Thick AlN Research Articles

Superconductor–insulator transition in two-dimensional NbN/MgO and NbN/AlN/MgO films

In this study, we investigate the dependence of superconducting transition temperature (Tc) on sheet resistance (Rsq) for two series of NbN films with different homogeneities which were prepared on either MgO substrates (NbN/MgO) or MgO substrate covered with a 1nm thick AlN layer (NbN/AlN/MgO). The Rsq-dependent Tc (Tc(Rsq)) behavior of the homogeneous NbN/MgO films could be explained with Finkel’stein’s formula, which is based on localization theory using reasonable fitting parameters. On the other hand, the Tc(Rsq) values of the NbN/AlN/MgO films were larger than that of the NbN/MgO films in the region To understand the origin of this deviation from the theory for homogeneous systems, we assumed that the NbN/AlN/MgO films have the potential to form an inhomogeneous percolation structure. To confirm this assumption, the temperature (T) dependence of the upper critical magnetic field ( near Tc was analyzed using the form which is based on classical percolation theory. From the dependence of Rsq ((Rsq)) of both series, it was found that the (Rsq) of the NbN/AlN/MgO films deviates in the region from in a homogeneous NbN/MgO films. By introducing an effective sheet resistance to account for the inhomogeneous structure of the NbN/AlN/MgO films, we were able to qualitatively explain the differences between the Tc(Rsq) behaviors of the two series.

Electrical properties of GaAs metal–oxide–semiconductor structure comprising Al2O3 gate oxide and AlN passivation layer fabricated in situ using a metal–organic vapor deposition/atomic layer deposition hybrid system

This paper presents a compressive study on the fabrication and optimization of GaAs metal–oxide–semiconductor (MOS) structures comprising a Al2O3 gate oxide, deposited via atomic layer deposition (ALD), with an AlN interfacial passivation layer prepared in situ via metal–organic chemical vapor deposition (MOCVD). The established protocol afforded self-limiting growth of Al2O3 in the atmospheric MOCVD reactor. Consequently, this enabled successive growth of MOCVD-formed AlN and ALD-formed Al2O3 layers on the GaAs substrate. The effects of AlN thickness, post-deposition anneal (PDA) conditions, and crystal orientation of the GaAs substrate on the electrical properties of the resulting MOS capacitors were investigated. Thin AlN passivation layers afforded incorporation of optimum amounts of nitrogen, leading to good capacitance–voltage (C–V) characteristics with reduced frequency dispersion. In contrast, excessively thick AlN passivation layers degraded the interface, thereby increasing the interfacial density of states (Dit) near the midgap and reducing the conduction band offset. To further improve the interface with the thin AlN passivation layers, the PDA conditions were optimized. Using wet nitrogen at 600 °C was effective to reduce Dit to below 2 × 1012 cm−2 eV−1. Using a (111)A substrate was also effective in reducing the frequency dispersion of accumulation capacitance, thus suggesting the suppression of traps in GaAs located near the dielectric/GaAs interface. The current findings suggest that using an atmosphere ALD process with in situ AlN passivation using the current MOCVD system could be an efficient solution to improving GaAs MOS interfaces.

Thick AlN epilayer grown by using the HVPE method

A thick AlN epilayer is grown directly on a c-plane sapphire substrate by using the hydride vapor phase epitaxy (HVPE) method with a small quantity of Al. The new type (RF + hot-wall) flow HVPE reactor used in the AlN epilayer growth is custommade. The growth temperatures of the source and the growth zones are set at 950 °C and 1145 °C, respectively. The characteristics of the AlN layer grown on a sapphire substrate are investigated by using energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. The value of the full width at half maximum (FWHM) for the (002) peak from the AlN layer on the sapphire substrate is observed at 790 arcsec. From the small value of the FWHM, the AlN layer seems to be a well-arranged plane with a uniform (002) growth direction on a sapphire substrate.

Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface

The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200°C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240°C, the crystalline quality of the 105nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276arcsec to 149 and 194arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10−8m2K/W, measured at an elevated temperature of 160°C, in a GaN-on-SiC transistor structure was achieved.

