This work studies the effect of four different types of buffer layers on the structural and optical properties of InGaN layers grown on Si(111) substrates and their correlation with electrical characteristics. The vertical electrical conduction of n-InGaN/buffer-layer/p-Si heterostructures, with In composition near 46%, which theoretically produces an alignment of the bands, is analyzed. Droplet elimination by radical-beam irradiation was successfully applied to grow high quality InGaN films on Si substrates for the first time. Among several buffer choices, an AlN buffer layer with a thickness above 24 nm improves the structural and optical quality of the InGaN epilayer while keeping a top to bottom ohmic behavior. These results will allow fabricating double-junction InGaN/Si solar cells without the need of tunnel junctions between the two sub-cells, therefore simplifying the device design.
Photoelectrocatalytic (PEC) reduction of N2 to ammonia (NH3) is emerging as the potential alternative to overcome the standard Haber-Bosch approach. In this communication, solar N2 reduction was demonstrated with molybdenum carbide (Mo2C) co-catalyst assisted indium gallium nitride (InGaN) nanowires. The effect of aiding Mo2C on InGaN NWs arrests the dark current and demonstrated the saturation current under illumination was briefly elucidated. Large NH3 production of 7.2 gh-1cm-2 with high Faradaic efficiency of 12.6 % was realized at -0.2 V vs. reversible hydrogen electrode for the Mo2C/GaN/…
Spectroscopic Signature of Local Alloy Fluctuations in InGaN/GaN Multi-Quantum-Disk Light Emitting Diode Heterostructures and Its Impact on the Optical Performance
We report a correlative microscopy study of a sample containing three stacks of InGaN/GaN quantum dots (QDs) grown at different substrate temperature, each stack consisting of 3 layers of QDs. Decreasing the substrate temperature along the growth axis leads to the proliferation of structural defects. However, the luminescence intensity increases towards the surface, in spite of the higher density of threading dislocations, revealing that the QD layers closer to the substrate behave as traps for non-radiative point defects. During atom probe tomography experiments combined with in-situ micro-ph…
InGaN light-emitting diodes (LEDs) are more efficient and cost effective than incandescent and fluorescent lighting, but lattice mismatch limits the thickness of InGaN layers that can be grown on GaN without performance-degrading dislocations. In this work, we apply hybrid density functional theory calculations to investigate the thermodynamic stability, lattice parameters, and band gaps of wurtzite and zincblende quaternary BInGaN alloys. We find that the wurtzite phase is more stable and can be lattice-matched to GaN for BInGaN compositions containing up to ~30% boron. The lattice match with…
We show evidence that tunnel junctions (TJs) in GaN grown by metal-organic vapor phase epitaxy are dominated by trap-assisted (Poole-Frenkel) tunneling. This stems from observations of the careful optimized doping for the TJs. Especially the p$^{++}$ and the n$^{++}$ layers are far from ideal. The n$^{++}$ layer induces 3D growth, which can be seen by a rising oxygen signal in Secondary Ions Mass Spectroscopy (SIMS). Furthermore, Mg segregation observed by SIMS indicates a depletion region of more than 10 nm. Still, we could realize TJ based LEDs with a low penalty voltage of 1.1 V and a speci…
Cubic silicon carbide (3C-SiC) is an emerging material for high power and new generation devices, but the development of high quality 3C-SiC layer still represents a scientific and technological challenge especially when grown on a Si substrate. In the present lecture, we discuss the use of a buffer layer between the epitaxial layer and the substrate in order to reduce the defectiveness and improve the overall quality of the SiC epi-film. In particular, we find that the morphology and the quality of the epi-film depends on the carbonization temperature and the concentration of Ge in close prox…
We demonstrate a new substrate cleaning and buffer growth scheme in $β$-Ga$_2$O$_3$ epitaxial thin films using metalorganic vapor phase epitaxy (MOVPE). For the channel structure, a low-temperature (LT, 600 $^\circ$C) undoped Ga$_2$O$_3$ buffer is grown followed by transition layers to a high-temperature (HT, 810 $^\circ$C) Si-doped Ga$_2$O$_3$ channel layers without growth interruption. The (010) Ga$_2$O$_3$ Fe-doped substrate cleaning uses solvent cleaning followed by an additional HF (49% in water) treatment for 30 mins before the epilayer growth. This step is shown to compensate the parasi…
A free-standing bulk gallium nitride layer with a thickness of 365 $μ$m and a diameter of 50 mm was obtained by hydride vapor phase epitaxy on a sapphire substrate with a carbon buffer layer. The carbon buffer layer was deposited by thermal decomposition of methane $\textit{in situ}$ in the same process with the growth of a bulk GaN layer. The bulk GaN layer grown on the carbon buffer layer self-separated from the sapphire substrate during the cooling after the growth. The dislocation density was $8\cdot10^{6}$ cm$^{-2}$. The (0002) X-Ray rocking curve full width at half maximum was 164 arcsec…
Gallium oxide films were grown by HVPE on (0001) sapphire substrates with and without $α$-Cr$_2$O$_3$ buffer produced by RF magnetron sputtering. Deposition on bare sapphire substrates resulted in a mixture of $α$-Ga$_2$O$_3$ and $ε$-Ga$_2$O$_3$ phases with a dislocation density of about $2\cdot10^{10}$ cm$^{-2}$. The insertion of $α$-Ga$_2$O$_3$ buffer layers resulted in phase-pure $α$-Ga$_2$O$_3$ films and a fourfold reduction of the dislocation density to $5 \cdot 10^9$ cm$^{-2}$.
