Residual Carrier Concentration Research Articles

Effects of crystalline quality on the ultraviolet emission and electrical properties of the ZnO films deposited by magnetron sputtering

The ZnO films were deposited on c-plane sapphire, Si (0 0 1) and MgAl 2O 4 (1 1 1) substrates in pure Ar ambient at different substrate temperatures ranging from 400 to 750 °C by radio frequency magnetron sputtering. X-ray diffraction, photoluminescence and Hall measurements were used to evaluate the growth temperature and the substrate effects on the properties of ZnO films. The results show that the crystalline quality of the ZnO films improves with increasing the temperature up to 600 °C, the crystallinity of the films is degraded as the growth temperature increasing further, and the ZnO film with the best crystalline quality is obtained on sapphire at 600 °C. The intensity of the photoluminescence and the electrical properties strongly depend on the crystalline quality of the ZnO films. The ZnO films with the better crystallinity have the stronger ultraviolet emission, the higher mobility and the lower residual carrier concentration. The effects of crystallinity on light emission and electrical properties, and the possible origin of the n-type conductivity of the undoped ZnO films are also discussed.

Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers

The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit reduced energies (1580 cm(-1)) and increased widths (14 cm(-1)) compared to the response of graphene monolayers supported on the SiO(2)-covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2 x 10(11) cm(-2) for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show little local disorder, based on the very weak Raman D-mode response. The two-phonon 2D mode of the free-standing graphene monolayers is downshifted in frequency compared to that of the supported region of the samples and exhibits a narrowed, positively skewed line shape.

Optical and electrical characterization of sputter-deposited FeSi2 and its evolution with annealing temperature

Optical and electrical properties of sputter-deposited FeSi2 thin films on p-Si(100) and SiO2/p-Si(100) substrates as well as their evolution with rapid thermal annealing (RTA) temperature have been investigated. Optical absorption measurements were carried out to determine the absorption spectra of FeSi2 based on the proposed optical absorption model for the double-layer and triple-layer structures. A direct band gap behavior was concluded for both amorphous and polycrystalline semiconducting FeSi2. An absorption coefficient in the order of 105 cm−1 at 1 eV and a band gap value of ∼0.86 eV were obtained for the β-FeSi2. Hall effect measurements at room temperature indicate heavily doped and n-type conductivity for the FeSi2 films on p-Si, whose residual carrier concentration was found to be closely correlated with the observed subgap optical absorption via band tailing. The carrier mobility was shown to increase with decreasing residual carrier concentration when the RTA temperature was increased.

Structural and optical properties of InN/GaN nanodots grown by metalorganic chemical vapor deposition

AbstractInN nanodots grown on GaN by metalorganic chemical vapor deposition (MOCVD) using conventional growth mode as well as flow‐rate modulation epitaxy (FME) at various growth temperatures (550–730 °C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN dots and their photoluminescence (PL) properties. The best growth efficiency of InN was achieved by FME at around 650 °C. The residual carrier concentration and PL efficiency was also be improved when a high growth temperature was used. Our results indicated that InN nanodots can be grown at a temperature even higher than 700 °C while maintain their optical quality. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of deep levels in GaN grown by RF‐MBE on GaN template by capacitance DLTS

AbstractThe dependence of deep levels in GaN epitaxial layer grown by molecular beam epitaxy on the V/III ratio was studied by capacitance deep level transient spectroscopy (DLTS). Four peaks corresponding to the electron traps were observed in the unintentionally n‐doped GaN films grown at various growth conditions. The deep level concentrations of T1 (0.31 eV) and T4 (0.21 eV) traps increased with decrease of the V/III ratio. This suggests that T1 and T4 traps are related to the N‐vacancies. Those of T2 (0.84 eV) and T3 (1.13 eV) traps did not show obvious dependence on the V/III ratio, but the concentration of the T2 and T3 traps increased with increase of the residual carrier concentration. T2 and T3 traps might have some correlation with the residual donor species. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of growth temperature on InN∕GaN nanodots grown by metal organic chemical vapor deposition

InN nanodots grown on GaN by metal organic chemical vapor deposition using conventional growth mode as well as flow-rate modulation epitaxy at various growth temperatures (550–730°C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN nanodots and their photoluminescence (PL) properties. The sample grown at around 650°C showed the highest growth rate. For samples grown at higher temperatures, the residual carrier concentration was reduced and the PL efficiency was improved. Furthermore, we found that the growth of InN nanodots is still sustainable even at a temperature higher than 700°C while retaining their optical quality.

