High Hall Mobility Research Articles

Traps and Defects in AlGaN-GaN High Electron Mobility Transistors on Semi-Insulating SiC Substrates

AbstractHigh electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for high power, high speed, and high temperature operation. Especially, AlGaN-GaN HEMTs grown on semi-insulating (SI) SiC substrates are the most promising for both military and commercial applications. High performance characteristics from these devices are possible in part due to the presence of high two-dimensional electron gas charge sheet density maintaining a high Hall mobility at the AlGaN barrier-GaN buffer hetero-interface and in part due to high thermal conductivity of the SiC substrates. However, long-term reliability of these devices still remains a major concern because of the large number of traps and defects present both in the bulk as well as at the surface leading to undesirable characteristics including current collapse. We report on the study of traps and defects in two MOCVD-grown structures: Al0.27Ga0.73N HEMTs on SI SiC substrates and Al0.27Ga0.73N Schottky diodes on conducting SiC substrates. Our HEMT structures consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer show a charge sheet density of ∼1013/cm2 and a Hall mobility of ∼1500cm2/V·sec. Deep level transient spectroscopy (DLTS) was employed to study traps in AlGaN Schottky diodes and HEMTs fabricated with different Schottky contacts consisting of Pt/Au and Ni/Au. Focused ion beam was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM.

Properties of Amorphous Transparent Conductive In-Ga-Zn Oxide Films Deposited on Fused Quartz by the PLD Method

Transparent conductive oxide (TCO) films of In-Ga-Zn-O were deposited by sputtering, for applications involving shorter wavelengths than visible light wavelengths. The films were deposited on fused quartz at a substrate temperature of 250° by pulsed laser deposition (PLD) using a pulsed KrF excimer laser (λ=248 nm). The targets were synthesized by mixing In2O3-Ga2O3-6ZnO powders and Ga2O3+3ZnO powders. Weight % of the mixed Ga2O3+3ZnO powders was estimated by x, where x was changed from 0 to 0.2 in order to increase the fraction of Ga atoms in the film. Near the stoichiometric composition of In, Ga and Zn, we obtained low resistivity films and high carrier mobility. All the films deposited were amorphous. The optical bandgap energy was increased with increasing Ga content. With appropriate incorporation of extra Ga in the target, we could deposit stoichiometric In-Ga-Zn-O amorphous films. We obtained the lowest resistivity of 3 × 10-3 Ωcm and a high Hall mobility of 20-25 cm2/Vs. The carrier concentration was 1 × 1020 cm-3. The optical band gap energy was changed from 3.91 to 4.06 eV with increasing x value from 0 to 20 wt%. It was confirmed that the slight deviation from the stoichiometric composition did not influence the electrical properties. Even for a thickness of 50-80 nm, the film properties were the same as those of much thicker films. The present films are very homogeneous and the surface is very flat, due to the fact that the films are amorphous. [DOI: 10.1380/ejssnt.2009.273]

Growth of Bulky Single Crystalline Films of (Zn,Mg)O Alloy Semiconductors by Liquid Phase Epitaxy

Very thick (about 0.5 mm) single crystalline films of (Zn1−xMgx)O solid solution have been grown by a liquid phase epitaxy (LPE) technique. The source materials, ZnO and MgO, were dissolved in a molten PbO-Bi2O3 flux and deposited on ZnO substrates as epitaxial (Zn1−xMgx)O layers. The (Zn1−xMgx)O layers thus obtained showed high crystallinity, similar to that of the ZnO substrate, and exhibited n-type conductivity with relatively high Hall mobilities (>90 cm2 V−1 s−1 for x = 0.1 at room temperature). Moreover, the activation energy of the mobile electrons was about 50 meV, and this value was independent of the MgO fraction. Since LPE is an appropriate technique for growing large area films, we examined the growth of (Zn,Mg)O thick films on 2 in. diameter ZnO substrates. The Mg concentration in the LPE grown layer was quite uniform, and the Li concentration was two or three orders lower than an ordinary ZnO substrate.

