Study of the electrical properties of individual (Ga,Mn)As nanowires

Arrays of (Ga, Mn)As nanowire crystals are synthesized by molecular-beam epitaxy. Electronbeam lithography made possible the fabrication of electric contacts to individual nanowires. The influence of the annealing temperature on the properties of the contacts is studied. The optical annealing temperature is determined to be 160°C. It is found that an increase in the annealing temperature yields structure degradation. From studies of the current-voltage characteristics of the individual nanowire structures, a number of their electrical parameters, such as the resistivity and the mobility of charge carriers are determined.     

Study of processes of self-catalyzed growth of gaas crystal nanowires by molecular-beam epitaxy on modified Si (111) surfaces

The processes of growth of self-catalyzed GaAs crystal nanowires on Si (111) surfaces modified by three different methods are studied. For the technology of production of the GaAs nanowires, molecular-beam epitaxy is used. It is found that, in the range of substrate temperatures between 610 and 630°C, the surface density of nanowires and their diameter sharply increases, whereas the temperature dependence of the nanowire length exhibits a maximum at 610°C. An increase in the temperature to 640°C suppresses the formation of nanowires. The method that provides a means for the fabrication of purely cubic GaAs nanowires is described. A theoretical justification of the formation of the cubic phase in self-catalyzed GaAs nanowires is presented.     

The effect of Cs-O activation temperature on the surface escape probability of NEA (In,Ga)As photocathodes

It has been found that the temperature TAat which Cs and O are applied to (In,Ga)As negative electron affinity photocathodes has a strong influence on the surface escape probability B. Over the temperature range investigated, 25-100°C, B changes by a factor of approximately 30.     

Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Needle‐like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga‐doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c‐axis direction. The X‐ray diffraction (XRD) θ‐scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c‐axes of the nanorods are along the normal direction of the substrate surface. The investigation using high‐resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room‐temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn‐on field for the ZnO nanowires was found to be about 18 V μm^–1 at a current density of 0.01 μA cm^–2. The emission current density from the ZnO nanowires reached 0.1 mA cm^–2 at a bias field of 24 V μm^–1.     

氨化Ga2O3/Mg薄膜制备簇状GaN纳米线

通过在1050°C时氨化Ga2O3/Mg薄膜制备出簇状GaN纳米线。用X射线衍射(XRD),傅里叶红外吸收光谱(FTIR)扫描电子显微镜(SEM)和高分辨电子显微镜(HRTEM)对样品进行测试分析。结果表明,GaN纳米线为六万纤锌矿结构单晶相并且成族生长,直径在200～500nm米左右,其长度可达5～10μm。几乎所有纳米线的直径均有逐渐缩小的趋势。对Mg膜的作用进行了初步的分析。     

Ion implantation damage in GaAs: A TEM study of the variation with ion species and stoichiometry

GaAs samples have been implanted with a dose of 2 × 10¹⁴ cm^?2 of each ion in the following combinations: Ga, As, Ga + As, Se, Ga + Se and As + Se. Implantation was at 200°C, and post implantation annealing at 700°C. Subsequent examination by transmission electron microscopy (TEM) showed clear and reproducible differences in the dislocation loop size and density, depending on the ion species implanted. The simplest results were obtained with the single implants, particularly Ga and As. These observed variations could be explained in terms of point defect populations, and hence rates of annealing at a given anneal temperature, being affected significantly by the stoichiometric effect of the implant. These simpler aspects were also seen to be incorporated in the more complex “dual” implants.     

Structural effects of heating gold-based contacts to gallium phosphide

The formation of α′ AuGa was observed by X-ray diffraction in situ upon heating Au/Ni/Au-Ge/GaP(111) in nitrogen to ≥ 438 ± 12°C and subsequent cooling to 300°C. No AuGa phase formation was observed without cooling, suggesting that the reaction product was a liquid at the reaction temperature. The amount of α′ increased with increasing reaction temperature for the same reaction time. The peritectic transformation of α′ to α and liquid AuGa solution was observed reversibly upon subsequent heating to 388 ± 12°C. After the AuGa phase formation in Au/Ni/Au-Ge/GaP(111), scanning electron microscopy and X-ray spectroscopy revealed ～ 20 μm interconnected Au-rich islands and ～ 2μm Ni-rich islands in a matrix which was Ga- and P-rich. In Au/GaP, Au-Ga phase formation was observed ex situ after heating in hydrogen to ≥ 475 ± 25°C, and a change from Schottky to near ohmic behavior was observed ex situ after heating in hydrogen to ≥ 400°C; neither AuGa phase formation nor appreciable change in the Schottky characteristic was observed when heated in nitrogen up to 550°C.     

