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It is shown that substrate pixelisation before epitaxial growth can significantly impact the emission color of semiconductor heterostructures. The wavelength emission from InxGa1−xN/GaN quantum wells can be shifted from blue to yellow simply by reducing the mesa size from 90 × 90 µm2 to 10 × 10 µm2 of the patterned silicon used as the substrate. This color shift is mainly attributed to an increase of the quantum well thickness when the mesa size decreases. The color is also affected, in a lesser extent, by the trench width between the mesas. Cathodoluminescence hyperspectral imaging is used to map the wavelength emission of the InxGa1−xN/GaN quantum wells. Whatever the mesa size is, the wavelength emission is red-shifted at the mesa edges due to a larger quantum well thickness and In composition.

We have shown that in an AIXTRON Close Coupled Showerhead MOVPE system, InAlN layers are strongly contaminated with gallium, giving increasing contamination when thicker GaN layers are grown below the InAlN. We have also shown that as the gallium contamination increases, there is reduced indium in the layers and thicker layers. To explain these phenomena we propose a quantitative model for these observations, suggesting that the TMIn precursor reacts with gallium on the showerhead surface to release TMGa or MMGa, which is then incorporated into the layers. We grew InAlN layers around 14 nm thick with a 1 nm AlN spacer to form High Electron Mobility Transistor (HEMT) structures. For the InAlN barrier layers grown directly on a GaN template, the 2D electron gas (2DEG) was degraded by the regrowth interface, but this 2DEG degradation was reduced by the growth of thin GaN layers. However, for thicker layers of GaN, there was a strong gallium contamination and so a change in alloy composition, which increased the sheet resistance. Thus we need an optimum regrowth thickness to have the best sheet resistance values.

International audience; A multi-microscopy investigation of a GaN tunnel junction (TJ) grown on an InGaN-based light emitting diode (LED) has been performed. The TJ consists of a heavily Ge-doped n-type GaN layer grown by ammonia-based molecular-beam epitaxy on a heavily Mg-doped p-type GaN thin layer, grown by metalorganic vapor phase epitaxy. A correlation of atom probe tomography, electron holography and secondary ion mass spectrometry has been performed in order to investigate the nm-scale distribution of both Mg and Ge at the TJ. Experimental results reveal that Mg segregates at the TJ interface, and diffuses into the Ge-doped layer. As a result, the dopant concentration and distribution differ significantly from the nominal values. Despite this, electron holography reveals a TJ depletion width of ~7 nm, in agreement with band diagram simulations using the experimentally determined dopant distribution.

InGaN-based devices have attracted a lot of attention, thanks to versatile optoelectronic applications. We show that the growth of these layers in an AIXTRON Close Coupled Showerhead is complicated due to the presence of gallium pollution in the chamber after the growth of GaN buffer layers. This pollution reacts with the TMIn precursor in the growth chamber to give thicker InGaN layers with reduced indium incorporation. We overcame this problem by limiting the presence of the metallic gallium in the growth chamber, resulting in more stable and predictable growth of InGaN layers.

The use of tunnel junctions (TJs) is a potential solution in blue light-emitting diodes (LEDs) to poor p-contacts, replacing it with another n-contact. TJs are even more advantageous for UV-emitting structures, which suffer from considerably low injection efficiency in high Al concentration UV LEDs. In this article, we report our work on Ge n-doped GaN and AlGaN TJs grown on top of blue and UV LEDs, respectively, by a hybrid growth method. We have achieved state-of-the-art mobility (67 cm2/V s) and resistivity (1.7 × 10−4 Ω cm) at a free electron concentration of 5.5 × 1020 cm−3 in Ge-doped GaN. With an emission wavelength of 436 nm, the GaN TJ slightly increased the optical power of the blue LED. The AlGaN TJs, on the other hand, improved the optical power of the UV LED (304 nm) by at least a factor of 3, suggesting the enhancement of the hole injection efficiency by the use of TJs in UV-emitting structures.

