InGaN Epilayers Research Articles

Temperature and excitation dependence of stimulated emission and spontaneous emission in InGaN epilayer**Project supported by the National Key Research Program of China (Grant No. 2016YFB0501604) and the National Natural Science Foundation of China (Grant Nos. 10874127 and 61227902).

Temperature and excitation dependent photoluminescence (PL) of InGaN epilayer grown on c-plane GaN/sapphire template by molecular beam epitaxy (MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission (SE) located at 430 nm and two spontaneous emissions (SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states. The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL (TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.

Self-Powered, Broad Band, and Ultrafast InGaN-Based Photodetector.

A self-powered, broad band and ultrafast photodetector based on n+-InGaN/AlN/n-Si(111) heterostructure is demonstrated. Si-doped (n+ type) InGaN epilayer was grown by plasma-assisted molecular beam epitaxy on a 100 nm thick AlN template on an n-type Si(111) substrate. The n+-InGaN/AlN/n-Si(111) devices exhibit excellent self-powered photoresponse under UV-visible (300-800 nm) light illumination. The maximum response of this self-powered photodetector is observed at 580 nm for low-intensity irradiance (0.1 mW/cm2), owing to the deep donor states present near the InGaN/AlN interface. It shows a responsivity of 9.64 A/W with rise and fall times of 19.9 and 21.4 μs, respectively. A relation between the open circuit voltage and the responsivity has been realized.

Suppression of indium-composition fluctuations in InGaN epitaxial layers by periodically-pulsed mixture of N2 and H2 carrier gas**Project supported by the Science and Technology Major Project of Guangdong Province, China (Grant No. 2015B010112001) and the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030312011).

Indium-composition fluctuations in InGaN epitaxial layers are suppressed by using periodically-pulsed mixture (PPM) of N2 and H2 carrier gas. Photoluminescence, optical transmission, reciprocal space map and space-resolved cathodoluminescence are employed to characterize the InGaN epilayers. It is shown that the lateral In-fluctuations mainly occur as hillock-like In-rich regions. Both the number and size of In-rich regions are reduced by introducing the PPM carrier gas. Moreover, the measurements first experimentally demonstrate that the H2 carrier gas has a stronger decomposition effect on the In-rich region. As the duration time of the PPM carrier gas increases, the reduction of In-content in the In-rich region reaches up to 12%, however, only 2% for the In-homogeneous region. These factors lead to the suppression of In-fluctuations.

Correlation between pit formation and phase separation in thick InGaN film on a Si substrate

We demonstrate improved surface pit and phase separation in thick InGaN grown on a GaN/Si (111) substrate, using plasma-assisted molecular beam epitaxy with an indium modulation technique. The formation of surface pit and compositional inhomogeneity in the InGaN epilayer are investigated using atomic force microscopy, scanning electron microscopy and temperature-dependent photoluminescence. Indium elemental mapping directly reveals that poor compositional homogeneity occurs near the pits. The indium-modulation epitaxy of InGaN minimizes the surface indium segregation, leading to the reduction in pit density and size. The phase separation in InGaN with a higher pit density is significantly suppressed, suggesting that the pit formation and the phase separation are correlated. We propose an indium migration model for the correlation between surface pit and phase separation in InGaN.

Effect of different buffer layers on the quality of InGaN layers grown on Si

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.

Peculiarities of plastic relaxation of (0001) InGaN epilayers and their consequences for pseudo-substrate application

In this paper, we study the plastic relaxation of InGaN layers deposited on (0001) GaN bulk substrates and (0001) GaN/sapphire templates by molecular beam epitaxy. We demonstrate that the InGaN layers relax by the formation of (a+c)-type misfit dislocations gliding on pyramidal planes in the slip system ⟨112¯3⟩{112¯2} down to the interface where they form a trigonal dislocation network. Combining diffraction contrast and large angle convergent beam electron diffraction analyses performed using a transmission electron microscope, we reveal that all (a+c)-type dislocations belonging to one subset of the network exhibit Burgers vectors with the same c-component. This relaxation mechanism leads to partially relaxed InGaN layers with smooth surfaces and threading dislocation densities below 109 cm−2. Such layers are of potential interest as pseudo-substrates for the growth of InGaN heterostructures.

Impact of surface morphology on the properties of light emission in InGaN epilayers

Scanning near-field optical microscopy was used to study the influence of the surface morphology on the properties of light emission and alloy composition in InGaN epitaxial layers grown on GaN substrates. A strong correlation between the maps of the photoluminescence (PL) peak energy and the gradient of the surface morphology was observed. This correlation demonstrates that the In incorporation strongly depends on the geometry of the monolayer step edges that form during growth in the step-flow mode. The spatial distribution of nonradiative recombination centers — evaluated from PL intensity maps — was found to strongly anticorrelate with the local content of In atoms in the InGaN alloy.