Effect of growth conditions on incorporation of Si into Ga and As sublattices of GaAs during molecular-beam epitaxy

The effect of dopant concentration and growth-surface crystallographic orientation on the incorporation of Si into Ga and As sublattices was investigated during GaAs molecular-beam epitaxy. The epitaxial layers (epilayers) were grown on GaAs substrates with (100), 2°(100), 4°(100), and 8°(100) orientations at a temperature of 520°C and with (111)A, 2°(111)A, 2°(111)A, 5°(111)A, 6°(111)A, and 8°(111)A (where A = Ga) orientations at a temperature of 480°C. The Sidopant concentration was varied within 10¹⁷–10¹⁹ cm^?3. Through electrical and photoluminescent methods of investigation, the Si impurity was found to occur at the sites of both GaAs-layer sublattices not only as simple donors and acceptors (Si_Ga and Si_As), but also as Si_Ga-Si_As, Si_Ga-V_Ga, and Si_As-V_As complexes. The concentration of Si impurity in various forms depends on the doping level of the layers and on the growth-surface orientation. Amphoteric properties of Si manifest themselves more prominently on the (111)A face than on the (100) one. It is shown that impurity defects form at the stage of layer crystallization and depend on the growth-surface structure.     

Characteristics of the formation of (Al, Ga)Sb/InAs heterointerfaces in molecular-beam epitaxy

A thermodynamic model is given for the molecular-beam epitaxy formation of InSb, GaAs, and AlAs heterointerfaces in (Al, Ga)Sb/InAs heterostructures. The maximum critical temperature of formation of a planar InSb-type heterointerface on an (Al, Ga)Sb layer, T≈390 °C, is determined from a comparison of the pressure of Sb₄ molecules in the external flux with their equilibrium value above a stressed monolayer on a heterointerface and is found to be in good agreement with existing experimental data. In contrast, the critical temperature of formation of a heterointerface of the AlAs (GaAs) type, corresponding to the onset of rapid reevaporation of As, is much higher than the growth temperatures normally used in molecular-beam epitaxy (350–550 °C). Fiz. Tekh. Poluprovodn. 31, 1242–1245 (October 1997)     

Annealing behavior of undoped bulk GaAs

Heat treatments of bulk undoped GaAs in a sealed ampoule (800° C, 0.5 or 1 h) and in a covered LPE boat under H₂ (695° C, 51 h) were performed with different surface conditions,i.e., exposed surface, proximate surface and a Ga(As) solution covered surface. Four characterization techniques, namely Polaron profile, secondary ion mass spectroscopy (SIMS), photoluminescence (PL) and deep level transient spectroscopy (DLTS), were used to detect the changes in impurities and defects in the surface region of the samples. The carrier concentration profiles, the accumulation of impurities (such as C, Si, Mn and Cu), the PL features and the PL profiles of Mn and Cu related emissions, as well as the trap profiles of EL6 and EL2 for the different surface conditions all demonstrate the important role of both impurities and native defects in surface degradation and the influence of native defects on impurity incorporation.     

Properties of the GaSb:Mn layers deposited from laser plasma

The X-ray diffraction and electrical and magnetic properties of GaSb:Mn layers deposited on GaAs (100) substrates from a laser plasma in free space are studied. It is shown that the films deposited at 200–440°C are epitaxial mosaic single crystals. Manganese-doped layers (up to ～4 at % Mn) had a hole concentration higher than 1 × 10¹⁹ cm^?3. Structures with the GaSb:Mn layers grown at 200°C had an anomalous Hall effect. A normal Hall effect was observed for the GaSb:Mn layers grown at 440°C. The exposure of these layers to a laser pulse (wavelength λ = 0.68 μm, duration 25 ns) caused an increase in the hole concentration and the emergence of the anomalous Hall effect at room temperature. Magnetic ultrahigh-frequency measurements confirmed that the films were ferromagnetic up to 293 K and revealed a magnetism anisotropy.     