Investigation of electrical and optical properties of ZnO thin films grown with O2/O3 gas mixture

The electrical and optical properties of ZnO thin films grown with an O2/O3 gas mixture are compared with samples grown with pure oxygen gas. The ZnO films were grown on sapphire(0001) by pulsed laser deposition. The residual background carrier concentration is reduced by using an O2/O3 gas mixture as compared to pure molecular oxygen. In particular, a one order of magnitude reduction in residual background carrier density (6.15×1016 cm-3) is achieved by using an O2/O3 gas mixture. The lower donor defect density is attributed to the generation of acceptor defects compensating for the residual donor defects. Photoluminescence results show that the deep level emission increased and the band edge emission decreased for the ZnO films grown with ozone, as compared to the samples grown with pure oxygen gas.

Effect of a Fe 3Si buffer layer for the growth of semiconducting β-FeSi 2 thin film on stainless steel substrate

This paper reports a Fe 3Si buffer layer for the growth of semiconducting β-FeSi 2 film on stainless steel (SS) substrate. It was formed through the silicidation reaction of Fe and Si at a Fe-rich interface at an elevated temperature of 600 °C. β-FeSi 2 film was deposited on it at low temperature around 400 °C. Auger electron spectroscopy depth profile showed that the inter-diffusions of Fe, Si and impurities contained in the SS substrate were effectively blocked by the buffer layer. The adhesive ability to SS substrate and crystallinity of β-FeSi 2 film were clearly improved by the introduction of the buffer layer, thus also induced the decrease of residual carrier concentration, which was revealed by capacity–voltage measurement.

Origins of n‐type residual carriers in RF‐MOMBE grown InN layers

AbstractWe have investigated quantitatively on the origins of residual electrons in InN layers to make clear some roles of oxygen incorporation for band‐gap widening. It has been found out that a linear relation was observed between oxygen and residual electron concentrations for InN layers grown by RF‐MOMBE using TMIn source, although the residual electron concentration is super‐linearly dependent on oxygen concentration for InN layers grown by RF‐MBE using metal In source. The experimental results strongly indicate that oxygen atoms and/or nitrogen vacancies induced by oxygen incorporation are major origins of the residual carrier concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High breakdown field of pnp GaN/InGaN/AlGaN DHBTs with AlGaN collector

We have investigated the characteristics of pnp GaN/InGaN/(Al)GaN double heterojunction bipolar transistors (DHBTs) grown by metalorganic vapour phase epitaxy. In GaN-collector DHBTs, the collector–base breakdown voltage under the open-emitter condition increased with increasing collector thickness. By fitting the experimental results to the theoretical model of the punch-through diode, we obtained the breakdown field of 3.1 MV/cm and the residual carrier concentration of 8.5 × 1016 cm–3 for the GaN collector layer. On the other hand, in a 500 nm thick AlGaN collector, the breakdown voltage was 190 V. Assuming that the bias was uniformly applied in the AlGaN collector, the corresponding breakdown field was calculated to be 3.8 MV/cm. The results indicate that the use of an AlGaN collector in HBTs is a promising way to increase the breakdown voltage without increasing the collector thickness. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of the oxygen vacancy in ZnO

As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (VO) in the material. Annealing the samples in O2 at about 1000 °C (2 atm, 20 h) reduces the H and VO donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the VO 0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the VO 2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations.

A quantitative study of suppression effect for oxygen contamination by Ga beam irradiation in InN RF-MOMBE growth

We have quantitatively investigated the suppression effect for the oxygen contamination into the InN layers by simultaneous irradiation of Ga beam during RF-MOMBE growth using the combination of the TMIn and the RF-plasma nitrogen sources. The O/C ratio and residual carrier concentrations decreased with increase in the Ga beam flux, and a linear relation was observed between O/C ratio and residual carrier concentrations. Present results strongly indicate that the origin of residual carrier concentrations is the incorporated oxygen atoms into the InN layers during growth.