Optical properties of InGaN thin films in the entire composition range

The optical properties of thick InGaN epilayers, with compositions spanning the entire ternary range, are studied in detail. High structural quality, single phase InxGa1-xN (0001) films were grown heteroepitaxially by radio-frequency plasma assisted molecular-beam epitaxy on freestanding GaN substrates. Their emission characteristics were investigated by low temperature photoluminescence spectroscopy, whereas variable angle spectroscopic ellipsometry was applied to determine the complex dielectric function of the films, in the 0.55–4.0 eV photon range. Photoluminescence lines were intense and narrow, in the range of 100 meV for Ga-rich InGaN films (x < 0.3), around 150 meV for mid-range composition films (0.3 < x < 0.6), and in the range of 50 meV for In-rich alloys (x > 0.6). The composition dependence of the strain-free emission energy was expressed by a bowing parameter of b = 2.70 ± 0.12 eV. The films' optical dielectric function dispersion was obtained by the analysis of the ellipsometric data employing a Kramers-Kronig consistent parameterized optical model. The refractive index dispersion was obtained for alloys in the entire composition range, and the corresponding values at the band edge show a parabolic dependence on the InN mole fraction expressed by a bowing parameter of b = 0.81 ± 0.04. The bowing parameter describing the fundamental energy bandgap was deduced to be equal to 1.66 ± 0.07 eV, consistent with the ab initio calculations for statistically random (non-clustered) InGaN alloys.

Hard X-ray photoelectron spectroscopy probes deeper into InGaN epilayers

A new study combines hard and soft X-ray photoelectron spectroscopy to investigate the chemical states and electronic structures of InGaN, revealing the effects of magnesium doping.

Anomalous indium incorporation and optical properties of high indium content InGaN grown by MOCVD

The optical properties of thick InGaN epilayers grown by metal-organic chemical vapor deposition with indium (In) content range from 10% to 42% were investigated. With the increase of In content, the crystal quality of InGaN epilayers significantly degraded, particularly at increasing the edge-type dislocations density for high In content InGaN. Combining high-resolution X-ray diffraction reciprocal space mapping, the splitting of the photoluminescence (PL) peak into several PL bands was found to be caused by compositional fluctuations rather than by the strain effects. Based on the PL and optical absorption spectra, the composition-dependent Stokes shift was observed and a linear relationship between Stokes shift and Urbach tailing energy was established. Additionally, a drop of In incorporation along the growth direction was observed in InGaN epilayers with high In content. Such drop behavior of In incorporation was attributed to the formation of In droplets, which offers a competing pathway for In incorporation by gathering the incoming In atoms hence causing a decrease in the surrounding In incorporation. This will present a broader view about the nature of InGaN alloys with high In content.

Indium Incorporation Induced Morphological Evolution and Strain Relaxation of High Indium Content InGaN Epilayers Grown by Metal–Organic Chemical Vapor Deposition

The indium (In) incorporation induced morphological evolution and strain relaxation of high In content InGaN epilayers grown by metal–organic chemical vapor deposition (MOCVD) were investigated. With the decrease of growth temperature from 753 to 627 °C, In incorporation increases from 0.10 to 0.42, and the surface morphology evolves from initially mound-like three-dimensional surface roughness to progressive smoothness. Such morphology evolution mechanism can be well accounted for by a self-regulating model of islands’ proportions considering both strain relaxation and In surfactant effect comprehensively. Additionally, the strain relaxation and microstructural defects of such InGaN epilayers were investigated by X-ray diffraction reciprocal space mapping and cross-sectional transmission electron microscopy, respectively. It is found that, with the increase of In content, plastic relaxation via generating random stacking faults along with the surface sawtooth roughening becomes a more important strain re...

Effect of growth temperature on InGaN/GaN heterostructures grown by MOCVD

InGaN/GaN heterostructured samples were grown at different temperatures varying from 680°C to 760°C on c-plane sapphire substrates using a horizontal flow Metal Organic Chemical Vapor Deposition (MOCVD) reactor. Systematic investigation of structural, optical, morphological and electrical properties has been carried out using High Resolution X-Ray Diffraction (HRXRD), Photoluminescence (PL), Atomic Force Microscope (AFM) and Hall Measurement respectively. From the HRXRD results, the respective Indium (In) composition in the InGaN/GaN heterostructures can be calculated using InGaN (0002) peaks. In incorporation is very much sensitive to the temperature and incorporation of In in InGaN epilayer was found to decrease with increase in growth temperature. PL spectra reveal a shift towards higher energy side because of In incorporation with increasing reactor temperature. AFM images of the sample grown at 680°C shows the In rich clusters due to compositional inhomogeneity. With further increase in the growth temperature, decomposition of InN starts and hence decreases the surface roughness. Hall Effect studies confirm that with increasing growth temperature bulk electron concentration of the InGaN thin films decrease and mobility increases. Maximum bulk electron concentration of ~4.5×1019cm−3 and a mobility of ~295cm2/V-s was obtained for 19% of In. Efforts are being made to optimize the incorporation of In content in InGaN/GaN heterostructures for photovoltaic application.