In situ X-ray diffraction study of melting in gold contacts to gallium arsenide

By in situ X-ray diffraction, we have obtained direct evidence for the distinction in melting temperature between the first and subsequent heating in ～ 1000 Å gold contacts to GaAs. During the first heating, melting occurred at 456–500°C and is attributed to the melting of a reaction product tentively identified as the orthorhombic AuGa compound (50 at.% Ga), which formed in the solid state. The minimum temperature required for the solid-state reaction and the melting temperature decreased with decreasing hydrostatic pressure. During second and subsequent heating, melting occurred at 410–415°C and is attributed to the melting of β AuGa (or Au₇Ga₂), which formed after the first melting-solidification cycle. At high cooling rates (e.g. 40°C/min) during the first solidification, β was observed together with a phase (tentative Au₂Ga) which increased in proportion with increasing cooling rate.     

Preparation and properties of Mn-doped epitaxial gallium arsenide

The preparation of manganese-doped GaAs epitaxial layers growth from the liquid-phase, and Hall effect and resistivity measurements in the temperature range from 60 to 300°K, are described. An anomalous solubility of GaAs in the Ga + GaAs + Mn solution was observed and occurrence of a Mn₂As-phase in solution was identified. From the analysis of the Hall effect measurements the thermal activation energy of the manganese acceptor was determined. The energy decreases from 0·092 to 0·084 eV with the Mn-concentration in the epitaxial layers increasing from 6 × 10¹⁷ cm^?3 to 3 × 10¹⁹ cm^?3. The solubility of Mn in GaAs at the temperature of the preparation (850°C) was found to be about 4 × 10¹⁹ cm^?3.     

Advances in new generation diluted magnetic semiconductors with independent spin and charge doping

As one branch of spintronics, diluted magnetic semiconductors (DMSs) are extensively investigated due to their fundamental significance and potential application in modern information society. The classical materials (Ga,Mn)As of III–V group based DMSs has been well studied for its high compatibility with the high-mobility semiconductor GaAs. But the Curie temperature in (Ga,Mn)As film is still far below room temperature because the spin &; charge doping is bundled to the same element that makes the fabrication very difficult. Alternatively, the discovery of a new generation DMSs with independent spin and charge doping, such as (Ba,K)(Zn,Mn)₂As₂ (briefly named BZA), attracted considerable attention due to their unique advantages in physical properties and heterojunction fabrication. In this review we focus on this series of new DMSs including (I) materials in terms of three types of new DMSs, i.e. the " 111”, " 122” and " 1111” system; (II) the physical properties of BZA; (III) single crystals &; prototype device based on BZA. The prospective of new type of DMSs with independent spin and charge doping is briefly discussed.     

The effect of heat treatment on unpassivated and passivated gallium arsenide surfaces

The effects of various heat treatments on the surface of GaAs crystals have been studied using Rutherford scattering and channelling of alpha particles. The heat treatment was in the form of isochronal anneals in the temperature range 350–750°C. The crystals were heated in both flowing dry nitrogen and in vacuum (～ 10^?6 torr). The changes in surface-stoichiometry and surface disorder were measured and the results, for both free and silicon oxide passivated surfaces, are discussed.It is shown that preferential loss of As occurs for the unpassivated surfaces heated in nitrogen. However, the unpassivated surfaces heated in vacuum show no degradation, even up to 750°C. The behaviour of the passivated surfaces was dominated by the mechanical properties of the silicon oxide layer, which suffered from adhesion and blistering problems. Only for annealing in nitrogen at 550°C did the silicon oxide behave successfully as a passivating layer and prevent the out-diffusion of Ga and the evaporation of As.     

Co-Implantation and autocompensation in close contact rapid thermal annealing of Si-implanted GaAs:Cr

Close contact rapid thermal annealing of semi-insulating GaAs:Cr implanted with Si, Si + Al, and Si + P has been studied using variable temperature Hall effect measurements and low temperature (4.2K) photoluminescence (PL) spectroscopy. Isochronal (10 sec) and isothermal (1000° C) anneals indicate that As is lost from the surface during close contact annealing at high anneal temperatures and long anneal times. Samples which were implanted with Si alone show maximum activation at an annealing temperature of 900° C, above which activation efficiency decreases. Low temperature Hall and PL measurements indicate that this reduced activation is due to increasing auto-compensation of Si donors by Si acceptors at higher anneal temperatures. However, co-implantation of column V elements can increase the activation of Si implants by reducing Si occupancy of As sites and increasing Si occupancy of Ga sites, and therebyoffset the effects of As loss from the surface. For samples implanted with Si + P, activation increases continuously up to a maximum at an anneal temperature of 1050° C, and both low temperature Hall and PL measurements indicate that autocompensation does not increase in this case as the anneal temperature increases. In contrast, samples implanted with Si + Al show very low activation and very high compensation at all anneal temperatures, as expected. The use of column V co-implants in conjunction with close contact RTA can produce excellent donor activation of Si implanted GaAs.     

