Effect of repetition nanoindentation of GaN epilayers on a‐axis sapphire substrates

In this work, gallium nitride (GaN) epilayers were deposited on a‐axis sapphire substrate by means of metal organic chemical vapor deposition (MOCVD). Berkovich nanoindentation was used to explore the repetition pressure‐induced impairment of the GaN film. The observation of load‐displacement vs stress‐strain curves concludes that basal slip is implicated in the deformation on the A plane GaN. The increase in the hardness (H) and elastic modulus (E) was determined from cyclic nanoindentation, and resulted in a crack due to the formation of incipient slip bands and/or the to‐and‐fro motion of mobile dislocation. It is indicated that the generation of individual dislocation and residual deformation of the GaN films are showed by CL mapping analysis. From the morphological studies, it is revealed that the crack was found by means of atomic force microscope (AFM) technique at nine loading/reloading cycles even after the indentation beyond the critical depth on the residual indentation impression. Copyright © 2010 John Wiley & Sons, Ltd.     

Depth profiling of light elements in PAMBE‐grown GaN and helium‐implanted titanium with heavy ion time‐of‐flight elastic recoil detection

The heavy ion time‐of‐flight elastic recoil detection analysis (HI‐ERDA) technique was used to investigate the possibility of measuring near‐surface elemental depth profiles of light and mid‐Z elements in thin films of plasma‐assisted molecular beam epitaxy (PAMBE)‐grown GaN and helium‐implanted titanium. The great advantage of HI‐ERDA is the ability to measure mass‐separated elemental depth profiles simultaneously. However for some materials it is not certain whether HI‐ERDA can be used successfully because significant sputtering or other beam‐induced damage may occur. The damage to the surfaces by a 77 MeV iodine beam was assessed using RBS, AFM and profilometry. The results show that for thin PAMBE‐grown polycrystalline GaN films and for titanium that has been heavily implanted with helium a significant modification of the near‐surface region is caused by the probing heavy ion beam. For the PAMBE‐grown GaN films the most significant loss trend is observed for nitrogen. Surprisingly this was not accompanied by a change in surface topology. In contrast, an almost complete removal of the heavily helium‐implanted surface layer was measured for the titanium specimens. The investigation shows that reference measurements with additional techniques such as RBS, AFM and profilometry have to be performed to ascertain sample integrity before HI‐ERDA data can be used. Copyright © 2004 John Wiley & Sons, Ltd.     

Surface‐Polarity‐Induced Spatial Charge Separation Boosts Photocatalytic Overall Water Splitting on GaN Nanorod Arrays

Photocatalytic overall water splitting has been recognized as a promising approach to convert solar energy into hydrogen. However, most of the photocatalysts suffer from low efficiencies mainly because of poor charge separation. Herein, taking a model semiconductor gallium nitride (GaN) as an example, we uncovered that photogenerated electrons and holes can be spatially separated to the nonpolar and polar surfaces of GaN nanorod arrays, which is presumably ascribed to the different surface band bending induced by the surface polarity. The photogenerated charge separation efficiency of GaN can be enhanced significantly from about 8 % to more than 80 % via co‐exposing polar and nonpolar surfaces. Furthermore, spatially assembling reduction and oxidation cocatalysts on the nonpolar and polar surfaces remarkably boosts photocatalytic overall water splitting, with the quantum efficiency increased from 0.9 % for the film photocatalyst to 6.9 % for the nanorod arrays photocatalyst.     

氮化镓粉末的溶胶凝胶法制备及其结构

报道了一种新颖而有效的二步制备氮化镓粉末的方法. 以乙氧基镓Ga(OC2H5)3作前驱体, 利用溶胶-凝胶法和高温氨化法相结合, 在950 ℃氨化温度下, 将凝胶与流动的NH3反应20 min, 合成了GaN粉末. XRD、FTIR、TEM及SAED的测量结果表明, GaN粉末是六方纤锌矿结构的单晶晶粒, 粉末粒度较均匀, FTIR吸收谱有明显的宽化现象.     

