N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk wafers by plasma-assisted molecular beam epitaxy growth. The current-voltage characteristics show high-quality rectification and electroluminescence characteristics with a high on/off current ratio and interband photon emission. The measured electroluminescence spectrum is dominated by strong near-band edge emission, while deep level luminescence is greatly suppressed. A very low dislocation density leads to a high reverse breakdown electric field. The low leakage current N-polar diodes open up several potential applications in polarization-engineered photonic and electronic devices.
Several ten $μ$m GaN have been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates have been covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features allow for an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear,…
To develop a quantitative reaction simulator, data assimilation was performed using high-resolution time-of-flight mass spectrometry (TOF-MS) data applied to GaN metalorganic vapor phase epitaxy system. Incorporating ab initio knowledge into the optimization successfully reproduces not only the concentration of CH$_4$ (an impurity precursor) as an objective variable but also known reaction pathways. The simulation results show significant production of GaH$_3$, a precursor of GaN, which has been difficult to detect in TOF-MS experiments. Our proposed approach is expected to be applicable to ot…
We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that, in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit a equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photolumines…
The effect of film morphology on its surface chemistry and band structure has been analyzed for gallium nitride epitaxial films grown by molecular beam epitaxy. The film morphology has been studied using scanning electron microscopy and atomic force microscopy, and the bandstructure, defect and emission properties have been studied by X ray photoelectron spectroscopy and cathodoluminescence spectroscopy. It was found that the highly porous GaN nanowall network shows the highest relative conductivity and does not have defect related luminescence. The flatter films were more resistive and showed…
The present study elucidates the correlation between the structural and optical properties of GaN nanowires grown on Si(111) substrate by plasma assisted molecular beam epitaxy (PA-MBE) technique under various growth conditions. GaN NWs exhibiting different shapes, sizes and distribution were grown at various substrate temperatures with same Ga-N (III-V) ratio of 0.4. We observe that sample grown at lower substrate temperature (~700 degC) results 2-dimensional island like structure with almost very little (~30%) circularity while increasing substrate temperature (>770 degC) leads to growth of …
RF plasma assisted MBE growth of Scandium Nitride (ScN) thin films on GaN (0001)/SiC, AlN (0001)/Al2O3 and on 6H-SiC (0001) hexagonal substrates is found to lead to a face centered cubic (rock-salt) crystal structure with (111) out-of-plane orientation instead of hexagonal orientation. For the first time, cubic (111) twinned patterns in ScN are observed by in-situ electron diffraction during epitaxy, and the twin domains in ScN are detected by electron backscattered diffraction, and further corroborated with X-ray diffraction. The epitaxial ScN films display very smooth, sub nanometer surface …
N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 °C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 °C. This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells e…
InGaN/GaN tunnel junction contacts were grown on top of an InGaN/GaN blue (450 nm) light emitting diode wafer using plasma assisted molecular beam epitaxy. The tunnel junction contacts enable low spreading resistance n-GaN top contact layer thereby requiring less top metal contact coverage on the surface. A voltage drop of 5.3 V at 100 mA, forward resistance of 2 x 10-2 ohm cm2 and a higher light output power are measured in tunnel junction LED. A low resistance of 5 x 10-4 ohm cm2 was measured in a MBE grown tunnel junction on GaN PN junction device, indicating that the tunnel junction LED de…
Dependence of the Mn sticking coefficient on Ga-rich, N-rich, and Ga/N-flux-free conditions in GaN grown by plasma-assisted molecular beam epitaxy
Cornell University（康奈尔大学）；Paul-Drude-Institut für Festkörperelektronik (PDI), Leibniz-Institut im Forschungsverbund Berlin e.V.（柏林莱布尼茨联合会保罗·德鲁德固态电子学研究所）
We report low-frequency noise characteristics of vertical GaN PIN diodes, focusing on the effects of the diode design, current and temperature. The as-grown and regrown diodes, with and without surface treatment have been studied. The noise in most of the GaN devices had a characteristic 1/f spectrum at high and moderate currents, while some devices revealed generation-recombination bulges at low currents (f is the frequency). The predominant trend of the noise spectral density, S, dependence on the current was S ~ I. All tested GaN PIN diodes had rather low normalized noise spectral densities…