International audience; Nanopendeo-epitaxy of gallium nitride (GaN) is considered in this study as a way of producing freestanding GaN with reduced strain and threading dislocation density (TDD) for optoelectronic applications. The novelty of this work lies in the use of silicon on insulator (SOI) substrates patterned into nano-pillars down to the buried oxide (BOX). We actually want to benefit from the creeping properties of SiO2 at the growth temperature of GaN for strain relaxation and grain-boundary dislocations reduction. In this paper, we report on the fabrication of 40×40 µm 2 and 300×300 µm 2 freestanding GaN platelets, up to 10 µm-thick, spontaneously separated from the initial pillars. Structural and optical characterizations show that the platelets are crack-free and almost fully relaxed, with a TDD of ∼ 4×10 8 /cm 2. We underline the different benefits of this approach, but most importantly, we believe that it will be the founding-brick for transferable GaN-based devices.

Tunnel junctions (TJs) are envisaged as potential solutions to improve the electrical injection efficiency of nitride emitters in the visible as well as in the UV range. Indeed TJs would solve the issues related to the poor contact with the top p type nitride layer, replacing it by an n type one. But if metal-organic chemical vapor deposition (MOCVD) is chosen to grow the n side of the TJ on a LED, one faces the problem of a potential re-passivation by hydrogen of the underlying p type layer. We propose a TJ epitaxial process whereby low growth temperatures, high growth rates and the type of carrier gas will minimize hydrogen incorporation in the underlying layers. In this view, n++/p++ GaN TJs with and without an (Ga,In)N intermediate layer are grown by MOCVD at varying temperatures (800°C and 1080°C), using N2 as a carrier gas under a very high growth rate of 2.5μm/h on top of blue (Ga,In)N/GaN LEDs. The LEDs made under N2 carrier gas and lower temperature growth conditions are operational without the need for further thermal activation of the Mg acceptors. The light emission intensity from the top surface of the TJ-LEDs is improved compared to the reference LED without TJ: besides the more efficient carrier injection this is also attributable to the larger photon extraction efficiency because of the rough surface of the low temperature grown n-GaN contact layer of the TJ-LEDs.

International audience; Gallium contamination is a well-known problem for InAlN layers grown in close coupled showerhead metal-organic vapor phase epitaxy reactors, and we have previously suggested a model explaining this gallium incorporation and the associated reduction in indium [Mrad, J. Cryst. Growth 507, 139 (2019)]. Here we propose the hypothesis that increasing the showerhead face temperature during GaN growth should encourage the growth of more stable GaN rather than metallic gallium, and reduce the reactions between tri-methyl indium and gallium on the showerhead. Using a hot deposition shield on the showerhead, we have confirmed this, to grow for the first time almost entirely gallium free InAlN layers in a showerhead reactor.

International audience; We demonstrate the growth of almost strain-free (10-11) semipolar GaN on silicon-on-insulator (SOI) substrates, with no meltbacketching and with a defect density strongly reduced compared to semipolar templates grown on patterned silicon substrates.This is carried out using SOI substrates with a very thin (∼150 nm) 6° off (001) Si top layer. By resorting to very smallnucleation (111) facets, revealed through chemical etching of the topmost thin Si layer, we are able to diminish significantly theoverall dislocation density. Cathodoluminescence and scanning electron microscopy images at different stages of the growthillustrate how the defect density reduction operates and confirm the complete suppression of meltback etching over the whole2 in. wafer. Low temperature photoluminescence and optical reflectivity indicate that complete strain relaxation is closelyachieved (D0X at 3.473 ± 0.001 eV), compared to semipolar epilayers grown onto “bulk” silicon (D0X at 3.460 eV). Thanks to thisefficient strain relaxation, very thick layers, up to 9 μm, could be obtained crack-free.

International audience

We show that gallium pollution in an AIXTRON Close Coupled Showerhead (CCS) Metal-Organic Vapor Phase Epitaxy (MOVPE) reactor can strongly affect the growth process stability of InGaN layers. By comparing growth with and without gallium pollution from the showerhead, we find a decrease in the indium incorporation and an increase in layer thickness of up to three times when the pollution is present. This phenomenon varies with the quantity of gallium on the showerhead, with increased gallium due to thicker GaN growth causing a stronger effect. Higher TMIn flows also give a more pronounced effect. These data support our previous hypothesis of TMIn reacting with metallic gallium present on the showerhead surface to release TMGa, which is then incorporated as Ga into the grown layers. This study of the effect of gallium contamination on InGaN layers in CCS MOVPE reactors leads us to suggest that the growth in CCS reactors should be performed free from gallium pollution to give more stable and comprehensible results.