Influence of rapid thermal annealing (RTA) on the structural and electrical properties of SnS films

The performance of optoelectronic devices critically depends on the quality of active layer. An effective way to obtain a high quality layers is by creating excess of metal atoms through various heat treatments. Recently, rapid thermal annealing (RTA) has proved a versatile technique for the post-treatment of semiconductor materials as compared to other techniques due to its precise control over the resources. Thus, we carried out a set of experiments on SnS films to explore the influence of RTA treatment on their properties. From these experiments we noticed that the films treated at 400 °C for 1 min in N2 atmosphere have a low electrical resistivity of ~5 Ωcm with relatively high Hall mobility and carrier density of 99 cm2/Vs and 1.3 × 1016 cm−3, respectively. The observed results, therefore, emphasise that it is possible to obtain good quality SnS films through RTA treatment without disturbing their crystal structure.

Carrier Transport in Homo- and Heteroepitaxial Zinc Oxide Layers

AbstractHomo- and heteroepitaxial ZnMgO:Al, ZnCaO:Al and ZnO:Ga films have been grown on sapphire and ZnO substrates by RF (13.56 MHz) reactive magnetron sputtering from oxidic targets. The films grow epitaxially, i.e. with a preferred in-plane and out-of plane orientation. However, the heteroepitaxial films on sapphire exhibit a much higher crystallographic defect density, compared to the homepitaxial films. The ZnMeO films (Me – metal)on a-plane sapphire exhibit a lower defect density leading to higher Hall mobilities. Both, homo- and heteroepitaxial ZnO:Ga films with carrier concentrations N>1020 cm−3 exhibit the same mobility values, which increase with increasing carrier concentration. This behaviour is typical for electrical grain barrier limited transport, as decribed recently for polycrystalline ZnO:Al(Ga) films on glass. For the ZnCaO:Al films, deposited at similar conditions as the ZnO:Ga films, much lower carrier concentrations were measured, both for sapphire as well as for ZnO substrates. The mobilities of the ZnCaO:Al films on ZnO are much higher than that on the sapphire single crystals. The measured Hall mobilities are compared to single crystalline ZnO transport data.Additionally, the work functions of the ZnMeO layers have been measured by X-ray and ultra-violet photoelectron spectroscopy. As expected, the work functions are lower compared to unalloyed ZnO, which can be used for ZnO band gap and band alignment engineering.

Zinc Oxide Thin Films with Reduced Native Compensative Defects Grown by Ultrasonic Spray Pyrolysis at Atmosphere

ZnO films were deposited on Si (100) substrate by ultrasonic spray pyrolysis at atmosphere. The film grown at optimum conditions is well crystallized with uniform, smooth and dense microstructure. Photoluminescence measurement shows a strong near band edge UV emission at 379nm and an almost undetectable deep-level emission band centered at 502nm. The resistivity of ZnO film is reduced by an order after N-In codoping, which produces p-type conduction with high hole concentration and hall mobility.

Alternative deposition methods of copper and silver indium ternary sulphides with spinel structure

We compared properties of CuIn 5S 8 and AgIn 5S 8 spinel thin films deposited on soda lime glass and sapphire substrates by three methods: a) conventional sulphurisation of a metallic precursor (I), b) alternative evaporation of the binary sulphides with a post-annealing step in sulphur vapour (II), and c) with a post-annealing in decomposition products of bulk spinel in an evacuated ampoule (III). X-ray diffractometry reveals the polycrystalline nature of the films and demonstrates the presence of cubic spinel phases in the films prepared by all the methods. However, the morphology and the electrical transport properties of the spinel films are essentially different. A high Hall mobility of electrons of 10–50 cm 2/(V s) was determined in AgIn 5S 8 films prepared from binary sulphides.

