Density Of V-pits Research Articles

Tunable structured AlGaN-based nanoporous distributed Bragg reflectors for light-coupling enhancement in monolayer MoS2

In this work, we demonstrated tunable structured AlGaN-based nanoporous (NP) distributed Bragg reflectors (DBRs) with high-reflectivity through one-step vertical-lateral electrochemical etching (ECE). The aperture of nanopores in the doping layer and the size and density of V-pits on the surface of NP-DBRs can be tuned by controlling the etching bias. Furthermore, by further tuning the thickness of alternating layers, we achieved high reflectivity of NP-DBRs at multiple center wavelengths within the wavelength range from 280 to 800 nm. To explore its potential applications, MoS2 with a photoluminescence (PL) emission in the red light region was grown for the first time on the NP-DBRs. Due to the light localized effect of surface V-pits and the high reflectivity of the bottom DBRs, the PL intensity of MoS2 was significantly enhanced by a factor of 37.3 compared to the region without NP-DBRs. The PL enhancement of MoS2 on porous AlGaN and different periodic NP-DBRs further validated the PL enhancement mechanism. The corresponding theoretical simulations also revealed that the PL enhancement should be attributed to the combined effect of enhanced light absorption and NP-DBRs induced emission interference. The fabricated AlGaN-based NP-DBRs provide a new approach for improving the quantum efficiency of two-dimensional layered materials, which holds significant importance for advancing the development of miniaturized and integrated 2D/3D heterogeneous structure systems in the field of optoelectronics.

Reduction of V-pit density and depth in InGaN semibulk templates and improved LED performance with insertion of high temperature semibulk layers

Highly relaxed InGaN templates with an effective In-content of ∼10% that exhibit reduced V-pit density and an improved surface roughness are reported using the semibulk (SB) growth approach. This was achieved by the insertion of five period high temperature SB (HTSB) InGaN SB regions. This report demonstrates that better quality InGaN templates can be achieved by the insertion of HTSB within the templates, rather than by ending the templates with a superlattice structure or by refilling the pits with GaN interlayers. Three SB samples were grown with and without the HTSB layers. Using secondary-ion mass spectrometry, photoluminescence, and x-ray diffraction, the effective In-content of the templates was determined to be 9.6%, 5.8%, and 8.7%. Using atomic force microscopy, the surface roughness was found to improve from 4.4 to 1.7 nm by using the two HTSB regions, and the average V-pit density and depth improved from 7.6 × 10−7 to 4.5 × 10−7 cm−2 and 8.2 to 2.8 nm, respectively. Also, the maximum V-pit depth was reduced from about 30.5 nm to about 9.6 nm in the sample with the HTSB regions. Two LEDs were studied, one with both HTSB regions, and one with only the topmost HTSB. The optical power density of the LED with both HTSB regions was 1.4 times higher at the peak injection current, displayed a ∼1.3 times higher external quantum efficiency peak, and a delay of the EQE droop onset. These results show that higher In-content SB templates can be improved with the implementation of a modified growth approach.

V-pits formation in InGaN/GaN: influence of threading dislocations and indium content

Two sets of InGaN/GaN MOVPE-grown samples were studied by high-resolution x-ray diffraction techniques together with statistical analysis of atomic force microscope images in order to determine the impact of In concentration and threading dislocations (TDs) density on the V-pit formation. It was shown that in our samples, the density of V-pits in the epilayer matched the TD density with a screw component in the substrate. Pure edge TDs do not affect the V-pit density. The In concentration influences the size of the V-pits, but not their density.

Re-engineering transition layers in AlGaN/GaN HEMT on Si for high voltage applications