AlN-on-Si can be considered as a model system for heteroepitaxial growth of highly mismatched materials. Indeed, AlN and Si drastically differ in terms of chemistry, crystalline structure, and lattice parameters. In this paper, we present a transmission electron microscopy and grazing incidence X-ray diffraction study of the microstructure of AlN layers epitaxially grown on Si (111) by molecular beam epitaxy. The large interfacial energy due to the dissimilarities between AlN and Si results in a 3D Volmer-Weber growth mode with the nucleation of independent and relaxed AlN islands. Despite a well-defined epitaxial relationship, these islands exhibit in-plane misorientations up to 6°–7°. We propose a model which quantitatively explains these misorientations by taking into account the relaxation of the islands through the introduction of 60° a-type misfit dislocations. Threading dislocations (TDs) are formed to compensate these misorientations when islands coalesce. TD density depends on two parameters: the islands' misorientation and density. We show that the former is related to the mismatch between AlN and Si, while the latter depends on the growth parameters. A large decrease in TD density occurs during the 3D growth stage by overlap and overgrowth of highly misoriented islands. On the other hand, the TD density does not change significantly when the growth becomes 2D. The proposed model, explaining the misorientations of 3D-grown islands, may be extended to other (0001)-oriented III-nitrides and more generally to any heteroepitaxial system exhibiting a 3D Volmer-Weber growth mode with islands relaxed thanks to the introduction of mixed-type misfit dislocations.AlN-on-Si can be considered as a model system for heteroepitaxial growth of highly mismatched materials. Indeed, AlN and Si drastically differ in terms of chemistry, crystalline structure, and lattice parameters. In this paper, we present a transmission electron microscopy and grazing incidence X-ray diffraction study of the microstructure of AlN layers epitaxially grown on Si (111) by molecular beam epitaxy. The large interfacial energy due to the dissimilarities between AlN and Si results in a 3D Volmer-Weber growth mode with the nucleation of independent and relaxed AlN islands. Despite a well-defined epitaxial relationship, these islands exhibit in-plane misorientations up to 6°–7°. We propose a model which quantitatively explains these misorientations by taking into account the relaxation of the islands through the introduction of 60° a-type misfit dislocations. Threading dislocations (TDs) are formed to compensate these misorientations when islands coalesce. TD density depends on two parameters: the...

With the objective of introducing further GaN semipolar orientations adequate for the fabrication of optoelectronic devices on silicon, we report on the selective area growth of semipolar (2021) GaN layers on patterned silicon (114) 1° off substrates. The patterning and subsequent anisotropic etching of the Si substrates allows exposing Si {111}-type facets on which GaN grows along the [0001] direction. Upon coalescence of neighboring GaN stripes, a continuous (2021) semipolar layer is obtained, displaying a facetted surface morphology dominated by more thermodynamically-stable crystallographic planes, i.e. semipolar (1011) and nonpolar (1010). Transmission electron microscopy shows that dislocation bending in the early growth stages determines the structural quality of the final semipolar (2021) layer. This is confirmed by X-ray rocking curves displaying full width at half-maximum values comparable to those of (2021) GaN epilayers of equivalent thickness grown on patterned sapphire. Optical characterization displays regions having threading dislocations and stacking faults alternating with almost defect-free regions, which correlate well with the transmission electron microscopy study.

Meltback etching, a deteriorating chemical reaction occurring between gallium and silicon under typical metal organic chemical vapor deposition growth conditions, is a common problem that often limits the development of GaN on silicon substrates, in particular, patterned substrates, and therefore must be circumvented. To further understand this reaction, energy dispersive X-ray spectroscopy was performed in cross-section, and a proposed 2-dimensional model on how meltback etching evolves throughout the growth process is discussed, which indicated an inter-diffusion reaction occurring primarily between gallium and silicon where gallium from GaN diffuses into the silicon substrate while silicon from the substrate diffuses out and incorporates into the GaN crystal. Moreover, we demonstrate an anisotropic behavior of the gallium penetrating the silicon substrate, which has shown to be delimited by the Si {111} planes. Finally, an approach to prevent meltback etching by changing the fractions of nitrogen and h...