Structure and composition of non‐polar (11‐20) InGaN nanorings grown by modified droplet epitaxy (Phys. Status Solidi B 5/2016)

Nitride-based quantum dots (QDs) show promise as sources for single photon emission, enabling comparably high temperature emission and access to the blue and green spectral region. Some droplets forming during modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN are associated with underlying ring-like structures. The work by Springbett et al. (pp. 840–844) discusses droplet etching as a possible mechanism for ring formation, whereby In and Ga atoms are incorporated into the droplet and diffuse to the edges. Thereafter, recrystallization occurs, leading to the formation of a ring. It is hypothesised that the droplet then creeps in a direction determined by the crystallographic anisotropy and the surface energetics. Once this movement halts, etching continues. The resulting structure consists of a recession enclosed by a double ring. Transmission electron microscopy (TEM) analysis shows the droplets move along the ⟨0001⟩ c-axis, and energy dispersive X-ray spectroscopy (EDXS) indicates that they have a very high In content. These studies may help reveal the underlying QD formation mechanism during modified droplet epitaxy.

Structure and composition of non‐polar (11‐20) InGaN nanorings grown by modified droplet epitaxy

Droplets grown by modified droplet epitaxy on non‐polar (11‐20) surfaces of InGaN epilayers on GaN have been seen to be associated with underlying ring‐like structures. This work discusses droplet etching as a possible mechanism for ring formation, and droplet creeping as a possible explanation for the droplets sitting askew of the ring centre. Transmission electron microscopy (TEM) analysis shows the droplets to move along the c‐axis, and indicates that they have a very high indium content.The image shows atomic force microscopy (AFM) data of a double‐ring structure, rendered in 3D.

Defects, strain relaxation, and compositional grading in high indium content InGaN epilayers grown by molecular beam epitaxy

We investigate the structural properties of a series of high alloy content InGaN epilayers grown by plasma-assisted molecular beam epitaxy, employing the deposition temperature as variable under invariant element fluxes. Using transmission electron microscopy methods, distinct strain relaxation modes were observed, depending on the indium content attained through temperature adjustment. At lower indium contents, strain relaxation by V-pit formation dominated, with concurrent formation of an indium-rich interfacial zone. With increasing indium content, this mechanism was gradually substituted by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal stacking faults and threading dislocations in the rest of the film. We show that this interfacial layer is not chemically abrupt and that major plastic strain relaxation through defect introduction commences upon reaching a critical indium concentration as a result of compositional pulling. Upon further increase of the indium content, this relaxation mode was again gradually succeeded by the increase in the density of misfit dislocations at the InGaN/GaN interface, leading eventually to the suppression of the strained InGaN layer and basal stacking faults.

(Invited) InGaN/GaN Multiple Quantum Well Solar Cells for Energy and Hydrogen Generation

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last decade. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white LEDs, violet/blue LDs, UV detectors, and high power/temperature transistors, have reached high performance level. This talk will provide a brief overview on recent advances made by our group and others in the area of III-nitrides for energy and hydrogen generation. The presentation will concentrate on topics of material and device challenges of III-nitride solar cells and photoelectrochemical cells (PECs). The bandgap of InGaN expands from about 0.65 to 3.4 eV, which covers the entire solar spectrum. In principle, a multi-junction solar cell based on multiple layers of InGaN with different In-contents is capable of capturing the entire spectrum of the sunlight and can thus be highly efficient. To count for the relatively low materials quality for high In content InGaN alloys, InGaN/GaN multiple quantum wells (MQWs) have been adopted for the construction of nitride solar cells. We have grown, fabricated, and characterized InGaN based solar cells utilizing the InGaN/GaN MQWs p-i-n structures.1-3The fabricated solar cells exhibited open circuit voltage of about 2 V, a fill factor of about 60% and overall efficiency of about 3% under AM 1.5 irradiation. The performance of these nitride solar cells under concentrated sunlight has also been investigated. In additional to its bandgap match to the solar spectrum, the bandgap of InGaN alloys can also be engineered to optimally match with conditions necessary for direct hydrogen generation by water splitting using sunlight.4-6 The results on hydrogen gas generation via solar water splitting accomplished using n- and p-type InGaN epilayer as working electrodes will be discussed. Direct solar water splitting and hydrogen gas generation under zero bias has been realized using an InGaN/GaN multiple quantum well solar cell as a photoelectrochemical cell. Under the irradiation by a simulated sunlight (AM1.5G solar simulator), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a new benchmark of employing III-nitrides as photoreactive electrodes for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period revealed an excellent chemical stability of InGaN in aqueous solution. Our results indicate that InGaN is an excellent artificial photosynthesis material which is capable to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input. InGaN materials growth effort was supported by NSF (DMR-1206652) and the energy device effort was by DOE (FG02-09ER46552). Jiang & Lin are grateful to AT&T Foundation for the support of Ed & Linda Whitacre endowed chairs. R. Dahal, B. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN multiple quantum well solar cells with long operating wavelengths,” Appl. Phys. Lett. 94, 063505 (2009). R. Dahal, J. Li, K. Aryal, J. Y. Lin, and H. X. Jiang, "InGaN/GaN multiple quantum well concentrator solar cells," Appl. Phys. Lett. 97, 073115 (2010). B N. Pantha, J. Y. Lin, and H. X. Jiang, “III-nitride nanostructures for energy generation,” Proc. SPIE 7608, 76081I (2010). J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93, 162107 (2008). K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, "Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells," Appl. Phys. Lett. 96, 052110 (2010). R. Dahal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis,” Appl. Phys. Lett. 104, 143901 (2014)