Etching characteristics of GaN by plasma chemical vaporization machining

A high‐quality bulk gallium nitride (GaN) substrate, which is suitable for high‐quality homoepitaxial growth, is indispensable for realizing high‐performance GaN devices. With improvement in the quality of the bulk GaN substrate, the removal of subsurface damage induced during surface polishing has become increasingly necessary. To remove the subsurface damage from the bulk GaN substrate, a chemical finishing method that does not produce further damage is required. We applied plasma chemical vaporization machining (CVM) to remove the subsurface damage from the bulk GaN substrate. In this study, we investigated the etching characteristics of GaN by plasma CVM applying atmospheric pressure Cl₂/He plasma. The maximum removal rate in the depth direction by plasma CVM was 9100 nm/min, which is seven times greater than that of reactive ion etching (RIE). The activation energy in plasma CVM was estimated to be 2.1 kcal/mol, which is 1.75 times greater than that in RIE. It is supposed that some of the energy required for the removal reaction in RIE is supplied by ion bombardment, but plasma CVM depends on only a chemical reaction without high‐energy ion collision. This result suggests that plasma CVM is a finishing method that causes less subsurface damage than RIE. Copyright © 2008 John Wiley & Sons, Ltd.     

Charge Separation in Wurtzite/Zinc‐Blende Heterojunction GaN Nanowires

The electronic properties of wurtzite/zinc‐blende (WZ/ZB) heterojunction GaN are investigated using first‐principles methods. A small component of ZB stacking formed along the growth direction in the WZ GaN nanowires does not show a significant effect on the electronic property, whereas a charge separation of electrons and holes occurs along the directions perpendicular to the growth direction in the ZB stacking. The later case provides an efficient way to separate the charge through controlling crystal structure. These results have significant implications for most state of the art excitonic solar cells and the tuning region in tunable laser diodes.     

GaN纳米棒的催化合成及其发光特性

使用稀土元素Tb作催化剂, 通过氨化溅射在Si(111)衬底上的Ga2O3/Tb薄膜, 成功制备出GaN纳米棒. X射线衍射测试显示, GaN纳米棒具有六方结构. 利用扫描电子显微镜和高分辨透射电子显微镜观察分析得出, 纳米棒为单晶GaN, 纳米棒的直径为50-150 nm, 长度约10 μm. 光致发光谱在368.6 nm处有一强的紫外发光峰, 说明纳米棒具有良好的发光特性. 讨论了GaN纳米棒的生长机制.     

Structural,electronic, and magnetic properties of 3d transition metal doped GaN nanosheet: A first‐principles study

We have performed the first‐principles calculations on the structural, electronic, and magnetic properties of 3d transition‐metal? (Cr, Mn, Fe, Co, and Ni) atoms doped 2D GaN nanosheet. The results show that 3d TM atom substituting one Ga leads to a structural reconstruction around the 3d TM impurity compared to the pristine GaN nanosheet. The doping of TM atom can induce magnetic moments, which are mainly located on the 3d TM atom and its nearest‐neighbor N atoms. It is found that Mn‐ and Ni‐doped GaN nanosheet with 100% spin polarization characters seem to be good candidates for spintronic applications. When two Ga atoms are substituted by two TM dopants, the ferromagnetic (FM) ordering becomes energetically more favorable for Cr‐, Mn‐, and Ni‐doped GaN nanosheet with different distances of two TM atoms. On the contrary, the antiferromagnetic (AFM) ordering is energetically more favorable for Fe‐doped GaN nanosheet. In addition, our GGA + U calculations show the similar results with GGA calculations. © 2016 Wiley Periodicals, Inc.     