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919., The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

Deep level transient spectroscopy measurements were per- formed on three non-intentionally doped n-type ZnO samples grown by different techniques in order to investigate the electronic properties of E3 electron trap. The ionization energy and the capture cross-section are found respectively at 0.275 eV from the conduction band and 2.3×10^−16 cm2 with no electric field dependence. This center is present irrespective of the synthesis method. In view of its physical properties and recent works published in the literature, its physical origin is discussed. Based mainly on its insensibility to the macroscopic electric field, the best candidates turn out to be dual defects with opposite charges on adjacent sites, like the dual vacancy VO -VZn .

A direct detection X-ray imager is presented. It uses polycrystalline cadmium telluride (CdTe) grown by close space sublimation technique for the X-ray photoconductor. A 15 mm×15 mm CdTe layer is connected to a 200×200 pixel readout CMOS by indium bumping. X-ray performance at 16 frames/s rate is measured. In particular a readout noise of 0.5 X-ray, an MTF of 50% at 4 lp/mm and a DQE of 20% at 4 lp/mm are obtained.

The spectroscopic properties of ZnO epilayers grown by molecular beam epitaxy are investigated. Three samples are compared: a homoepitaxial layer grown on a Crystec substrate, a sample directly grown on c-sapphire and a layer grown on c-sappire using an MgO buffer. In the latter case, in spite of the high dislocation density, temperature-dependent photoluminescence measurements show only a small decrease of the luminescence intensity between 4 and 300 K, with an activation energy of 108 meV. Cathodoluminescence presents an inhomogeneous emission on the micrometric scale: a stronger emission is measured in small nanometric areas. A tentative explanation of this behavior is proposed. The optical properties of these three samples are also compared to those of two-dimensional homoepitaxial layers and ZnO nanowires grown on sapphire and p-type GaN by metal organic vapor-phase epitaxy.

We report on ZnO nanorods grown by catalyst-free metal-organic chemical vapour deposition (MOCVD) in a commercial Epigress reactor using diethylzinc and N"2O as precursors. Well-aligned ZnO nanorods with uniform diameter, length and density have been grown perpendicularly to the sapphire (0001) surface. Scanning electron microscopy (SEM) has been used to observe the morphology of the ZnO nanorods and X-ray diffraction and transmission electron microscopy (TEM) to investigate the crystalline structure of the nanorods. TEM observation as well as photoluminescence measurements confirm the very good crystalline quality of the nanorods. SEM observation on samples that have been prepared with various deposition times has been used in order to investigate the growth mechanism. Three types of ZnO morphologies have been identified: a thin two-dimensional ZnO layer formed at the sapphire surface, covered by three-dimensional hexagonal-shaped islands and hexagonal nanorods on top of them.

We report on the integration and characterizations of vertically aligned ZnO nanowires (NWs). Technological processes have been developed both for mineral and organic planarizations with suitable morphologies. Optical and electrical characterizations of these integrated NWs have been performed. A comparison of the photoluminescence (PL) spectra at 300K of the as-grown and integrated NWs has shown no significant impact of the integration process on the crystal quality of the NWs. Photoconducting properties in the UV-visible range have been investigated through collective electrical contacts. A small increase of resistivity in ZnO NWs under sub-bandgap illumination has been observed and discussed. The electrical transport properties of vertically integrated single NWs have also been investigated by scanning spread resistance microscopy (SSRM). Surface depletion layers in ZnO NWs have been evidenced.

An analytical model is presented in order to derive a general expression for the flow stress in polycrystalline films which encompasses and correlates dimensional constraints and strengthening effects. The model is based on the Thompson approach, which is extended to take into account both different grain aspect ratios and distinct strengthening contributions. It allows an accurate prediction of the growth textures in polycrystalline CdTe thick films when grain growth is driven by strain energy minimization. The model also matches the experimental data concerning the grain size and film thickness dependences of the yield stress in polycrystalline Cu thin films either deposited on a substrate or freestanding. Interestingly, the yield stress is found to be fitted by a modified Hall–Petch relation resulting in a d − n dependence in which the exponent n varies between ½ and 1 as a function of the grain size for a given thickness.

Polycrystalline Cadmium Telluride samples were electrically characterized using two high resolution imaging techniques: Electron and Ion Beam Induced Current. Using different probes, electrons and protons, both surface and bulk transport properties were obtained. The grain structure was observed and grain boundaries effects were studied. The material tends to have a homogeneous response under low excitation, with only few weakly responding grains and no dead areas. Under higher excitation, the material exhibits some particular behavior, like grain sub-structures. The IBIC experiment gives a measure of Charge Collection Efficiency under different sample bias voltages. In addition to the measurement of the response dispersion, it leads to a discussion of the charge transport properties and a mobility-lifetime product calculation.

