ODENDOR can be described with asymmetry parameter η=0. This provides evidence that the donors are in the crystallites rather than near grain boundaries.

Shallow donors in epitaxial GaN

Photoluminescence (PL) of GaN is commonly dominated by the annihilation of excitons bound to a 34.5 meV deep donor appearing at 3.472 eV (for strain free GaN at 4 K). With PL we were able to resolve two additional donor bound exciton transitions. The excitons have localization energies of 3.7 ± 0.3 and 11.3 ± 05 meV, respectively. Using Haynes rule the respective donors lie 56.5 and 18.5 meV below the conduction band. The applicability of Haynes rule could nicely be confirmed by IR absorption where the 1s-2p transition of the 34.5 and 58 meV donor are observed. The issue of residual donors in GaN will be discussed in this context.