Fabrication of lateral lattice-polarity-inverted GaN heterostructure

Fabrication of the lateral polarity-inverted GaN heterostructure on sapphire (0 0 0 1) using a radio-frequency plasma enhanced molecular beam epitaxy is demonstrated. Its microscopic properties, which are closely related to the local polarity distribution, such as surface potentials, piezoelectric polarizations and residual carrier concentrations were investigated by Kelvin force microscopy and micro-Raman scattering. The successful inversion from Ga-polarity to N-polarity of GaN in a specific domain and its higher crystal perfection had been confirmed clearly by these microscopic analyses. The results were also fairly consistent with that of KOH etching experiments, which suggest the applicability of these processes to the fabrication of photonic nanostructures composed of nitride semiconductors.

Evaluation of incorporation efficiency of group V source gases in metal organic chemical vapor deposition of GaInNAs for high quality 1.21 μm quantum-well ridge wave guide lasers

For the growth of GaAs and AlGaAs by metalorganic chemical vapor deposition (MOCVD), the residual carrier concentration was confirmed to follow the relation of [Carrier concentration] ∝ [V/III] −4/3. As this relation fit various growth conditions, it provided a basis for a new parameter of “effective TBAs concentration” which represents the concentration of the tertiarybutylarsine (TBAs) effectively contributing to the growth of epitaxial layer. By using this new parameter, a large spatial variation of the efficiency of TBAs across a wafer in a direction of the gas flow was indicated for the growth of Ga(Al)As at low temperature of 500 °C. With using this parameter, the decomposition of 1,1-dimethylhydrazin (DMHy) was suggested to occur only on the wafer at this temperature. The optimized TBAs flow rate for the growth of GaInNAs deduced by considering the distribution of the effective TBAs concentration coincided with the optimized actual input TBAs flow rate. This coincidence supported the appropriateness of the method of measuring the efficiency of TBAs on the wafer with the effective TBAs concentration. The optimized TBAs flow rate obtained by using the new parameter was applied to the growth of a GaInNAs quantum well (QW) and to the fabrication of a high quality laser diode emitting at 1.21 μm.

Growth of short-period InAs∕GaSb superlattices

The purpose of this work is to explore materials for midinfrared detectors that can operate at room temperature. Shorter-period InAs∕GaSb superlattices (SLs) have larger intervalance band separations, which are beneficial for reducing Auger recombination and tunneling current, thus making room temperature operation possible. To test these possibilities, several short-period SLs ranging from 50to11Å were grown and their morphological properties were carefully monitored by transmission electron microscopy. The effect of structural degradation caused by the period reduction on the optical properties was studied using low-temperature photoluminescence (PL). The samples with larger periods (50–32Å) showed excellent structural qualities and produced narrow full width at half maximum (FWHM) of the PL peak (5meV). As the period approached 24Å, slight layer thickness undulations within the SLs were observed and these undulations intensified as the period further reduced to 17Å. These structural degradations strongly influence optical properties causing significant broadening in the FWHM and higher residual carrier concentration in the shorter-period SLs. With slower growth rates, samples with periods as thin as 19Å were grown without significant layer undulations.