Blue light-emitting diodes (LEDs) consisting of a buried n+-p+ GaN tunnel junction, (In,Ga)N multiple quantum wells (MQWs) and a n+-GaN top layer are grown on single-crystal Ga-polar n+-GaN bulk wafers by plasma-assisted molecular beam epitaxy. The (In,Ga)N MQW active regions overgrown on the p+-GaN show chemically abrupt and sharp interfaces in a wide range of compositions and are seen to have high structural and optical properties as verified by X-ray diffraction and spatially resolved cathodoluminescence measurements. The processed LEDs reveal clear rectifying behavior with a low contact an…
Molecular beam epitaxy of GaN/AlGaN quantum wells on bulk GaN substrate in the step-flow or step meandering regime: influence on indirect exciton diffusion
本文在块体 GaN 衬底上通过分子束外延生长 GaN/AlxGa1-xN 量子阱，通过优化生长温度等条件最小化光致发光峰展宽，利用微光致发光装置测量间接激子扩散，提取扩散常数并确定了研究间接激子液体量子集体效应的最佳生长条件。
We compare the temperature dependence of optical and electrical characteristics of commercially available GaN light-emitting diodes (LEDs) grown on silicon and sapphire substrates. Contrary to conventional expectations, LEDs grown on silicon substrates, commonly referred to as GaN-on-Si LEDs, show less efficiency droop at higher temperatures even with more threading dislocations. Analysis of the junction temperature reveals that GaN-on-Si LEDs have a cooler junction despite sharing identical epitaxial structures and packaging compared to LEDs grown on sapphire substrates. We also observe a dec…
Electrical and optical characterization of two-dimensional/three-dimensional (2D/3D) p-molybdenum disulfide/n-gallium nitride (p-MoS2/n-GaN) heterojunction diodes are reported. Devices were fabricated on high-quality, large-area p-MoS2 grown by chemical vapor deposition (CVD) on hexagonal sapphire substrates. The processed devices were transferred onto GaN/sapphire substrates, and the transferred films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). On-axis XRD spectra and surface topology obtained from AFM scans were consistent with previously grown high-quali…
High quality semipolar and nonpolar GaN is crucial in achieving high-performance GaN-based optoelectronic devices, yet it has been very challenging to achieve large-area wafers that are free of basal-plane stacking faults (BSFs). In this work, we report an approach to prepare large-area, stacking-fault-free (SF-free) semipolar GaN on (4-inch) sapphire substrates. A root cause of the formation of BSFs is the emergence of N-polar (000-1) facets during semipolar and non-polar heteroepitaxy. Invoking the concept of kinetic Wulff plot, we succeeded in suppressing the occurrence of N-polar GaN (000-…
Using in situ grazing-incidence x-ray scattering, we have measured the diffuse scattering from islands that form during layer-by-layer growth of GaN by metal-organic vapor phase epitaxy on the (1010) m-plane surface. The diffuse scattering is extended in the (0001) in-plane direction in reciprocal space, indicating a strong anisotropy with islands elongated along [1 $\overline{2}$ 10] and closely spaced along [0001]. This is confirmed by atomic force microscopy of a quenched sample. Islands were characterized as a function of growth rate G and temperature. The island spacing along [0001] obser…
Resonant tunneling diodes are formed using AlN/GaN core-shell nanowire heterostructures grown by plasma assisted molecular beam epitaxy on n-Si(111) substrates. By using a coaxial geometry these devices take advantage of non-polar (m-plane) nanowire sidewalls. Device modeling predicts non-polar orientation should enhance resonant tunneling compared to a polar structure and that AlN double barriers will lead to higher peak-to-valley current ratios compared to AlGaN barriers. Electrical measurements of ensembles of nanowires show negative differential resistance appearing only at cryogenic tempe…
We investigate the influence of modified growth conditions during the spontaneous formation of GaN nanowires on Si(111) in plasma-assisted molecular beam epitaxy. We find that a two-step growth approach, where the substrate temperature is increased during the nucleation stage, is an efficient method to gain control over the area coverage, average diameter, and coalescence degree of GaN nanowire ensembles. Furthermore, we also demonstrate that the growth conditions employed during the incubation time that precedes nanowire nucleation do not influence the properties of the final nanowire ensembl…
An HVPE reactor for the growth of bulk GaN crystals with a diameter of 50 mm was developed. Growth rate non-uniformity of 1% was achieved using an axisymmetric vertical gas injector with stagnation point flow. Chemically-resistant refractory materials were used instead of quartz in the reactor hot zone. High-capacity external gallium precursor sources were developed for the non-stop growth of the bulk GaN layers. A load-lock vacuum chamber and a dry in-situ growth chamber cleaning were implemented to improve the growth process reproducibility. Freestanding GaN crystals with a diameter of 50 mm…
White light emitting diodes based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, in particular in the green/yellow range ov the visible spectrum. This range, howev…