BGaN micro-islands as novel buffers for growth of high-quality GaN on sapphire

We discuss the growth mechanism of GaN films and report very high two-dimensional electron gas (2DEG) mobility in AlGaN/AlN/GaN heterostructures fabricated on sapphire using BGaN micro-islands as novel buffers by metalorganic vapor phase epitaxy. The three-dimensional growth of BGaN (formation of BGaN micro-islands) occurs due to the phase separation of BGaN. However, the surface of the overgrown GaN on the BGaN micro-islands becomes smooth and continuous through the epitaxial lateral overgrowth process. The threading dislocations (TDs) in GaN consist mainly of pure edge-type ones and are effectively annihilated using single and double layers of BGaN micro-islands from 2×10 10 to 2×10 9 and 2×10 8 cm −2, respectively. An n-type GaN film shallowly doped with Si exhibits an electron concentration and high Hall mobility of 3.0×10 16 cm −3 and 669 cm 2/Vs at room temperature (RT). Very high Hall 2DEG mobility in an Al 0.10Ga 0.90N/AlN/GaN heterostructure is obtained: 1910 and 20,600 cm 2/Vs at RT and 77 K, respectively. The sheet carrier density had almost constant values of 6.9−5.7×10 12 cm −2 in the temperature range from 77 to 500 K, indicating that the parallel conduction due to the residual electrons in the GaN underlying layer was negligible.

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Abstract Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer. Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the ( 10 1 5 ) reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer. GaN layers, n -doped with silicon in the range from 1015 to 1019 cm−3 exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm2/V s at 300 K and 10 220 cm2/V s at 77 K for a sheet carrier density of 9.6E12 cm−2. The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrodinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.

Chemical vapor deposition of homoepitaxial diamond films

AbstractThis paper presents a review of state‐of‐the‐art reports of homoepitaxial diamond growth. Crystal growth and characterization methods that are commonly used for homoepitaxial diamond studies are first described briefly. Then, effects of each process parameter on the growth mode of homoepitaxial diamond are discussed. Methane concentration and substrate temperature were two dominant parameters that determine the growth rate and crystalline quality of diamond films, while conditions of diamond substrates including crystal orientation and pretreatments should be optimized for high‐quality diamond deposition. Attempts to increase growth rate of homoepitaxial diamond films while keeping their crystalline quality are discussed in the later section. Increase of microwave power density for plasma‐assisted chemical vapor deposition was found to be effective for the high‐rate growth of diamond. Free‐exciton recombination emission was intensively observed from homoepitaxial films grown at higher growth rate of 3.5 μm/h under the high power density condition. In addition, boron‐doped samples grown under the high‐power conditions showed high Hall mobility of 1620 cm2 V s−1 at room temperature. The plasma power density is thus one of the important parameters of diamond deposition from the viewpoint of electronic device application. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polycrystalline films of tungsten-doped indium oxide prepared by d.c. magnetron sputtering

Electrical and optical properties of polycrystalline films of W-doped indium oxide (IWO) were investigated. These films were deposited on glass substrate at 300 °C by d.c. magnetron sputtering using ceramic targets. The W-doping in the sputter-deposited indium oxide film effectively increased the carrier density and the mobility and decreased the resistivity. A minimum resistivity of 1.8 × 10 − 4 Ω cm was obtained at 3.3 at.% W-doping using the In 2O 3 ceramic targets containing 7.0 wt.% WO 3. The 2.2 at.% W-doped films obtained from the targets containing 5.0 wt.% WO 3, showed the high Hall mobility of 73 cm 2 V − 1 s − 1 and relatively low carrier density of 2.9 × 10 20 cm − 3 . Such properties resulted in novel characteristics of both low resistivity (3.0 × 10 − 4 Ω cm) and high transmittance in the near-infrared region.

Epitaxial GaN Film Grown at Low Temperature by Hydrogen-Plasma Assisted MOCVD

A comparative study of the structural and electrical properties of GaN films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD) at 700°C, with and without AlN buffer layer is presented . Hydrogen plasma was used in addition to nitrogen plasma to produce GaN. The introduction of H-plasma is found to influence the properties of the films. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall Effect measurements show that the sample with AlN buffer layer possesses a smoother and more homogenous morphological characteristics as well as a lower background electron and higher Hall mobility as compared to the sample without buffer layer. X-ray diffraction (XRD) reveals that hydrogenation is capable of producing the epitaxial GaN films at reduced temperatures with the full width at half maximum (FWHM) of the x-ray rocking (XRC) of GaN (0002) reflection was found to be 54.8 and 256 arcmin for samples with buffer layer and without buffer layer, respectively.