Direct Protein Detection in Solutions of High Ionic Strength using Polyethylene Glycol-modified AlGaN/GaN High Electron Mobility Transistors

Herein, we proposed a simple 3-(aminopropyl) triethoxysilane/polyethylene glycol (APTES/PEG)-AlGaN/GaN high electron mobility transistors (HEMT) biosensor that enabled direct detection in high ionic strength solutions. Unlike previous modification strategies, the PEG-modified on the GaN surface established a stable screening system by forming a porous biopolymer layer, which can overcome Debye shielding. In our work, the APTES/PEG modification enabled immunoglobulin G (IgG) to be easily detected in solutions with concentrations of up to 30 mM (3x) of phosphate buffered saline (PBS), indicating that the APTES/PEG modification strategy had the potential for direct detection of biomolecules in physiological solutions, which provided a new method for bio-surface modification on GaN and advanced the practical application of AlGaN/GaN HEMT biosensors.     

Potential-dependent recombination kinetics of photogenerated electrons in n- and p-type GaN photoelectrodes studied by time-resolved IR absorption spectroscopy

Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H(2) and O(2) evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20 μs and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photoelectrochemical systems.     

Theoretical study of gallium nitride molecules, GaN2 and GaN4

The electronic and geometric structures of gallium dinitride GaN 2, and gallium tetranitride molecules, GaN 4, were systematically studied by employing density functional theory and perturbation theory (MP2, MP4) in conjunction with the aug-cc-pVTZ basis set. In addition, for the ground-state of GaN 4( (2)B 1) a density functional theory study was carried out combining different functionals with different basis sets. A total of 7 minima have been identified for GaN 2, while 37 structures were identified for GaN 4 corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces and bonding mechanisms for some low-lying electronic states of GaN 4. The dissociation energy of the ground-state GaN 2 ( X (2)Pi) is 1.1 kcal/mol with respect to Ga( (2)P) + N 2( X (1)Sigma g (+)). The ground-state and the first two excited minima of GaN 4 are of (2)B 1( C 2 v ), (2)A 1( C 2 v , five member ring), and (4)Sigma g (-)( D infinityh ) symmetry, respectively. The dissociation energy ( D e) of the ground-state of GaN 4, X (2)B 1, with respect to Ga( (2)P) + 2 N 2( X (1)Sigma g (+)), is 2.4 kcal/mol, whereas the D e of (4)Sigma g (-) with respect to Ga( (4)P) + 2 N 2( X (1)Sigma g (+)) is 17.6 kcal/mol.     

Interface analysis of Ti/Al/Ti/Au ohmic contacts with regrown n+‐GaN layers using molecular beam epitaxy

The regrowth technique of highly doped n‐type GaN layers is reliable and effective for lowering the ohmic contact resistance. The interface between metal contacts with Ti/Al/Ti/Au and regrown n⁺‐GaN/GaN layers were analyzed in detail with transmission electron microscopy. During the annealing process, Ti metals and N atoms diffusing from GaN layers formed TiN epitaxial layers between metal alloys and n⁺‐GaN layers. The orientational relationship between GaN and TiN was [1 0 0]_GaN//[?1 1 0]_TiN verified by nano‐beam diffraction. Al atoms diffused through the GaN layers and formed thin AlGaN phase. Al content was confirmed as 60% by high‐resolution transmission electron microscopy images. Electron energy loss spectroscopy showed that Si dopants were confined within n⁺‐GaN layers. These results show that in regrowth technique both TiN layers and Si dopants affect the contact properties because the formation of TiN layers can induce nitrogen vacancies from GaN, while Si‐doped GaN layers can enhance the tunneling effect through the metal contacts resulting in reduced contact resistance. Copyright © 2011 John Wiley & Sons, Ltd.     

