Thick Ga Research Articles

Improved-sensitivity integral SQUID magnetometry of (Ga,Mn)N thin films in proximity to Mg-doped GaN

Nominally 45 nm GaN:Mg / 5 nm (Ga,Mn)N / 45 nm GaN:Mg trilayers structures prepared by molecular beam epitaxy on GaN-buffered Al2O3 substrates are investigated to verify whether the indirect co-doping by holes from the cladding layers can alter the spin-spin interaction in (Ga,Mn)N. The four investigated structures, differing with the Mg doping level, are carefully characterized at a nanoscale by high-resolution transition electron microscopy (HRTEM), energy-dispersive x-ray spectroscopy, and secondary ion mass spectrometry. Importantly, HRTEM decisively excluded a presence of foreign Mn-rich phases. Magnetic studies of these structures are aided by the employment of a dedicated experimental approach of the in situ compensation of the magnetic contribution from the substrate, allowing an up to about fifty-fold reduction of this contribution. This technique, dedicated to these structures, simultaneously provides a tenfold reduction of long-term instabilities of the output of the magnetometer and lowers the experimental jitter to merely 5 × 10−7 emu at 70 kOe, vastly increasing the precision and the credibility of the results of the standard integral (volume) superconducting quantum interference device magnetometry, particularly in high magnetic fields. The magnetic characteristics of the trilayer structures established here prove identical with the already known properties of the thick (Ga,Mn)N single layers, namely (i) the low temperature ferromagnetism among Mn3+ ions driven by superexchange and (ii) purely paramagnetic response at higher temperatures. The possible cause of the lack of any effects brought about by the adjacent Mg-doping is a presence of residual Mn in the cladding layers, resulting in the deactivation of the p-type doping intended there. This finding points out that a more intensive technological effort has to be exerted to promote the co-doping-driven carrier-mediated ferromagnetic coupling in Mn-enriched GaN, especially at elevated temperatures.

Metal–insulator transition in epitaxial Ga-doped ZnO films via controlled thickness

Understanding and tuning of metal–insulator transition (MIT) in oxide systems is an interesting and active research topics of condensed matter physics. We report thickness dependent MIT in Ga-doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film (6 nm) exhibits a resistivity of 0.05 Ω cm, lying in the insulating regime, the thickest (51 nm) has resistivity of 6.6 × 10−4 Ω cm which shows metallic type of conduction. Our analysis reveals that the Mott’s variable range hopping model governs the insulating behavior in the 6 nm film whereas the 2D weak localization (WL) phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO films. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while WL is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaOx buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm−2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaOx semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Enhanced photoresponse performance in Ga/Ga2O3 nanocomposite solar-blind ultraviolet photodetectors**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674405 and 11675280) and the Fund from the Laboratory of Microfabrication in Institute of Physics, Chinese Academy of Sciences.

In the present work, we explore the solar-blind ultraviolet (UV) photodetectors (PDs) with enhanced photoresponse, fabricated on Ga/Ga2O3 nanocomposite films. Through pre-burying metal Ga layers and thermally post-annealing the laminated Ga2O3/Ga/Ga2O3 structures, Ga/Ga2O3 nanocomposite films incorporated with Ga nanospheres are obtained. For the prototype PD, it is found that the photocurrent and photoresponsivity will first increase and then decrease monotonically with the thickness of the pre-buried Ga layer increasing. Each of all PDs shows a spectrum response peak at 260 nm, demonstrating the ability to detect solar-blind UV light. Adjustable photoresponse enhancement factors are achieved by means of the surface plasmon in the nanocomposite films. The PD with a 20 nm thick Ga interlayer exhibits the best solar-blind UV photoresponse characteristics with an extremely low dark current of 8.52 pA at 10-V bias, a very high light-to-dark ratio of ∼ 8 × 105, a large photoresponsivity of 2.85 A/W at 15-V bias, and a maximum enhancement factor of ∼ 220. Our research provides a simple and practical route to high performance solar-blind UV PDs and potential applications in the field of optoelectronics.

Morphological and chemical instabilities of nitrogen delta-doped GaAs/(Al, Ga)As quantum wells

The microstructure and the element distribution across tensile-strained nitrogen δ-doped GaAs/(Al,Ga)As quantum wells (QW) are investigated by transmission electron microscopy. We find that the nitrogen sub-monolayer insertion results in a several monolayer thick Ga(As,N) layer with thickness and lateral composition fluctuations. The thickness and composition fluctuations are not arbitrary, but they are anticorrelated, i.e., the Ga(As,N) layer is thinner in areas of higher nitrogen content and vice versa. Thus, regardless of the specific position along the QW, the amount of incorporated nitrogen remains constant and close to its nominal value. The increase in the nitrogen content at the insertion promotes an anisotropic shape transition towards highly faceted three-dimensional structures. Our experimental observations indicate that the two-dimensional to three-dimensional morphological transition is determined by intrinsic factors associated with the different Ga-N and Ga-As bonds and hence occurs regardless of the epitaxial strain state of the layers.

