ZnO nanowires grown on Al2O3-ZnAl2O4 nanostructure using solid-vapor mechanism

We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given.Comment: Conference: 13th Polish-Japanese Joint Seminar on Micro and Nano Analysi

All-wurtzite (In,Ga)As-(Ga,Mn)As core-shell nanowires grown by molecular beam epitaxy

Structural and magnetic properties of (In,Ga)As-(Ga,Mn)As core-shell nanowires grown by molecular beam epitaxy on GaAs(111)B substrate with gold catalyst have been investigated.(In,Ga)As core nanowires were grown at high temperature (500 {\deg}C) whereas (Ga,Mn)As shells were deposited on the {1-100} side facets of the cores at much lower temperature (220 {\deg}C). High resolution transmission electron microscopy images and high spectral resolution Raman scattering data show that both the cores and the shells of the nanowires have wurtzite crystalline structure. Scanning and transmission electron microscopy observations show smooth (Ga,Mn)As shells containing 5% of Mn epitaxially deposited on (In,Ga)As cores containing about 10% of In, without any misfit dislocations at the core-shell interface. With the In content in the (In,Ga)As cores larger than 5% the (In,Ga)As lattice parameter is higher than that of (Ga,Mn)As and the shell is in the tensile strain state. Elaborated magnetic studies indicate the presence of ferromagnetic coupling in (Ga,Mn)As shells at the temperatures in excess of 33 K. This coupling is maintained only in separated mesoscopic volumes resulting in an overall superparamagnetic behavior which gets blocked below ~17 K.Comment: 37 pages, 8 figure

Optimized ohmic contacts for InAlGaN/GaN HEMTs

International audienceDuring the last years, the most significant improvement of the contact resistance forAlInN/GaN high electron mobility has been the use of a highly doped n+ GaN layer grown bymolecular beam epitaxy for the source and drain terminals. In a first report [1], the ohmiccontact processing was carried out as follows, first the device surface was passivated using aSiN layer and then protected by SiO2. Then the contact windows were opened through theprotection and the InAlN(5.6 nm) /GaN heterostructures was then etched out to a depth of 12nm followed by a regrowth of a 40 nm thick n+ doped GaN layer by MOCVD. After removalof the residual polycrystalline GaN from the SiO2 surface, a Ti/Al/Ni/Au metal stack wasdeposited by electron-beam evaporation and annealed at 850°C for 30s in N2 which resultedin an improved contact resistance of 0.40 Ohm.mm. In a more recent work using an identicalprocedure [2], the same authors deposited, a different metallic stack based on Mo/Au andmeasured a lower contact resistance of 0.16 Ohm.mm. In this work, the Ohmic contact isfabricated in a more conventional way by first removing the passivation layers through anoptimized surface chemical cleaning procedure in ohmic region, next a Ti/Al/Ni/Au metallicstack is deposited by electron-beam evaporation and the sample is submitted to rapid thermalannealing at 875°C for 30 seconds. In our report, we shall discuss the results measured on twosamples; S1 was processed using standard surface cleaning procedure which leads routinely toa reproducible contact resistance of about 0.5-0.6 Ohm.mm. For S2, we carefully optimizedthe AlInGaN barrier surface preparation prior to the metal stack deposition resulting incontact resistances as low as 0.15-0.16 Ohm.mm. Through a detailed analysis by transmissionelectron microscopy, it is shown that, for both samples the metallic stack is transformed into amultiphase alloy, although in a different way. In S1, there is an extensive phase separationwith two dominant alloys, one AuAl rich, and the second NiAl rich. In S2, the metallicreaction leads to a homogeneous alloy of the four starting metals. Moreover, at the interfacewith the barrier extended epitaxial relationships are present with the metallic particles.Although a diffusion of Ti, Ni, and Au can be seen along screw or mixed type dislocations,the surface layer of the barrier presents extended reaction with the metals. All thiscombination is probably at the origin of the measured low contact resistance, which becomecomparable to the case where the localized MBE growth of n+ GaN has been used