Polycrystalline CdTe samples were characterized using photo-induced current transient spectroscopy (PICTS). The spectra obtained show that the traps are not discrete ones, or localized bands in the gap like in monocrystalline samples, indeed the PICTS signal presents large peaks. With an improvement of the calculation method presented here, these spectra reveal two types of traps. The first ones are active at low temperature and have energies between 0.3 and 0.5 eV. The second ones are deep level traps located between 0.65 and 0.8 depending on the sample. Focusing on these deep level traps, their influence on the photoconductive response of the material is discussed.

An extensive optical spectroscopy investigation of Polycrystalline Cl doped CdTe layers brings new insights into the compensation and passivation processes typical of such structurally imperfect materials. 10 K Photoluminescence shows that if some of the main Cl related compensation processes are similar in poly- and mono-crystalline materials, specific mechanisms are evidenced in polycrystalline layers involving complexes between chlorine atoms and extrinsic point defects in the vicinity of structural defects (e.g. extended defects). Using low temperature Cathodoluminescence imaging, it is also demonstrated that structural defects can be efficiently passivated by chlorine atoms, leading to improved electro-optical properties. Finally, using cross-section photoluminescence on thick layers, we correlate the evolution of the defect levels to that of the structural properties along the growth axis. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; In this work, we present the realization and the characterization of an optical waveguide made of AlN and GaN layers grown by MBE on Si(111) substrate. For the fundamental mode at 633nm, the propagation losses are in the order of 2 dB/cm, which is a good number for SC waveguides at this wavelength. The propagation losses dramatically increase with the mode order. A careful comparison of measurements and modeling of the complete structure allows identifying the part of the losses due to absorptionin the Si substrate, and showing that propagation losses could be further reduced by using well chosen SOI substrates.

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.

The Smart CutTM technology, now recognised as the technology of choice for the manufacturing of SOI substrates (Silicon On Insulator) [G. G. Shahidi, IBM J. Res. & Dev. 46, No. 2/3 (March and May 2002)], has got the unique potential to propose high quality Compound Semiconductors On Silicon (CSOS) substrates as a complementary technology to epitaxy. The use of this technology has led to the development of SiCOI (SiC On Insulator) structures where a thin film of single SiC crystal is bonded onto an oxidized substrate such as, for instance, a silicon substrate [L. Di Cioccio et al., Mater. Sci. Eng. B 46, 349 (1997)]. In this paper, we present the recent developments of SiCOI substrates developed using the Smart CutTM technology as well as the first results of GaN epitaxy performed onto these new structures. As a proof of concept and demonstration of feasibility, a Molecular Beam Epitaxy (MBE) GaN layer has been grown onto SiCOI substrate small samples and has been compared with similar growth on SiC bulk substrate. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the growth of AlN films on vicinal substrates of SiC(0001). The layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) and examined in-situ using reflection high-energy electron diffraction (RHEED). Morphological and structural properties were characterized by atomic force microscopy (AFM) and High Resolution X-Ray Diffraction (HRXRD), respectively. The obtained results by AFM show two main differences with AlN films grown on nominally flat SiC substrates: a) the absence of spiral growth hillocks, and b) the formation of predominantly straight steps. HRXRD measurements on misoriented substrate rocking curves have revealed good structural quality of AlN films (tilt and twist), as well as a disorientation of the c-planes of the AlN with respect to the substrate.

The organisation of GaN quantum dots on vicinal (0001) AlN surfaces has been investigated. Such surfaces were obtained using (0001)-misoriented SiC substrates. It is shown that AlN growth leads to a step bunching instability that can be kinetically controlled by temperature. The study of the GaN growth mechanism reveals a preferential alignment along AlN step edges and facets. Finally, the possibility of obtaining an efficient control of the island spatial distribution via step density and periodicity is demonstrated.