A Study of Defects and Surface Properties in AlGaN/GaN High Electron Mobility Transistors (HEMTs) Grown by MOCVD on Semi-Insulating SiC Substrates

ABSTRACTAlGaN/GaN HEMTs (High Electron Mobility Transistors) grown on semi-insulating (SI) SiC substrates are very promising for high power, high speed, and high temperature operation with great potential for both military and commercial applications. These high performance characteristics are possible due to presence of high two-dimensional electron gas (2 DEG) charge sheet density maintaining a high Hall mobility at the AlGaN barrier/GaN buffer hetero-interface. However, reliability of AlGaN HEMTs still remains a major concern because of the large number of defects and traps present both in the bulk as well as at the surface leading to current collapse. We report on the study of defects and surface properties in MOCVD-grown Al0.27Ga0.73N HEMT structures on SI SiC substrates. Our HEMT structures consist of a 25nm thick undoped AlGaN barrier layer and a 3μm thick undoped GaN buffer layer grown on a 100nm thick AlN nucleation layer. Hall measurements showed a charge sheet density of ∼1013/cm2 and a Hall mobility of ∼1500cm2/V·sec. Both cross-sectional and plan view TEMs were employed to study defects in the heterostructures and XPS (X-ray Photoelectron Spectroscopy) and AES (Auger Electron Spectroscopy) employed to study surface properties in both GaN and AlGaN layers. DC characterization results from AlGaN Schottky diodes with Pt/Au Schottky contacts are also reported along with results from AlGaN/GaN HEMT devices.

Electrical transport properties of GaAs structures with a pair of Be and donor impurity delta-doped layers

Electrical transport properties of GaAs structures with a pair of acceptor and donor impurity delta-doped layers are studied. Beryllium is used as an acceptor impurity, while Se or Si is used as a donor impurity. The Be and donor impurity delta-doped layers are separated from each other with a 1-nm-thick spacer layer, and Be and donor impurity concentrations are varied among samples. In the samples with the p-type conduction where the Be doping concentration is higher than the donor impurity doping concentration, a transition from thermally activated conduction to metallic conduction occurs at room temperature with the resistivity at the transition being close to the quantum unit of resistance 12h∕e2. In the samples with the n-type conduction where the donor impurity concentration is higher than the Be doping concentration, no such transition is observed. The different transport properties between two groups of samples indicate that heavy holes in the Be delta-doped layer behave as a quasi-two-dimensional system at room temperature, while electrons in the donor impurity delta-doped layer can no longer be considered as a quasi-two-dimensional system at high temperatures due to significant spreading of their wave functions in the direction normal to the delta-doped layer. The temperature dependence of resistivity of p-type pair delta-doped samples with high Hall mobility exhibits a broad peak in the low-temperature range.

Dependences of the Al thickness and annealing temperature on the structural, optical and electrical properties in ZnO/Al multilayers

High-quality (0001) oriented ZnO (300 Å) film and [ZnO(100 Å)/Al(tAl)]3 (tAl=0.6, 1.7, 2.8 Å) multilayers have been established at room temperature on Al2O3 (0001) substrates by ion-beam sputtering. The structural, optical and electrical properties of multilayers as functions of both the Al thickness and annealing temperature are reported. We have verified that Al thickness and annealing temperature are the key factors to optimize transparency-conducting property in ZnO/Al multilayers. The optimum Al thickness and annealing temperature for ZnO/Al multilayer of 300 Å thin is 1.7 Å (about one Al atomic layer) and 400 °C, respectively, leading to the relatively lower resistivity (2.8×10−3 Ω cm) and higher Hall mobility (10 cm2/V·s) without suppression of the visible transmittance (above 85%).