Ultrathin barrier AlN/GaN high electron mobility transistors grown at a dramatically reduced growth temperature by pulsed metal organic chemical vapor deposition

Ultrathin-barrier AlN/GaN heterostructures were grown on sapphire substrates by pulsed metal organic chemical vapor deposition (PMOCVD) using indium as a surfactant at a dramatically reduced growth temperature of 830 °C. Upon optimization of growth parameters, an electron mobility of 1398 cm2/V s together with a two-dimensional-electron-gas density of 1.3 × 1013 cm−2 was obtained for a 4 nm thick AlN barrier. The grown structures featured well-ordered parallel atomic steps with a root-mean-square roughness of 0.15 nm in a 5 × 5 μm2 area revealed by atomic-force-microscopic image. Finally, the potential of such structures for device application was demonstrated by fabricating and testing under dc operation AlN/GaN high-electron-mobility transistors. These results indicate that this low temperature PMOCVD growth technique is promising for the fabrication of GaN-based electronic devices.

Influence of AlN buffer layer thickness on material and electrical properties of InAlN/GaN high-electron-mobility transistors

The influence of the thickness of a high-temperature AlN (HT-AlN) buffer layer on the properties of an InAlN/GaN high-electron-mobility transistor (HEMT) grown on a sapphire substrate was investigated. As revealed by atomic force microscope analysis, a rougher surface and larger grain size were observed with a thicker buffer layer. The larger grains promoted the two-dimensional (2D) growth mode of the GaN layer at the initial growth stage. This suppressed oxygen incorporation at the GaN/HT-AlN interface and thus improved the resistivity of the GaN layer. Moreover, the lower grain density also resulted in enhanced GaN crystal quality of the GaN layer. As a consequence, the electrical properties of the InAlN/GaN HEMT device, such as output current, transconductance and off-state breakdown voltage, were improved by increasing the HT-AlN buffer layer thickness.

Fabrication of vertical Schottky barrier diodes on n-type freestanding AlN substrates grown by hydride vapor phase epitaxy

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ∼8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.

Unintentional gallium incorporation in AlN and its impact on the electrical properties of GaN/AlN and GaN/AlN/AlGaN heterostructures

Thin AlN interlayers are widely used in (In,Al,Ga)N based high-electron-mobility transistors to improve the mobility of the two-dimensional electron gas forming at the GaN/(In,Al,Ga)N interface. AlN layers grown by metal-organic chemical vapor deposition, however, were recently shown to contain high amounts of gallium caused by carry over reactions, resulting in AlxGa1−xN layers with x ∼ 0.5 under typical deposition conditions. By modifying the AlN growth conditions in this study, layers with an Al mole fraction up to 0.78 were obtained. The unintentional Ga incorporation had a negligible effect on the electronic properties of GaN/AlN/AlGaN structures with nominally 0.7 nm thick AlN interlayer and sheet carrier densities in the order of 1 × 1013 cm−3. It resulted, however, in low electron mobility values for samples with thicker nominal AlN layers and/or higher sheet carrier densities.

The influences of AlN/GaN superlattices buffer on the characteristics of AlGaN/GaN-on-Si (1 1 1) template

The influence of AlN/GaN superlattices (SL) buffer on the characteristics of AlGaN/GaN-on-Si (111) template was studied in detail. There existed an optimized Relative AlN Thickness (RAT) in the superlattices buffer which can not only further filtering the edge- and screw-type dislocations to the upper epilayer and lead to a good crystal quality with narrowest (0002) and (10−12) full width of half maximum (FWHMs), 439″ and 843″, but also improve the surface roughness to enhance the Two dimensional electron gas (2DEG) mobility and superior electrical properties were achieved. Moreover, an optimized RAT in SL can induce a proper compressive stress to the subsequently grown GaN epilayer and protect it from crack during the cooling step, which can also lead to a better wafer bending.

Surface polariton spectroscopy of AlN films grown by ammonia MBE on (0001) Al2O3 substrate

AbstractSurface polariton (SP) spectra of nitridated sapphire and rather thick (1 µm) AlN films on sapphire have been measured using attenuated total reflection technique. The high sensitivity of SP allows to see the sapphire spectra change after nitridation of sapphire and even variations parameters of the nitridation procedure. SP dispersion curves depend on degree of conversion of an initial sapphire surface layer into crystalline AlN during the nitridation procedure. The TO frequencies of formed AlN films are higher than the frequency reported for the AlN single crystal indicating the increase of the lattice constant. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of inserted AlN layer on the two-dimensional electron gas in AlxGa1-xN/AlN/GaN