The electrical properties of ZnO mono-crystalline materials, either in the form of bulk crystals or epitaxial films, were investigated for a large range of un-intentional or intentional doping concentrations extending from 4.0 × 10 15 cm − 3 up to 1.3 × 10 20 cm − 3. Hall and resistivity measurements were carried out from 10 K to 300 K, yielding the temperature dependent carrier densities and carrier mobilities. This allowed for an unambiguous determination of the dopant ionization energies, taking into account the concentration of compensation centers. The ionization energy variation as a function of dopant concentration was found to follow Mott's law, being consistent with the hydrogenic behavior of all involved donors; an effective critical Mott's concentration for the insulator to metal transition was found to be around 4.2 × 10 18 cm − 3, while the apparent value of the isolated donor ionization energy was determined as being 60 meV.

LED reliability and lifetime prediction is a key point for Solid State Lighting adoption. For this purpose, one hundred and fifty LEDs have been aged for a reliability analysis. LEDs have been grouped following nine current-temperature stress conditions. Stress driving current was fixed between 350mA and 1A and ambient temperature between 85C and 120°C. Using integrating sphere and I(V) measurements, a cross study of the evolution of electrical and optical characteristics has been done. Results show two main failure mechanisms regarding lumen maintenance. The first one is the typically observed lumen depreciation and the second one is a much more quicker depreciation related to an increase of the leakage and non radiative currents. Models of the typical lumen depreciation and leakage resistance depreciation have been made using electrical and optical measurements during the aging tests. The combination of those models allows a new method toward a quicker LED lifetime prediction. These two models have been used for lifetime predictions for LEDs.

Current and temperature aging have been conducted on flip chip high power light emitting diodes (LEDs) with Al-Ni-Ti-Au n-contacts. Electrical and optical characteristics have been monitored during aging and a forward voltage increase has been observed. In order to understand this behavior, cross sections have been made on representative aged samples. For LEDs with a forward voltage shift, in the n-contact area, Au-Al inter-diffusion leading to a possible formation of Au-Al intermetallic compounds has been observed. We propose here to analyze the failure modes; the related failure mechanism(s) and consequences on LED flip chip reliability.

International audience; In this work, statistical formulations of the temperature dependence of ionized and neutral impurityconcentrations in a semiconductor, needed in the charge balance equation and for carrier scatteringcalculations, have been developed. These formulations have been used in order to elucidate aconfusing situation, appearing when compensating acceptor (donor) levels are located sufficientlyclose to the conduction (valence) band to be thermally ionized and thereby to emit (capture) anelectron to (from) the conduction (valence) band. In this work, the temperature dependent Hall carrierdensity and Hall mobility data adjustments are performed in an attempt to distinguish the presence ofa deep acceptor or a deep donor level, coexisting with a shallower donor level and located near theconduction band. Unfortunately, the present statistical developments, applied to an n-typehydrothermal ZnO sample, lead in both cases to consistent descriptions of experimental Hall carrierdensity and mobility data and thus do not allow to determine the nature, donor or acceptor, of the deeplevel. This demonstration shows that the emission of an electron in the conduction band, generallyassigned to a (0/+1) donor transition from a donor level cannot be applied systematically and couldalso be attributed to a (-1/0) donor transition from an acceptor level. More generally, this result canbe extended for any semiconductor and also for deep donor levels located close to the valence band(acceptor transition)

ZnO epilayers usually exhibit high n-type residual doping which is one of the reasons behind the difficulties to dope this material p-type. In this work, we aimed at determining the nature of the involved impurities and their potential role as dopant in ZnO thin films grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire and ZnO substrates. In both cases, secondary ion mass spectroscopy (SIMS) measurements give evidence for a strong diffusion of impurities from the substrate to the epilayer, especially for silicon and aluminum. In the case of samples grown on sapphire substrates, aluminum follows Fick's diffusion law on a wide growth temperature range (800−1000°C). Thus, the saturation solubility and the diffusion coefficient of aluminum in ZnO single crystals have been determined. Furthermore, the comparison between SIMS impurity and effective dopant concentrations determined by capacitance-voltage measurements highlights, on one hand a substitutional mechanism for aluminum diffusion, and on the ...

International audience; Fourier transform deep level transient spectroscopy has been performed between 80 and 550 Kin five n-type ZnO samples grown by different techniques. The capture cross section andionization energy of four electron traps have been deduced from Arrhenius diagrams. A trap1 eV below the conduction band edge is systematically observed in the five samples with alarge apparent capture cross section for electrons (1.6 ± 0.4 × 10−13 cm2) indicating a donorcharacter. The assignment of this deep level to the oxygen vacancy is discussed on the basis ofavailable theoretical predictions.

International audience