Evolution of the local structure in GaN:O thin films grown by ion‐assisted deposition with film thickness

Optical and optoelectronic properties of gallium nitride strongly depend on the synthesis procedure, which may be related to specific structural characteristics of GaN inherent to each preparation condition. Amorphous and nanocrystalline GaN films have been prepared by ion‐assisted deposition (IAD). The films prepared at 10^?5 Torr for <50 min have shown exploitable optoelectronic properties, in spite of the high concentration of oxygen of these films (up to 25 at.%). We study here the evolution of the local structure around Ga atoms as the deposition time increases. Five IAD GaN films of thickness ranging between 140 and 450 nm on silicon substrates were analysed by x‐ray absorption fine structure (XAFS) at the Ga K‐edge. The first and second shells of neighbouring atoms are clearly identified in the radial distribution functions at approximately 1.9 and 3.2 Å, respectively. In all of the films, Ga seems to be tetrahedrally coordinated to four nitrogen atoms, some of which may be substituted by oxygen. For deposition times <50 min, analysis of both x‐ray adsorption near‐edge structure (XANES) and extended x‐ray adsorption fine structure (EXAFS) regions indicates that the material is highly amorphous. Above this threshold, a peak corresponding to the first coordination sphere of Ga atoms becomes discernible and increases in intensity for longer deposition times, indicating that the second shell of atoms is now more ordered. The pseudo Debye–Waller factor of the Ga shell is used for monitoring the average degree of amorphization in an ～100 nm thick top layer, which seems to be related to the film oxygen content. The XAFS results are compatible with a layered distribution of crystallinity, as has been suggested previously for these films. Copyright © 2005 John Wiley & Sons, Ltd.     

Self-assembled monolayers of alkylphosphonic acid on GaN substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.     

Mg掺杂GaN纳米线的结构及其特性

利用类似Delta掺杂技术在硅衬底上沉积Mg:Ga2O3薄膜, 然后在850 ℃下对薄膜进行氨化, 反应后制备出大量Mg掺杂GaN纳米线. 采用扫描电子显微镜(SEM)、X射线衍射(XRD)、傅里叶变换红外(FTIR)光谱和高分辨透射电子显微镜(HRTEM)对样品进行分析.结果表明, Mg掺杂GaN纳米线具有六方纤锌矿单晶结构, 纳米线的直径在30-50 nm范围内, 长度为几十微米.     

Electrical,structural and morphological characteristics of rapidly annealed Ni/Pd Schottky rectifiers on n‐type GaN

Schottky rectifiers are fabricated on n‐type GaN using Ni/Pd metallization scheme and its characteristics have been investigated by current‐voltage (I‐V), Capacitance‐Voltage (C‐V), X‐Ray Diffraction (XRD) and SIMS measurements as a function of annealing temperature. The calculated Schottky barrier height of the as‐deposited contact was found to be 0.60 eV (I‐V), 0.71 eV (C‐V) with an ideality factor of 1.44. However, the barrier height slightly increases after annealing at 300, 400 and 500 °C. On the basis of the experimental results, a high‐quality Schottky contact with barrier height and ideality factor of 0.81 eV (I‐V), 0.88 eV (C‐V) and 1.13 respectively, can be obtained after annealing at 600 °C for 1 min in a nitrogen atmosphere. Further, after annealing at 700 °C, it is found that the barrier height slightly decreased to 0.74 eV (I‐V) and 0.85 eV (C‐V). From the above observations, one can note that Ni/Pd Schottky contact exhibits excellent electrical properties after a rapid thermal annealing at 600 °C. According to the SIMS and XRD analysis, the formation of gallide phases at the Ni/Pd/n‐GaN interface could be the reason of the barrier height increase at elevated annealing temperatures. The Atomic Force Microscopy (AFM) results show that the overall surface morphology of Ni/Pd Schottky contacts on n‐GaN is fairly smooth. The above observations reveal that Ni/Pd Schottky metallization scheme was a good choice for the fabrication of high‐temperature and high‐power device applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Interface formation of Al2O3 on n‐GaN(0001): Photoelectron spectroscopy studies