MOVPE and characterization of InAsN/GaAs multiple quantum wells

InAsN/GaAs multiple quantum wells (MQWs) were grown on GaAs (0 0 1) substrates by metalorganic vapor phase epitaxy (MOVPE) using 1,1-dimethylhydrazine (DMHy) and tertiarybutylarsine (TBAs) as the N and As precursors, respectively. The photoluminescence (PL) peaks of the MQWs clearly show a red shift with increasing DMHy flow rates. This result is opposite to that of the InAsN thin film whose absorption edge shows a blue shift due to the Burstein–Moss (B–M) effect (or band filling effect) caused by the high electron concentration associated with the N incorporation. The red shift of the PL peak is attributed to the huge bandgap bowing appearing with the N incorporation, because in contrast to the result of bulk InAsN, the step-like functional form of the two-dimensional density of states of quantum wells suppresses the B–M effect even in the case of a rise in residual carrier concentration. With increasing DMHy flow rate, the increase of the Stokes shift between the PL and photoluminescence excitation (PLE) peaks was observed. As a result of the excitation power dependence of the PL spectrum, it was found that the Stokes shift is attributed to the compositional fluctuation. These results indicate that the incorporation of the N into InAs in the MOVPE growth of InAsN induces a bandgap reduction due to the bandgap bowing, and the increase of the N content in the InAsN alloy causes the compositional fluctuation.

Formation of β-FeSi2 thin films on non-silicon substrates

β-FeSi2 films were prepared on non-silicon substrates by sputtering. The crystalline growth, stress induced cracks and adhesive ability to the substrate were investigated on substrate temperature and thermal expansion coefficient of substrate materials. It was found that crack formation in β-FeSi2 films was dependent on the thermal expansion coefficients of CaF2, MgO and quartz glass insulating materials. High-density cracks were observed from β-FeSi2 films on CaF2 and quartz glass substrates with large difference of the thermal expansion coefficient between β-FeSi2 film and substrate materials, and it was crack-free on MgO substrate with a thermal expansion coefficient close to that of β-FeSi2 films. Polycrystalline β-FeSi2 films grew on Mo, Ta, W, Fe and stainless steel (SS) substrates at low substrate temperature around 400 °C. There was no α-FeSi2 phase confirmed in the films. All the films had continuous structures without noticeable cracks even though they have different thermal expansion coefficients. Capacity–voltage measurements showed that β-FeSi2 films formed on SS substrates has n-type conductivity, with residual carrier concentrations of about 1.3∼6.4×1018 cm−3. Auger electron spectroscopy depth profile measurements identified homogeneous distribution of Fe and Si atoms in the film region, but with a large interface region between the film and the substrate.

NH3/TMI molar ratio dependence of electrical and optical properties for atmospheric‐pressure MOVPE InN

AbstractThe electrical and optical properties for atmospheric‐pressure MOVPE InN have been studied as a function of NH3/TMI molar ratio during the growth. Residual carrier (electron) concentration is decreased with increasing NH3/TMI molar ratio. PL peak energy is also shifted to a low energy side with increasing NH3/TMI molar ratio. Hall mobility of InN is almost independent on NH3/TMI molar ratio and is rather decreased at a high NH3/TMI molar ratio. The highest mobility is obtained for a sample grown at a relatively low NH3/TMI molar ratio. The grain size of the InN films is decreased with increasing NH3/TMI molar ratio. The lowest carrier concentration of 4.5×1018 cm–3 and highest mobility of 1100 cm2/Vs obtained here are the best data for MOVPE InN ever reported. Based on these results, the most probable candidate for donors and the dominant carrier‐scattering mechanism for atmospheric‐pressure MOVPE InN are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on influence of atomic hydrogen irradiation on growth and property of N‐polarity InN

AbstractEpitaxial growth of N‐polarity InN films under atomic‐hydrogen irradiation was systematically investigated. It was found that any difference could not be observed in electron concentrations of InN films with and without the atomic‐H irradiation, where typical residual carrier concentrations were from 3 to 5×1018 cm–3. These results suggest that the atomic H may not be an origin of residual donors at least in the InN films with carrier concentrations of 3‐5×1018 cm–3. On the other hand, the atomic‐H irradiation was effective in improving surface morphology of the InN films, which suggested an effect of surfactant on InN epitaxy. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intrinsic and extrinsic point-defects in vapor transport grown ZnO bulk crystals

Vapor transport grown ZnO bulk crystals were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga, and oxygen vacancies in the material. Annealing the samples in O 2 at about 1000 °C (2 atm, 20 h) reduces the H and V O donor concentration by typical one order of magnitude. The photoluminescence and DLTS results suggest a correlation between the broad unstructured emission at 2.45 eV (green band) and a donor level 530 meV below the conduction band (E4).