Pure spin current transport in gallium doped zinc oxide

We study the flow of a pure spin current through zinc oxide by measuring the spin Hall magnetoresistance (SMR) in thin film trilayer samples consisting of bismuth-substituted yttrium iron garnet (Bi:YIG), gallium-doped zinc oxide (Ga:ZnO), and platinum. We investigate the dependence of the SMR magnitude on the thickness of the Ga:ZnO interlayer and compare to a Bi:YIG/Pt bilayer. We find that the SMR magnitude is reduced by almost one order of magnitude upon inserting a Ga:ZnO interlayer and continuously decreases with increasing interlayer thickness. Nevertheless, the SMR stays finite even for a 12 nm thick Ga:ZnO interlayer. These results show that a pure spin current indeed can propagate through a several nm-thick degenerately doped zinc oxide layer. We also observe differences in both the temperature and the field dependence of the SMR when comparing tri- and bilayers. Finally, we compare our data to the predictions of a model based on spin diffusion. This shows that interface resistances play a crucial role for the SMR magnitude in these trilayer structures.

Surface photovoltage and photoluminescence study of thick Ga(In)AsN layers grown by liquid-phase epitaxy

We present an experimental and theoretical study of Ga(In)AsN layers with a thickness of around 1 μm grown by liquid-phase epitaxy (LPE) on n-type GaAs substrates. The samples are studied by surface photovoltage (SPV) spectroscopy and by photoluminescence spectroscopy. Theoretical calculations of the electronic structure and the spectral dependence of the dielectric function are carried out for different nitrogen concentrations using a full-band tight-binding approach in the sp3d5s*sN parameterisation. The SPV spectra measured at room temperature clearly show a red shift of the absorption edge with respect to the absorption of the GaAs substrate. This shift, combined with the results of the theoretical calculations, allows assessing the nitrogen concentration in different samples. The latter increases with increasing the In content. The analysis of the SPV phase spectra provides information about the alignment of the energy bands across the structures. The photoluminescence measurements performed at 2 K show a red shift of the emission energy with respect to GaAs, in agreement with the SPV results.

Ab initiostudy of Ga-GaN system: Transition from adsorbed metal atoms to a metal–semiconductor junction

Ab initio studies of a GaN(0001)-Ga system with various thicknesses of a metallic Ga layer were undertaken. The studied systems extend from a GaN(0001) surface with a fractional coverage of gallium atoms to a Ga-GaN metal–semiconductor (m–s) contact. Electronic properties of the system are simulated using density functional theory calculations for different doping of the bulk semiconductor. It is shown that during transition from a bare GaN(0001) surface to a m–s heterostructure, the Fermi level stays pinned at a Ga-broken bond highly dispersive surface state to Ga–Ga states at the m–s interface. Adsorption of gallium leads to an energy gain of about 4 eV for a clean GaN(0001) surface and the energy decreases to 3.2 eV for a thickly Ga-covered surface. The transition to the m–s interface is observed. For a thick Ga overlayer such interface corresponds to a Schottky contact with a barrier equal to 0.9 and 0.6 eV for n- and p-type, respectively. Bond polarization-related dipole layer occurring due to an electron transfer to the metal leads to a potential energy jump of 1.5 eV, independent on the semiconductor doping. Additionally high electron density in the Ga–Ga bond region leads to an energy barrier about 1.2 eV high and 4 A wide. This feature may adversely affect the conductivity of the n-type m–s system.

Magnetic anisotropy investigations of (Ga,Mn)As with a large epitaxial strain

Magnetic properties of 20nm thick (Ga,Mn)As layer deposited on (Ga,In)As buffer with very large epitaxial tensile strain are investigated. Ga1−xInxAs buffer with x=30% provides a 2% lattice mismatch, which is an important extension of the mismatch range studied so far (up to 0.5%). Evolution of magnetic anisotropy as a function of temperature is determined by magnetotransport measurements. Additionally, results of direct measurements of magnetization are shown.

Molecular-beam epitaxy of high-quality diluted magnetic semiconductor (Ga, Mn)Sb single-crystalline films

Diluted magnetic semiconductor (Ga, Mn)Sb and its related hetero-structures have attracted much attention in recent years since they are predicted to have some novel properties, such as the quantum anomalous Hall effect etc. However, it is not easy to grow high-quality (Ga, Mn)Sb films due to their narrow growth window. In this article, a series of 10 nm thick (Ga, Mn)Sb films with different Mn contents from 0.016 to 0.039 have been grown by molecular-beam epitaxy at low temperaturs (～230 ℃). The films have high crystalline quality as confirmed by in situ reflection high-energy electron diffraction and ex situ atomic force microscopy, and no MnSb phase could be observed. Curie temperature up to 30 K has been obtained in one (Ga, Mn)Sb film after post-growth thermal annealing. The magneto-resistance and anomalous Hall effect of this film have also been investigated at different temperatures.