We investigate the effect of the p-type top contact on the optoelectronic characteristics of light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. We compare devices fabricated with either Ni/Au or indium tin oxide (ITO) top contact. The NW-LEDs with ITO exhibit a number density of NWs emitting electroluminescence about ten times higher, significantly lower turn-on voltage and series resistance, and a relative external quantum efficiency more than one order of magnitude higher than the sample with Ni/Au. These results show …
Strain-released epitaxy of GaN enabled by compliant single-crystalline metal foils
Zhejiang University（浙江大学）；Westlake University（西湖大学）；Zhongke Crystal Materials (Dongguan) Technology Co., Ltd.（中科晶材（东莞）科技有限公司）；Institute of Semiconductors, Chinese Academy of Sciences（中国科学院半导体研究所）；Westlake Smoky-mountain Technologies（西湖烟山科技有限公司）；Peking University（北京大学）；University of Toronto（多伦多大学）；BOE Technology Group Co., Ltd.（京东方科技集团股份有限公司）
N-polar GaN/AlN resonant tunneling diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current-voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8$\pm$ 0.8 kA/cm$^2$ at a forward bias of ~8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ~$-$4 V. These resonant and threshold voltages are well explained with the polarization field which is opposite to that of the metal-polar counterpart, confirming the N-polarity of…
Recent experiments suggest that Mg condensation at threading dislocations induce current leakage, leading to degradation of GaN-based power devices. To study this issue, we perform first-principles total-energy electronic-structure calculations for various Mg and dislocation complexes. We find that threading screw dislocations (TSDs) indeed attract Mg impurities, and that the electronic levels in the energy gap induced by the dislocations are elevated towards the conduction band as the Mg impurity approaches the dislocation line, indicating that the Mg-TSD complex is a donor. The formation of …
We studied the persistent photoconductivity (PPC) effect in AlGaN/AlN/GaN heterostructures with two different Al-compositions (x=0.15 and x=0.25). The two-dimensional electron gas formed at the AlN/GaN heterointerface was characterized by Shubnikov-de Haas and Hall measurements. Using optical illumination, we were able to increase the carrier density of the Al0.15Ga0.85N/AlN/GaN sample from 1.6x10^{12} cm^{-2} to 5.9x1012 cm^{-2}, while the electron mobility was enhanced from 9540 cm2/Vs to 21400 cm2/Vs at T = 1.6 K. The persistent photocurrent in both samples exhibited a strong dependence on …
The Schottky diode, BN/GaN layered composite contacting to bulk aluminum, is theoretically plausible to harvest wireless energy above X-band. According to our first principle calculation, the insertion of GaN layers dramatically influences the optical properties of the layered composite. The relative dielectric constant of BN/GaN layered composite as a function of layer-to-layer separation is investigated where the optimized dielectric constant is 3.1. Furthermore, we design another Schottky diode via nanostructuring. Our first principle calculation suggests that the relative dielectric consta…
Images of the morphology of GaN (0001) surfaces often show half-unit-cell-height steps separating a sequence of terraces having alternating large and small widths. This can be explained by the $αβαβ$ stacking sequence of the wurtzite crystal structure, which results in steps with alternating $A$ and $B$ edge structures for the lowest energy step azimuths, i.e. steps normal to $[0 1 \bar{1} 0]$ type directions. Predicted differences in the adatom attachment kinetics at $A$ and $B$ steps would lead to alternating $α$ and $β$ terrace widths. However, because of the difficulty of experimentally id…
We are the first to report the frequency response and corresponding current density of a wurtzite phase Indium Nitride (InN)-based vertical configuration Gunn diode at 1 μm active length. Domain growths dynamics with respect to space and time, variation of domain velocity and the field outside the vicinity of the domain with respect to domain field have also been studied using the presented mathematical equations. It has been found that, InN-based Gunn diodes are capable to operate around 224 GHz with 691 kA/cm2 current density at 300 K. For comparison purpose, all these characteristics have b…
We carefully investigate three important effects including postgrowth activation annealing, delta (δ) dose and p+-GaN layer thickness and experimentally demonstrate their influence on the electrical properties of GaN p-n homojunction diodes with a tunnel junction (TJ)-based p-contact. The p-n diodes and TJ structures were monolithically grown by metalorganic chemical vapor deposition (MOCVD) in a single growth step. By optimizing the annealing time and temperature for magnesium (Mg) activation and introducing δ-doses for both donors and acceptors at TJ interfaces, a significant improvement in …
We report on the design and demonstration of polarization-engineered GaN/InGaN/GaN tunnel junction diodes with high current density and low tunneling turn-on voltage. Wentzel-Kramers-Brillouin (WKB) calculations were used to model and design tunnel junctions with narrow bandgap InGaN-based barrier layers. N-polar p-GaN/In0.33Ga0.67N/n-GaN heterostructure tunnel diodes were grown using molecular beam epitaxy. Efficient zero bias tunneling turn-on with a high current density of 118 A/cm2 at a reverse bias of 1V, reaching a maximum current density up to 9.2 kA/cm2 were obtained. These results rep…