低温退火对稀磁半导体(Ga,Mn)As性质的影响

利用低温分子束外延技术在GaAs(001)上外延生长出厚度为500nm的稀磁半导体(Ga,Mn)As薄膜. 双晶X射线衍射证明其为闪锌矿结构，晶格参数为0.5683nm，据此推导出其Mn含量为7%. 磁测量结果揭示其铁磁转变温度为65K. 观察了低温退火处理对(Ga,Mn)As磁性质的影响，发现生长后退火处理显著提高了其铁磁转变温度，可以达到115K.     

Morphological development during platinum/gallium arsenide interfacial reactions

The diffusion path and reaction kinetics of Pt/GaAs diffusion couples were investigated in the temperature range 400 to 1000° C using optical metallography, electron probe microanalysis (EPMA), and scanning electron microscopy (SEM). In all cases the formation of two phases, Pt₃Ga and PtAs₂, was observed. A layered structure Pt/Pt₃Ga/ PtAs₂/GaAs was observed in the temperature range 400 to 450° C and again between 700 and 800° C. However, in the intermediate temperature range from 450 to 650° C a two-phase mixture was observed with the sequence Pt/Pt₃Ga/(Pt₃Ga, PtAs₂)/GaAs. This two-phase microstructure resulted in faster reaction rates than would have been predicted from the layered kinetics. The reversion from the agglomerate two-phase to a layered microstructure between 650 and 675° C may be explained by a rapid rate of grain coarsening in the two-phase microstructure at high temperature. This return to a planar morphology resulted in a substantial decrease in reaction rate with increasing temperature.     

Effect of temperature on InGaAsP alloy composition

The influence of growth temperature on the composition of InGaAsP films grown by low pressure metalorganic vapor phase epitaxy (MOVPE) is reported for quaternary (Q) alloys having bandgap wavelengths of λ_g = 1.1, 1.3, and 1.5 μn. Films with these different Q-compositions were deposited lattice matched to InP at a growth temperature of 675°C. Subsequent growth experiments were then performed for each Q-composition in which the input gas flow rates were kept the same and only the temperature changed in 25°C decrements down to 600°C. Photoluminescence (PL) and lattice mismatch (LMM) measurements of the resulting films were used to determine the effect of growth temperature on film composition. The PL data indicate a temperature shift in the PL wavelength of −1.8 nm/ °C for the 1.5Q composition, −2.9 nm/°C for 1.3Q, and −4.3 nm/°C for 1.1Q. Negative shifts were also observed in LMM of −80 ppm/°C for 1.5Q, −150 ppm/°C for 1.3Q, and −250 ppm/°C for 1.1Q. The Ga/In and P/As ratios of the Q-filmswere measured by secondary ion mass spectroscopy and correlated with full-wafer maps of the PL wavelength and lattice mismatch to gain insight into the processes responsible for wafer nonuniformity in MOVPE.     

ZnO spintronics and nanowire devices

ZnO is a very promising material for spintronics applications, with many groups reporting room-temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during pulsed laser deposition (PLD), we find an inverse correlation between magnetization and electron density as controlled by Sn-doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for ferromagnetism include the bound magnetic polaron model or exchange that is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. The progress in ZnO nanowires is also reviewed. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for microlasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼−3 V, and a maximum transconductance of 0.3 mS/mm. Under ultraviolet (UV) illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼5 mS/mm. The channel mobility is estimated to be ∼3 cm²/Vss, comparable to that for thin film ZnO enhancement mode metal-oxide semiconductor field effect transistors (MOSFETs), and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. The Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25°C, very low reverse currents, and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25°C to 150°C, the resistivity of nanorods treated in H₂ at 400°C prior to measurement showed an activation energy of 0.089 eV and was insensitive to ambient used. By contrast, the conductivity of nanorods not treated in H₂ was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. Sensitive pH sensors using single ZnO nanowires have also been fabricated.