Ga lithography in sputtered niobium for superconductive micro and nanowires

This work demonstrates the use of focused ion beam (FIB) implanted Ga as a lithographic mask for plasma etching of Nb films. Using a highly collimated Ga beam of a FIB, Nb is implanted 12 nm deep with a 14 nm thick Ga layer providing etch selectivity better than 15:1 with fluorine based etch chemistry. Implanted square test patterns, both 10 μm by 10 μm and 100 μm by 100 μm, demonstrate that doses above than 7.5 × 1015 cm−2 at 30 kV provide adequate mask protection for a 205 nm thick, sputtered Nb film. The resolution of this dry lithographic technique is demonstrated by fabrication of nanowires 75 nm wide by 10 μm long connected to 50 μm wide contact pads. The residual resistance ratio of patterned Nb films was 3. The superconducting transition temperature (Tc) = 7.7 K was measured using a magnetic properties measurement system. This nanoscale, dry lithographic technique was extended to sputtered TiN and Ta here and could be used on other fluorine etched superconductors such as NbN, NbSi, and NbTi.

The Influence of Al Underlayer on the Optical and Electrical Properties of GZO/Al Thin Films

100 nm thick Ga doped ZnO (GZO) thin films were deposited with DC and RF magnetron sputtering at room temperature on glass substrate and Al coated glass substrate, respectively. and the effect of the Al underlayer on the optical and electrical properties of the GZO films was investigated. As-deposited GZO single layer films had an optical transmittance of 80% in the visible wavelength region, and sheet resistance of 1,516 , while the optical and electrical properties of GZO/Al bi-layered films were influenced by the thickness of the Al buffer layer. GZO films with 2 nm thick Al film show a lower sheet resistance of 990 , and an optical transmittance of 78%. Based on the figure of merit (FOM), it can be concluded that the thin Al buffer layer effectively increases the performance of GZO films as a transparent and conducting electrode without intentional substrate heating or a post deposition annealing process.

Ferromagnetic Interfacial Interaction and the Proximity Effect in aCo2FeAl/(Ga,Mn)AsBilayer

The magnetic properties of a Co2FeAl/(Ga,Mn)As bilayer epitaxied on GaAs (001) are studied both experimentally and theoretically. Unlike the common antiferromagnetic interfacial interaction existing in most ferromagnet-magnetic semiconductor bilayers, a ferromagnetic interfacial interaction in the Co2FeAl/(Ga,Mn)As bilayer is observed from measurements of magnetic hysteresis and x-ray magnetic circular dichroism. The Mn ions in a 1.36 nm thick (Ga,Mn)As layer remain spin polarized up to 400 K due to the magnetic proximity effect. The minor loops of the Co2FeAl/(Ga,Mn)As bilayer shift with a small ferromagnetic interaction field of +24 Oe and -23 Oe at 15 K. The observed ferromagnetic interfacial coupling is supported by ab initio density functional calculations. These findings may provide a viable pathway for designing room-temperature semiconductor spintronic devices through magnetic proximity effect.

On Equilibrium Ga Intergranular Films in Cr2GaC

The presence of equilibrium intergranular films (IGFs) in metallic systems has long been postulated; compelling evidence for this idea, however, is still lacking. Herein differential scanning calorimetry, in the−60°C to 70°C temperature range, on bulk and Cr2GaC powders was carried out. A sharp endothermic trough, roughly 50°C below the melting point of bulk pure Ga, in the powdered samples was taken to be strong evidence for the presence of IGFs in the latter. The troughs for the sintered body were much more diffuse suggesting that in this case, a distribution of grain boundary thicknesses exists. A temperature dependent adsorption energy is postulated to explain the reversible extrusion and resorption of 100 μm thick Ga micro-pillars.