We report on the study of step-graded AlGaN transition layers (TLs) in metalorganic chemical vapor deposition-grown GaN HEMT-on-silicon toward improving the breakdown field while minimizing buffer-induced current dispersion. The transition layers include three AlGaN epi-layers of 75%, 50%, and 25% Al-content, downgraded from bottom to top. The growth temperature and carbon doping are varied independently to assess the transition layer's role in reducing current collapse and leakage current. We observe that the introduction of High Temperature (HT) AlGaN increases the lateral but decreases the vertical leakage, the latter being attributed to the reduction of V-pit density. Temperature-dependent data indicate that the increased lateral (mesa) leakage current in HT AlGaN layers is due to space charge limited current, the activation energy of which yields the positions of the defect states within the bandgap. The increase in mesa leakage current in HT AlGaN layers is attributed to the formation of point defects such as oxygen in nitrogen site (ON) and VGa–ON complexes. The introduction of C-doping in the top AlGaN transition layer with 25% Al-content helps reduce lateral leakage in both mesa and 3-terminal configurations. The combination of HT AlGaN (75% Al-content) with C-doped AlGaN (25% Al-content) is found to be the optimal TL design that yielded a minimum buffer-induced current dispersion with a 65% channel recovery when the substrate was swept to −300 V and back; moreover, it also enabled a vertical breakdown field of 2.05 MV/cm defined at 1 A/cm2 for a buffer thickness of 1.65 μm.

Vertical breakdown of GaN on Si due to V-pits

Gallium nitride on silicon (GaN/Si) is an important technological approach for power electronic devices exhibiting superior performance compared to devices based on a pure silicon technology. However, the material defect density in GaN/Si is high, and identification of critical defects limiting device reliability is still only partially accomplished because of experimental difficulties. In this work, atomic force microscopy, scanning electron microscopy, secondary ion mass spectrometry, and cathodoluminescence were employed to investigate commonly occurring epitaxial overgrown V-pits and inhomogeneous incorporation of oxygen and carbon across layer stacking in the vertical direction. These experiments identified V-pits as regions with higher n-type carrier concentrations and paths for vertical leakage through the buffer, as directly probed by conductive atomic force microscopy. The deleterious effect of V-pits on device performance is demonstrated by evaluating test devices fabricated on two wafers with significantly diverse density of buried V-pits induced by varying growth conditions of the aluminum nitride nucleation layer. A clear correlation between observed vertical breakdown and density of V-pits within the C-doped GaN layer below the device structures is obtained. Back-gating transient measurements also show that the dynamic device behavior is affected by the V-pit density in terms of the detrapping time constants.

Improvement of crystallinity and luminescence of GaN-based laser diode structure with suppressed curvature variation in active layers

The influence of curvature variation on the crystallinity and luminescence of GaN-based blue laser diode (LD) structure is comprehensively investigated during the growth of InGaN/GaN active layers. Compared with InGaN/GaN multiple quantum well (MQWs) grown by the conventional 2-temperature deposition, curvature variation is successfully suppressed and a much gentler deformation from wells to barriers is obtained by employing same-temperature deposition method (STDM). With less curvature fluctuation, the V-pit density of InGaN/GaN MQWs grown on sapphire is effectively decreased from 4.5 × 108/cm2 to 2.8 × 108/cm2 with average root-mean-square roughness of 0.55 nm, while threading dislocation density of the GaN-based LD structures grown on FS-GaN is also reduced from 5.9 × 106/cm2 to 1.6 × 106/cm2. Additionally, an ~ 7 nm redshift in photoluminescence emission for LD structure is achieved, accompanied by a ~ 6 times higher emission intensity. A more uniform distribution of emission wavelength along the radial direction is observed and the full width at half maximum is also narrowed, indicating that the STDM is advantageous to effectively eliminate the negative influence of curvature variation and contribute to fabricating high-performance GaN-based light-emitting diodes and LDs.

Effect of TMAl flow rate on AlInGaN/GaN heterostructures grown by MOCVD

In this work, lattice-matched AlInGaN/GaN epilayers are grown on c-plane Al2O3 substrate by metal organic chemical vapor deposition technique. The structural and optical properties of AlInGaN/GaN epilayers have been studied and compared. TMIn and TMAl flow is believed to have a direct impact on the quality of AlInGaN epilayer. By varying In and Al composition in AlInGaN epilayer, dislocations (V-pit density) have been found to decrease, indicating lower threading dislocation density as confirmed by HRXRD and AFM. Reciprocal space mapping analysis has confirmed that the growth of AlInGaN on GaN buffer is a fully lattice matched growth. AlInGaN exhibits two emissions originating from In(Ga)N clusters and the AlInGaN random matrix is confirmed by photoluminescence. In the Raman spectra of AlInGaN, A1 (LO) mode at 751 cm−1 and In(Ga)N clustering A1(LO) mode at 680 cm−1 is observed. These findings open the new advancement in the field of opto-electronic applications.