Al₂O₃ insulator layers were deposited step by step by the physical vapor deposition (PVD) method onto gallium nitride in the wurtzite form, n‐type and (0001)‐oriented. The substrate surface and the early stages of Al₂O₃/n‐GaN(0001) interface formation were characterized in situ under ultra‐high vacuum conditions by X‐ray and ultraviolet photoelectron spectroscopy (XPS, UPS). The electron affinity (EA) of the substrate cleaned by annealing was 3.6 eV. Binding energies of the Al 2p (76.0 eV) and the O 1s (532.9 eV) confirmed the creation of the Al₂O₃ compound in the deposited film for which the EA was 1.6 eV. The Al₂O₃ film was found to be amorphous with a bandgap of 6.9 eV determined from the O 1s loss feature. As a result, the calculated Al₂O₃/n‐GaN(0001) valence band offset (VBO) is ?1.3 eV and the corresponding conduction band offset (CBO) 2.2 eV.     

Excellent field-emission properties of P-doped GaN nanowires

GaN nanowires with P doping were synthesized via a simple thermal evaporation process. The P-doped GaN nanowires have average diameters of approximately 100 nm and lengths up to tens of micrometers. Scanning electron microscope and high-resolution field-emission transmission electron microscope analyses revealed that P doping results in a rough surface morphology of GaN nanowires. Field-emission measurements showed that P doping effectively decreases the turn-on field of GaN nanowire to 5.1 V/mum, holding promise of application as an electron emitter. The rough surface is responsible for enhancement of the field-emission properties of GaN nanowires.     

利用Pd催化合成单晶GaN纳米线的光学特性(英文)

基于金属元素钯具有的催化特性,采用射频磁控溅射方法,在Si(111)衬底上沉积Pd:Ga2O3薄膜,然后在950℃下对薄膜进行氨化,制备出大量GaN纳米线.采用扫描电子显微镜(SEM)、X射线衍射(XRD)、透射电子显微镜(TEM)和高分辨透射电子显微镜(HRTEM)等技术手段对样品的结构、形貌和成分进行分析.结果表明,制备的样品为具有六方纤锌矿结构的单晶GaN纳米线,直径在20-60nm范围内,长度为几十微米,表面光滑无杂质,结晶质量较高.用光致发光光谱对样品的发光特性进行测试,分别在361.1、388.6和426.3nm处出现三个发光峰,且与GaN体材料相比近带边紫外发光峰发生了较弱的蓝移.对GaN纳米线的生长机制也进行了简单的讨论.     

Wearable Multiparameter Platform Based on AlGaN/GaN High‐electron‐mobility Transistors for Real‐time Monitoring of pH and Potassium Ions in Sweat

Biosensors based on field‐effect transistor (FET) structures have attracted considerable attention because they offer rapid, inexpensive parallel sensing and ultrasensitive label‐free detection. However, long‐term repeatable detection cannot be performed, and Ag/AgCl reference electrode design is complicated, which has hindered FET biosensors from becoming truly wearable health‐monitoring platforms. In this paper, we propose a novel wearable detection platform based on AlGaN/GaN high‐electron‐mobility transistors (HEMTs). In this platform, a sweatband was used to continuously collect sweat, and a pH detecting unit and a potassium ion detecting unit were formed by modifying different sensitive films to realize the long‐term stable and repeatable detection of pH and potassium ions. Experimental data show that the wearable detection platform based on AlGaN/GaN HEMTs has good sensitivity (pH 3–7 sensitivity is 45.72 μA/pH; pH 7.4–9 sensitivity is 51.073 μA/pH; and K⁺ sensitivity is 4.94 μA/lgα_K⁺), stability (28 days) and repeatability (the relative standard deviation (RSD) of pH 3–7 sensitivity is 2.6 %, the RSD of pH 7.4–9 sensitivity is 2.1 %, and the RSD of K⁺ sensitivity is 7.3 %). Our newly proposed wearable platform has excellent potential for predictive analytics and personalized medical treatment.