Optimized ohmic contacts for InAlGaN/GaN HEMTs

International audienceDuring the last years, the most significant improvement of the contact resistance forAlInN/GaN high electron mobility has been the use of a highly doped n+ GaN layer grown bymolecular beam epitaxy for the source and drain terminals. In a first report [1], the ohmiccontact processing was carried out as follows, first the device surface was passivated using aSiN layer and then protected by SiO2. Then the contact windows were opened through theprotection and the InAlN(5.6 nm) /GaN heterostructures was then etched out to a depth of 12nm followed by a regrowth of a 40 nm thick n+ doped GaN layer by MOCVD. After removalof the residual polycrystalline GaN from the SiO2 surface, a Ti/Al/Ni/Au metal stack wasdeposited by electron-beam evaporation and annealed at 850°C for 30s in N2 which resultedin an improved contact resistance of 0.40 Ohm.mm. In a more recent work using an identicalprocedure [2], the same authors deposited, a different metallic stack based on Mo/Au andmeasured a lower contact resistance of 0.16 Ohm.mm. In this work, the Ohmic contact isfabricated in a more conventional way by first removing the passivation layers through anoptimized surface chemical cleaning procedure in ohmic region, next a Ti/Al/Ni/Au metallicstack is deposited by electron-beam evaporation and the sample is submitted to rapid thermalannealing at 875°C for 30 seconds. In our report, we shall discuss the results measured on twosamples; S1 was processed using standard surface cleaning procedure which leads routinely toa reproducible contact resistance of about 0.5-0.6 Ohm.mm. For S2, we carefully optimizedthe AlInGaN barrier surface preparation prior to the metal stack deposition resulting incontact resistances as low as 0.15-0.16 Ohm.mm. Through a detailed analysis by transmissionelectron microscopy, it is shown that, for both samples the metallic stack is transformed into amultiphase alloy, although in a different way. In S1, there is an extensive phase separationwith two dominant alloys, one AuAl rich, and the second NiAl rich. In S2, the metallicreaction leads to a homogeneous alloy of the four starting metals. Moreover, at the interfacewith the barrier extended epitaxial relationships are present with the metallic particles.Although a diffusion of Ti, Ni, and Au can be seen along screw or mixed type dislocations,the surface layer of the barrier presents extended reaction with the metals. All thiscombination is probably at the origin of the measured low contact resistance, which becomecomparable to the case where the localized MBE growth of n+ GaN has been used

The critical role of N-vacancy on chemical composition fluctuations and degradation of InAlN layer

International audienc

Thermal annealing of molecular beam epitaxy-grown InGaN/GaN single quantum well

The effect of thermal annealing on In0.25Ga0.75N/GaN quantum wells grown by molecular beam epitaxy at 550 degrees C is investigated. A strong increase in the 300 K photoluminescence (PL) intensity is observed for samples annealed at 880 degrees C, while degradation occurs for higher temperatures. The improvement of the optical properties is ascribed to higher internal quantum efficiency (IQE), as indicated by temperature-dependent and time-resolved PL experiments. The effect of carrier localization due to possible quantum dot formation via indium clustering is ruled out based on high-resolution transmission electron microscopy imaging. IQE improvement is thus attributed to a reduction of point defects upon annealing

Influence of Growth Polarity Switching on the Optical and Electrical Properties of GaN/AlGaN Nanowire LEDs

For the development and application of GaN-based nanowire structures, it is crucial to understand their fundamental properties. In this work, we provide the nano-scale correlation of the morphological, electrical, and optical properties of GaN/AlGaN nanowire light emitting diodes (LEDs), observed using a combination of spatially and spectrally resolved cathodoluminescence spectroscopy and imaging, electron beam-induced current microscopy, the nano-probe technique, and scanning electron microscopy. To complement the results, the photo- and electro-luminescence were also studied. The interpretation of the experimental data was supported by the results of numerical simulations of the electronic band structure. We characterized two types of nanowire LEDs grown in one process, which exhibit top facets of different shapes and, as we proved, have opposite growth polarities. We show that switching the polarity of nanowires (NWs) from the N- to Ga-face has a significant impact on their optical and electrical properties. In particular, cathodoluminescence studies revealed quantum wells emissions at about 3.5 eV, which were much brighter in Ga-polar NWs than in N-polar NWs. Moreover, the electron beam-induced current mapping proved that the p–n junctions were not active in N-polar NWs. Our results clearly indicate that intentional polarity inversion between the n- and p-type parts of NWs is a potential path towards the development of efficient nanoLED NW structures