Role of compositional fluctuations and their suppression on the strain and luminescence of InGaN alloys

Advanced electron microscopy techniques are combined for the first time to measure the composition, strain, and optical luminescence, of InGaN/GaN multi-layered structures down to the nanometer scale. Compositional fluctuations observed in InGaN epilayers are suppressed in these multi-layered structures up to a thickness of 100 nm and for an indium composition of 16%. The multi-layered structures remain pseudomorphically accommodated on the GaN substrate and exhibit single-peak, homogeneous luminescence so long as the composition is homogeneous.

Influences of excitation power and temperature on photoluminescence in phase-separated InGaN quantum wells**Project supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120131110006), the Key Science and Technology Program of Shandong Province, China (Grant No. 2013GGX10221), the Key Laboratory of Functional Crystal Materials and Device (Shandong University, Ministry of Education) (Grant No.

The photoluminescence (PL) properties of a green and blue light-emitting InGaN/GaN multiple quantum well structure with a strong phase separated into quasi-quantum dots (QDs) and an InGaN matrix in the InGaN epilayer are investigated. The excitation power dependences of QD-related green emissions (PD) and matrix-related blue emissions (PM) in the low excitation power range of the PL peak energy and line-width indicate that at 6 K both PM and PD are dominated by the combined action of Coulomb screening and localized state filling effect. However, at 300 K, PM is dominated by the non-radiative recombination of the carriers in the InGaN matrix, while PD is influenced by the carriers transferred from the shallower QDs to deeper QDs by tunnelling. This is consistent with the excitation power dependence of the PL efficiency for the emission.

Growth of non-polar (11-20) InGaN quantum dots by metal organic vapour phase epitaxy using a two temperature method

Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An InGaN epilayer was grown and subjected to a temperature ramp in a nitrogen and ammonia environment before the growth of the GaN capping layer. Uncapped structures with and without the temperature ramp were grown for reference and imaged by atomic force microscopy. Micro-photoluminescence studies reveal the presence of resolution limited peaks with a linewidth of less than ∼500 μeV at 4.2 K. This linewidth is significantly narrower than that of non-polar InGaN quantum dots grown by alternate methods and may be indicative of reduced spectral diffusion. Time resolved photoluminescence studies reveal a mono-exponential exciton decay with a lifetime of 533 ps at 2.70 eV. The excitonic lifetime is more than an order of magnitude shorter than that for previously studied polar quantum dots and suggests the suppression of the internal electric field. Cathodoluminescence studies show the spatial distribution of the quantum dots and resolution limited spectral peaks at 18 K.

Spatially localised luminescence emission properties induced by formation of ring-shaped quasi-potential trap around V-pits in InGaN epi-layers

The correlation between surface structural and luminescence properties of InGaN epi-layers grown by metal organic chemical vapour deposition has been investigated. Ring-shaped regions with high In-content are observed around V-pits by spatial resolved cathodoluminescence and energy dispersive X-ray spectroscopy. These ring-shaped regions are found to form a potential energy well with depth about 300 meV with respect to the ordinary regions, which can attract carriers and suppress non-radiative recombination. The causes of the formation of high-In-content region are discussed. This localised characteristic of carriers is supported further by power and temperature dependent photoluminescence results. The understanding of the spatially emission characteristics might be helpful to improving the internal quantum efficiency of InGaN/GaN light emitting diodes (LEDs).