This paper investigates the changes of electron transport properties in AlxGa1-xN/GaN with an inserted AlN layer. The polarization charge density and two-dimensional electron gas (2DEG) sheet density in AlxGa1-xN/AlN/GaN double heterojunction high electron mobility transistors (HEMT) affected by the spontaneous polarization and piezoelectric polarization in AlxGa1-xN and AlN barrier are studied. Relations of interface roughness scattering and alloy disorder scattering with the AlN thickness are systematically analyzed. It is found that the alloy disorder scattering is the main scattering mechanism in AlxGa1-xN/GaN heterojunction high-electron-mobility transistors, while the interface roughness scattering is the main scattering mechanism in AlxGa1-xN/AlN/GaN double-heterojunction structure. It is also known that the 2DEG sheet density, interface roughness scattering and alloy disorder scattering are depended on the thickness of the inserted AlN layer. The 2DEG sheet density increases slightly and the mobility increases obviously by inserting an AlN layer about 13 nm. Taking Al mole fraction of 0.3 as an example, if without AlN layer, the 2DEG sheet density is 1.47 1013 cm-2 with the mobility limited by the interface roughness scattering of 1.15 104 cm2V-1-1, and the mobility limited by alloy disorder scattering of 6.07 102cm2V-1-1. After inserting an AlN layer of 1 nm, the 2DEG sheet density increases to 1.66 1013cm-2, and the mobility limited by the interface roughness scattering reduces to 7.88 103cm2V-1-1 while the mobility limited by alloy disorder scattering increases greatly up to 1.42 108 cm2V-1-1.

A study of 2DEG properties in AlGaN/GaN heterostructure using GaN/AlN superlattice as barrier layers grown by MOCVD

GaN/AlN superlattice (SL) structures working as quasi-AlGaN barrier layers for GaN-based high electron mobility transistor have been grown by metal–organic chemical vapor deposition. The influences of the SL period thickness on the electrical properties of two-dimensional electron gas (2DEG) have been investigated. It is found that the sheet carrier concentration increases as the increase in period thickness at a certain equivalent Al composition, and the electron mobility is strongly dependent on the AlN thickness in a period. We consider that AlN transits from two-dimensional growth to three-dimensional (3D) growth when AlN thickness exceeds its critical thickness. The 3D growth mode results in rough interface and surface morphology, which rapidly decreases the electron mobility due to the increase in interface roughness scattering and dislocation scattering to 2DEG.

Effect of AlN/GaN superlattice buffer on the strain state in GaN-on-Si(111) system

The effect of AlN/GaN superlattice (SL) buffer on the strain state in a GaN-on-Si(111) system was studied in detail by room-temperature micro-Raman scattering measurement. An abnormal satellite peak attached to a GaN E2 peak was observed, which was verified to stem from the compressively strained GaN in SLs. The results indicate that the strain-sensitive GaN E2 (high) peak in the GaN-on-Si system with AlN/GaN SLs splits into two peaks because the GaN stress state in the top GaN layer is different from that in SLs. The compressive stress in the GaN layer in SLs was introduced by the AlN layer in each SL period because of the lattice mismatch between GaN and AlN, which ultimately counterbalanced the tensile stress in the top GaN during cooling. Such a counterbalance interaction is strongly dependent on the stiffness coefficient of AlN/GaN SLs, which is proportional to the number of periods of SLs and the relative thickness of AlN in SLs. Such two E2 peaks from GaN enable us to monitor the strain state in the GaN-on-Si system with AlN/GaN SLs quantitatively.

Experimental Investigation on Thermoresistance between AlN, Bi-2223 and OFHC in High Tc- Direct Cooling Technology

In the development of high temperature superconducting (HTS) direct cooling technology, the high electric insulation high heat conducting AlN has become one of the important components. The thermal contact resistance between AlN, Bi-2223 and OFHC is investigated by experiment with a G-M cryocooler as the source of cooling. The heat conductivity of AlN is measured between 29 and 160 K temperatures. When the temperature on the interface layer side of Bi-2223 is 55 K, under the action of the contact pressure of 0.5469 MPa, the thermal contact resistance between AlN and Bi-2223 is 38.86 times to the thermal conduction resistance of a 10 mm thick AlN pad. Baced on micro-nanocryogenics, it is proposed that the thermal contact resistance is one of the crucial techniques to be attacked in HTS direct cooling technology.