This work demonstrates the first nonpolar vertical GaN on GaN pn power diodes grown on m-plane free standing substrates by MOCVD. The SEM and HRXRD results showed the good crystal quality of the homoepitaxial nonpolar structure with low defect densities. The CL result confirmed the nonpolar p GaN was of high quality with considerably reduced deep level states. At forward bias, the device showed good rectifying behaviors with a turn-on voltage of 4.0 V, an on-resistance of 2.3 mohmcm2, and a high on off ratio of 1e10. At reverse bias, the current leakage and breakdown were described by the trap…
By the insertion of thin InGaN layers into Nitrogen-polar GaN p-n junctions, polarization-induced Zener tunnel junctions are studied. The reverse-bias interband Zener tunneling current is found to be weakly temperature dependent, as opposed to the strongly temperature-dependent forward bias current. This indicates tunneling as the primary reverse-bias current transport mechanism. The Indium composition in the InGaN layer is systematically varied to demonstrate the increase in the interband tunneling current. Comparing the experimentally measured tunneling currents to a model helps identify the…
GaN-based resonators supporting whispering gallery modes are suggested as a promising approach for fabrication of efficient nanophotonic applications. A modification of excitonic emission due to coupling with cavity modes is observed experimentally in the GaN planar hexagonal microcavities grown by a selective area metal-organic vapor phase epitaxy. Low temperature cathodoluminescence spectra measured in-situ scanning electron microscope show two or three emission peaks split by 100 meV or 160 meV for the resonators formed by double- and single-wall micro-nuts, respectively. The results are co…
Cubic (zinc-blende) Ga1-xMnxN layers were grown by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. Some of the structures also contained cubic AlN buffer layers. Auger Electron Spectroscopy and Secondary Ion Mass Spectroscopy studies clearly confirmed the incorporation of Mn into cubic GaN, the Mn being uniformly distributed through the layer. X-ray diffraction studies demonstrated that the Mn-doped GaN films are cubic and do not show phase separation up to a Mn concentrations of x<0.1. P-type conductivity for the cubic Ga1-xMnxN layers was observed for a wide range of the Mn …
TBA-enabled spin-coating of a percolatively connected GO nanosieve for thru-hole epitaxy: tuning GO flake stacking and coverage to control GaN nucleation
High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current-voltage characteristics reveal a high on/off current ratio of 10^11 at 4 V and an ideality factor of 1.6. As the temperature increases to 200 C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley-Read-Hall recombination times of 0.32-0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminesce…
We report the influence of the In mole fraction on the material and electrical characteristics of p-i-n InxGa1-xN homojunctions (x = 0.10-0.40) synthesized by plasma-assisted molecular-beam epitaxy on GaN-on-sapphire substrates. Junctions terminated with p-InGaN present improved carrier extraction efficiency in comparison with devices capped with p-GaN, due to the deleterious effect of polarization discontinuities on the device performance. We demonstrate that the presence of Mg does not perturb the In incorporation in InGaN, and it leads to a significant reduction of the stacking fault densit…
Recently theorized hybrid II-IV-N{_2} / III-N heterostructures, based on current commercialized (In,Ga)N devices, are predicted to significantly advance the design space of highly efficient optoelectronics in the visible spectrum, yet there are few epitaxial studies of II-IV-N{_2} materials. In this work, we present heteroepitaxial ZnGeN{_2} grown on GaN buffers and AlN templates. We demonstrate that a GaN nucleating surface is crucial for increasing the ZnGeN{_2} crystallization rate to combat Zn desorption, extending the stoichiometric growth window from 215 {\degree}C on AlN to 500 {\degree…
We explore and systematically compare the morphological, structural and optical properties of GaN/(Al,Ga)N multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA-MBE) on freestanding GaN$(0001)$ and GaN$(000\bar{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and Al content. Regardless of the crystal orientation, the exciton decay in the MQWs at 10 K is dominantly radiative and the photoluminescence (PL) energy follows the quantum …
AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with…
Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report high-quality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350-450 C with complete surface coverage and smooth surface morphology. Higher growth tempe…
We report on the unambiguous detection of Auger electrons by electron emission spectroscopy from a cesiated InGaN/GaN light emitting diode (LED) under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high energy electron peaks simultaneously with an observed drop in electroluminescence efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the "droop current" - t…