A tandem GaAsP/SiGe solar cell has been developed employing group-IV reverse buffer layers grown on silicon substrates with a subsurface porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses, but ease the appearance of cracks, as observed in previous designs grown on regular Si substrates. In this new design, a porous silicon layer has been incorporated close to the substrate surface. The ductility of this layer helps repress the propagation of cracks, diminishing the problems of low shunt resistance and thus improving solar ce…
Thin (approximately 10 nm) oxide buffer layers grown over lead-halide perovskite device stacks are critical for protecting the perovskite against mechanical and environmental damage. However, the limited perovskite stability restricts the processing methods and temperatures (<=110 C) that can be used to deposit the oxide overlayers, with the latter limiting the electronic properties of the oxides achievable. In this work, we demonstrate an alternative to existing methods that can grow pinhole-free TiOx (x = 2.00+/-0.05) films with the requisite thickness in <1 min without vacuum. This techniqu…
Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this work we develop a p-type CuOx buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or…
The direct integration of GaN with Si can boost great potential for low-cost, large-scale, and high-power device applications. However, it is still challengeable to directly grow GaN on Si without using thick strain relief buffer layers due to their large lattice and thermal-expansion-coefficient mismatches. In this work, a GaN/Si heterointerface without any buffer layer is successfully fabricated at room temperature via surface activated bonding (SAB). The residual stress states and interfacial microstructures of GaN/Si heterostructures were systematically investigated through micro-Raman spe…
The integration of MOCVD grown group III-A nitride device stacks on Si (111) substrates is critically dependent on the quality of the first AlN buffer layer grown. A Si surface that is both oxide-free and smooth is a primary requirement for nucleating such layers. A single parameter, the AlN layer growth stress, is shown to be an early (within 50 nm), clear (<0.5 GPa versus >1 GPa) and fail-safe indicator of the pre-growth surface, and the AlN quality required for successful epitaxy. Grain coalescence model for stress generation is used to correlate growth stress, the AlN-Si interface and crys…
For the past two decades piezo MEMS technology are emerging for its low power consumption of ultrasonic sensing and actuation devices for biomedical applications. Still researchers are facing challenges in miniaturization of piezo-MEMS devices. But up-to-date there is no reports on achieving better sensitivity and actuation strength properties in the same piezoelectric material. As piezoelectric materials are anisotropic in nature, for that it is essential to grow different textured PZT thin film on a single silicon substrate. In the present investigation we have demonstrated the simple method…
We present the electrical characterization of N-polar AlN layers grown by metal-organic vapor phase epitaxy and the demonstration of N-polar AlN-channel metal-semiconductor field-effect transistors (MESFETs). A high concentration of silicon is unintentionally incorporated during the high-temperature growth of N-polar AlN, causing high buffer leak current. The silicon concentration reduces from 2x10^18 /cm^3 to 9x10^15 /cm^3 with decreasing growth temperature, reducing the buffer leak current to 5.6 nA/mm at a 100 V bias. The N-polar AlN MESFET exhibits an off-state drain current of 0.27 nA/mm …
Epitaxial 200nm BiFe$_{0.95}$Mn$_{0.05}$O$_{3}$ (BFO) film was grown by pulsed laser deposition on (111) oriented SrTiO3 substrate buffered with a 50nm thick SrRuO$_{3}$ electrode. The BFO thin film shows a rhombohedral structure and a large remnant polarization of Pr = 104 $μ$C/cm$^{2}$. By comparing I(V) characteristics with different conduction models we reveal the presence of both bulk limited Poole-Frenkel and Schottky interface mechanisms and each one dominates in a specific range of temperature. At room temperature and under 10mW laser illumination, the as grown BFO film presents short-…
A mechanism of controlling the degree of strain relaxation in GeSn epilayers, grown by molecular beam epitaxy on Ge/Si(001) substrates, is reported in this work. It is demonstrated that by suitably controlling the thickness and the growth recipe of the underlying Ge buffer layer, both fully-strained and highly-relaxed GeSn epilayers can be obtained, without significant Sn segregation. The strain relaxation of the GeSn epilayer is mediated by threading dislocations of the Ge buffer layer, propagating across the Ge-GeSn interface. Systematic estimation of the threading dislocation density in bot…
Alternate stacking of a highly conducting metallic layer with a magnetic triangular layer found in delafossite PdCrO2 provides an excellent platform for discovering intriguing correlated quantum phenomena. Thin film growth of the material may enable not only tuning the basic physical properties beyond what bulk materials can exhibit, but also developing novel hybrid materials by interfacing with dissimilar materials, yet this has proven to be extremely challenging. Here, we report the epitaxial growth of metallic delafossite PdCrO2 films by pulsed laser epitaxy (PLE). The fundamental role of t…