Investigation, characterization and effect of substrate position on thick AlN layers grown by high temperature chemical vapor deposition

Thick AlN layers are grown on c-sapphire at 1,300 °C in turbulent, laminar and 45° oblique laminar injection respectively by a homebuilt hot-wall high temperature chemical vapor deposition system (HTCVD) and characterized by SEM, XRD, Raman and optical absorption spectra. The results demonstrate that high quality and 20 μm thick (002) AlN epilayer with full width at half maximum of 1,010 arcsec is grown in 45° oblique laminar injection at growth rate of 24 μm/h, while 28 μm thick (002) AlN epilayer and 46 μm thick (002)-oriented columnar AlN layer with rough surface and larger misorientation are grown in laminar and turbulent injection at growth rate of 34 and 55 μm/h respectively. Meanwhile, the epilayer grown in 45° oblique laminar injection exhibits sharper absorption edge and larger optical band gap of 5.93 eV than that of other two layers. Furthermore, the proposed model suggests that column growth mode governs the growth in turbulent injection with high source concentrations, resulting in (002)-oriented columnar layer with large misorientation. In contrast, Stranski–Krastanov mode governs the growth in laminar injection and 45° oblique laminar injection with decreased source concentrations, leading to nonuniform and uniform (002) epilayer, respectively. Therefore, the HTCVD with 45° oblique laminar injection can be used to grow high quality thick (002) AlN epilayer at high growth rate and low cost.

Microscopic crystalline structure of a thick AlN film grown on a trench-patterned AlN/α-Al2O3 template

The microscopic crystalline structure (MCS) such as domain texturing, lattice tilting fluctuation and strain fluctuation in thick AlN films grown on trench-patterned AlN/α-Al2O3 templates was clarified by utilizing position-dependent X-ray microdiffraction measurements in combination with transmission electron microscopy observations. The results clearly demonstrate that the trench-patterned templates have a strong influence on the MCS in the thick AlN films. The MCS is anisotropic between the [112¯0] and [11¯00] directions. Periodic domain texturing was observed along the [112¯0] direction, corresponding to the periodicity of the patterning pitch of the template. Submicron crystal domains are tilted at different angles to the [112¯0] direction, and the crystal domain tilting becomes larger in the void-containing trench regions than in the terrace regions. This generates the periodicity in lattice tilting fluctuation and strain fluctuation along the [112¯0] direction. These were found to be caused primarily by two factors; one of which being the elastic strain relaxation that depends on the growth direction in the terrace regions, and the other being the inhomogeneity in distribution and morphology of dislocations that have Burgers vectors with a screw component in the void-containing trench regions.

Single phase (112¯2) AlN grown on (101¯0) sapphire by metalorganic vapour phase epitaxy

Heteroepitaxial growth of AlN buffer layers directly on (101¯0) sapphire substrates by metalorganic vapour phase epitaxy has been investigated. A single-step growth procedure without a sapphire nitridation was employed resulting in mirror-like crack free ≈1.1–1.6μm thick AlN layers of single phase (112¯2) orientation. Trimethylaluminum pre-dose time and reactor pressure were optimized for surface roughness and crystal quality. The crystal quality was found to degrade with increasing pre-dose time and also reactor pressure. The smallest full width at half maximum value for on-axis X-ray rocking curve of the (112¯2) AlN layers was about 610arcsec and 1480arcsec along [1¯1¯23]AlN and [11¯00]AlN, respectively. The surface roughness, measured by atomic force microscopy using a 10×10μm2 area, was in the range 2.6–3.5nm. A basal stacking fault density of (7±1)×105cm−1 was estimated by transmission electron microscopy.

Structural and electronic properties of SiC/AlN core/shell nanowires: a first-principles study

The structural stabilities and electronic properties of passivated and unpassivated SiC / AlN core/shell nanowires (CSNWs) along [0001] direction are investigated by using first-principles calculations with density functional theory. Our calculations demonstrate that thick AlN shell and small ratio of SiC core make the SiC / AlN CSNWs more stable. The band gaps decrease with the increasing of the CSNWs diameters. After passivation at the surface, type of SiC / AlN heterostructure changes and the mobility can be improved by increasing the CSNWs diameter and the SiC core ratio. These results provide an effective way to modulate the electronic properties of SiC / AlN structure, which is useful for fabrications and applications of CSNWs.

Growth of AlInN/GaN distributed Bragg reflectors with improved interface quality

We report on lattice-matched AlInN/GaN distributed Bragg reflectors (DBRs) with improved overgrowth of the AlInN layers. Typically, AlInN layers are exposed to high temperatures when changing to GaN growth conditions. Uncapped AlInN surfaces suffer from In desorption leading to formation of 2nm thick AlN interface layers with micro-cracks at the AlInN surface. Capping the AlInN with 5nm GaN at the same temperature and subsequent overgrowth with GaN at high temperatures resolves the In-desorption problem and DBRs with improved interface quality and smooth surfaces are achieved. Optical properties of high-reflectivity DBR structures such as maximum reflectivity and bandwidth are virtually unaffected whether or not unintentionally-grown AlN interlayers are present.

Impact of barrier thickness on transistor performance in AlN/GaN high electron mobility transistors grown on free-standing GaN substrates

A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.