Formation of two-dimensionally confined superparamagnetic (Mn, Ga)As nanocrystals in high-temperature annealed (Ga, Mn)As/GaAs superlattices

The annealing-induced formation of (Mn, Ga)As nanocrystals in (Ga, Mn)As/GaAs superlattices was studied by x-ray diffraction, transmission electron microscopy and magnetometry. The superlattice structures with 50 Å thick (Ga, Mn)As layers separated by 25, 50 and 100 Å thick GaAs spacers were grown by molecular beam epitaxy at low temperature (250 ° C), and then annealed at high temperatures of 400, 560 and 630 ° C. The high-temperature annealing causes decomposition to a (Ga, Mn)As ternary alloy and the formation of (Mn, Ga)As nanocrystals inside the GaAs matrix. The nanocrystals are confined in the planes that were formerly occupied by (Ga, Mn)As layers for the up to 560 ° C annealing and diffuse throughout the GaAs spacer layers at 630 ° C annealing. The two-dimensionally confined nanocrystals exhibit a superparamagnetic behavior which becomes high-temperature ferromagnetism (∼350 K) upon diffusion.

Alloy and lattice disorder in Hf implanted AlxGa1-xN(0≤x≤1)

Abstract The alloy and lattice disorder as well as the annealing behaviour of 0.5 μ m thick Al x Ga 1 - x N films on sapphire substrate after implantation of 181Hf(181Ta) was studied using the perturbed angular correlation (PAC) technique. Al x Ga 1 - x N samples with different molar fractions of AlN were implanted with a fluence of 1 × 10 13 at / cm 2 of the radioisotope 181Hf with an energy of 160 keV at the Bonn Isotope Separator. Subsequently the samples were annealed in a rapid thermal annealing apparatus at temperatures up to 1100 ∘ C in nitrogen atmosphere. The strength of the electric field gradient, which is caused by the wurtzite structure of the host lattice at the probe site, varies linearly with the concentration x of aluminium in the ternary compound. The uniformity of this hyperfine interaction has its minimum at x ≈ 0.6 .

Growth of thick GaInN on grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC

We succeeded in growing high-crystalline-quality thick (1 0 1¯ 1¯) Ga 0.92In 0.08N films on a grooved (1 0 1¯ 1¯) GaN/(1 0 1¯ 2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580 nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure.

Huge tunnelling anisotropic magnetoresistance in (Ga,Mn)As nanoconstrictions

We report here large anisotropic magnetoresistance (AMR) behaviours in single lateral (Ga,Mn)As nanoconstrictions of up to 1300%, along with large multistable telegraphic switching. The nanoconstriction devices are fabricated using high-resolution electron beam lithography of a 5 nm thick (Ga,Mn)As epilayer. The unusual behaviour exhibited by these devices is discussed in the context of existing theories for enhanced AMR ferromagnetic semiconductor nanoscale devices, particularly with regard to the dependence on the magnetotransport of the bulk material. We conclude that our results are most consistent with the Coulomb blockade AMR mechanism.

Vertical transport in GaAs/ Ga 1− yAl yAs barrier structures containing quantum wells: Current–temperature characteristics

Vertical transport in GaAs/ Ga 1− y Al y As barrier structures was investigated using current–temperature ( I – T ) measurements in the dark. The samples studied had 500 Å thick Ga 1− y Al y As (0≤ y ≤0.26 ) linearly graded barriers between the n +-GaAs contacts and the Ga 0.74Al 0.26As central barrier containing N w (=0, 4, 7 and 10) n-doped GaAs quantum wells of width 35 Å. The thickness of Ga 0.74Al 0.26As barrier between the wells was 310, 135 and 77 Å for the samples with N w = 4 , 7 and 10, respectively. The vertical current was measured as a function of temperature (3.5–295 K) at applied voltages in the range 0.01–0.7 V. The total barrier height ( Δ U m ) and the space charge density in the sample with N w = 0 , and the effective barrier height ( Δ E ) for the samples with N w = 4 , 7 and 10 were determined by analysing the experimental I – T characteristics using drift–diffusion theory for thermal current. The variation of Δ U m and Δ E with applied voltage was discussed in connection with various dark current mechanisms in barrier structures, including thermionic emission, Fowler–Nordheim tunnelling and sequential resonant tunnelling.

M‐plane III‐nitride materials for polarization sensitive devices grown by PAMBE with real time analysis by spectroscopic ellipsometry

AbstractIn this paper we present a study of the growth of M‐plane GaN epilayers on LiAlO2 substrates under Ga rich conditions and growth at Ga stable conditions by plasma assisted molecular beam epitaxy (PAMBE) coupled with in‐situ spectroscopic ellipsometry (SE). Previous studies on M‐plane materials have suggested that optimum growth conditions with respect to surface morphology are those giving rise to Ga trilayer coverage at low growth temperature. By studying Ga adsorption/desorption in vacuum and in the presence of active nitrogen, we find that the total coverage for M‐plane material is distinctly different for these two conditions. Under vacuum, the stable Ga coverage corresponded of 0.48 nm, while a thicker Ga stable coverage of 0.7 nm was determined N plasma. Using these GaN growth conditions, M‐plane InGaN layers were grown and fabricated into polarization‐sensitive photodetectors. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)