Fabrication and characteristics of transparent conducting In 2O 3–ZnO thin films by ultrasonic spray pyrolysis

Transparent conducting In 2O 3–ZnO thin films were prepared by ultrasonic spray pyrolysis technique. Indium nitrate trihydrate (In(NO 3) 3·3H 2O) and zinc acetate dihydrate (Zn(CH 3COO) 2·2H 2O) were used as precursors and a solvent was 2-methoxyethanol. The thin films, as a function of Zn/(Zn + In) atomic ratio (abbreviated to x), were annealed at 550 °C. Oxygen gas was used as both carrier and reactor gas. From analyzing X-ray diffraction patterns, In 2O 3 phase and ZnO phase were formed at x = 0.11 and 0.89, respectively, while homologous phases of In 2O 3–ZnO were observed between x = 0.5 and 0.67. The resistivity of the thin film increased until x = 0.33 and then decreased to be the lowest value (1.47 × 10 −2 Ω cm) at x = 0.5. In the range of x = 0.6–1, the resistivity increased again with x. The highest carrier concentration and the highest Hall mobility were 2.02 × 10 19 cm −3 at x = 0.6 and 15.89 cm 2/V s at x = 0.5, respectively. The optical transmittance in the visible region at x = 0.5 was 88–92%.

P-type doping by B ion implantation into diamond at elevated temperatures

We have studied B ion implantation at 400 °C into undoped homoepitaxial chemical vapor deposition diamond films and high-pressure and high-temperature (HPHT) synthetic IIa substrates. The highest Hall mobility at room temperature is 268 cm 2/Vs among B implanted homoepitaxial films, while it is 38 cm 2/Vs for the B implanted HPHT synthetic IIa substrate. The present result reveals that the quality of a doped layer is strongly dependent upon that of a diamond substrate employed for ion implantation.

The electrical and optical properties of molybdenum-doped indium oxide films grown at room temperature from metallic target

High performance molybdenum-doped indium oxide (IMO) films were deposited on slide glass substrates from metallic targets by using dc reactive magnetron sputtering at room temperature. The structural, electrical and optical properties have been investigated as functions of target composition and oxygen partial pressure. The deposited films were smooth and amorphous, as determined by scanning electron microscopy and x-ray diffraction, respectively. The results revealed that the as-deposited molybdenum-doped In2O3 films show good electrical property and high optical transmittance, as well as high infrared transmittance. The films prepared at oxygen partial pressure of 3.8 × 10−2 Pa and with 2 wt% Mo-doped target are characteristic of high Hall mobility of 20.2 cm2 V−1 s−1, carrier concentration of 5.2 × 1020 cm−3, and the average optical transmittance excess 90% in the visible region from 400 to 700 nm. Thus IMO films may be a potential material for novel optoelectrical devices such as an organic light-emitting diode.

Epitaxial growth of aluminium-doped zinc oxide films by magnetron sputtering on (001), (110), and (012) oriented sapphire substrates

Highly aluminium-doped zinc oxide (ZnO) films have been grown on differently oriented sapphire substrates by magnetron sputtering from an oxidic target. Rocking curve measurements, Rutherford backscattering analysis and transmission electron microscopy show that the films exhibit a disturbed film growth. However, despite the large nominal lattice mismatch between ZnO and sapphire (-31%), the films grow epitaxially on every sapphire orientation, even at room temperature. This was proven by pole figure analysis. The reason that epitaxial growth can be observed is an incommensurate lattice fitting between ZnO and sapphire by a mutual rotational alignment of their lattices. Films of the best crystallographic quality have been grown on (110)-oriented sapphire, which is also reflected by the highest Hall mobility in these layers.

Poly-crystalline Ge thin films prepared by RF sputtering method for thermo photo voltaic application

AbstractThe poly crystalline Ge films were prepared by the RF sputtering method with Ar and Ar-H2 mixture gases. The crystallization temperature increases from 400 °C to 500 °C due to the H2 introduction into the sputtering gases. On the other hand, the H2 introduction decreases the absorption coefficients in the long wavelength region corresponding to lower energy values than the energy gap of Ge, although much higher absorption coefficients are observed in the case of the Ar sputtering. Probably, the gap state density decreases due to the hydrogen termination of dangling-bonds in the grain-boundaries. The (220) preferential orientation is stressed and the highest Hall mobility is obtained with 75% of the gas flow ratio [Ar/ (Ar+H2)]. The preferential growth probably causes higher quality Ge films.