Influence of waveguide strain and surface morphology on AlGaN-based deep UV laser characteristics

The waveguide strain and the surface morphologies of AlGaN-based laser heterostructures emitting in the deep UV spectral range have been investigated. In particular, the impact of the AlGaN heterostructure design on the strain relaxation as well as the effect of the growth temperature on the surface morphology were explored. We found strain-induced plastic relaxation for laser heterostructures with 130 nm thick Al0.45Ga0.55N waveguide layers, whereas pseudomorphic growth was obtained for laser heterostructures with Al0.70Ga0.30N waveguide layers. Optically pumped lasing near 280 nm was demonstrated for the coherently grown laser heterostructures. A strong correlation of the surface morphology with the waveguide growth conditions was also observed. Low growth temperatures of 900 °C lead to a high density of V-pits originating from dislocations. By increasing the growth temperatures to 1070 °C the V-pit density significantly decreases, resulting in a more than two-fold reduction of the threshold power density of optically pumped lasers.

Porous AlGaN-Based Ultraviolet Distributed Bragg Reflectors.

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process to realise AlGaN-based ultraviolet distributed Bragg reflectors (DBRs). These are of interest because they could provide a pathway to non-absorbing UV reflectors to enhance the performance of UV LEDs, which currently have extremely low efficiency. We have demonstrated porous AlGaN-based UV DBRs with a peak reflectance of 89% at 324 nm. The uniformity of these devices is currently low, as the as-grown material has a high density of V-pits and these alter the etching process. However, our results indicate that if the material growth is optimised, the ECE process will be useful for the fabrication of UV reflectors.

Effect of strain modification on crystallinity and luminescence of InGaN/GaN multiple quantum wells grown by MOCVD

The effects of compressive strain modification on green-emitting InGaN/GaN multiple quantum wells (MQWs) are comprehensively investigated by inserting a bulk InGaN prestrain underlying layer with different indium doping contents. Systematic measurements of the structural, morphological and optical properties are conducted. Better crystallinities are obtained for both modified samples, in which the density of V-pits is reduced from 4.0 × 108 /cm2 in the original sample to 1.9 × 108 /cm2 and 3.3 × 108 /cm2. Compared with the conventional InGaN/GaN MQWs, the photoluminescence (PL) emission wavelength of the strain-modified MQW structure with an InGaN (In = 0.03) prestrain layer is blueshifted by ~ 6 nm as a result of the reduced built-in piezoelectric field, while the other MQW structure with an InGaN (In = 0.1) layer has an ~ 15 nm redshift, which is attributed to not only the ~ 1% increase in the In content in the QWs and the ~ 0.6 nm increase in the well width due to the composition pulling effect, but also the ~ 0.19 degree of strain relaxation in the InGaN QWs. The low-temperature PL (LTPL) intensities of both samples are approximately twice as large as that of the conventional one, whereas the internal quantum efficiency is three times larger at the maximum. These tremendous improvements are mainly due to the weakened quantum confined stark effect and stronger carrier localization states, as confirmed by LTPL and temperature-dependent PL (TDPL) measurements.

InGaN stress compensation layers in InGaN/GaN blue LEDs with step graded electron injectors

We investigate the effect of InGaN stress compensation layer on the properties of light emitting diodes based on InGaN/GaN multiple quantum well (MQW) structures with step-graded electron injectors. Insertion of an InGaN stress compensation layer between n-GaN and the step graded electron injector provides, among others, strain reduction in the MQW region and as a result improves epitaxial quality that can be observed by 15-fold decrease of V-pit density. We observed more uniform distribution of In between quantum wells in MQW region from results of electro- and photoluminescence measurement. These structural improvements lead to increasing of radiant intensity by a factor of 1.7–2.0 and enhancement of LED efficiency by 40%.

Effect of small flow hydrogen treatment at the upper well/barrier interface on the properties of InGaN/GaN multiple quantum wells

To enhance the quality of InGaN/GaN multiple quantum wells (MQWs), a small hydrogen flow is introduced to treat the upper well/barrier interface. High-resolution X-ray diffraction results indicate that hydrogen treatment improves the interface quality of MQWs. Room temperature photoluminescence (PL) tests show that integrated PL intensity is enhanced by 57.6% and line width is narrowed, while emission peak energy is almost unchanged. On the basis of temperature-dependent PL characteristics analysis, it is concluded that hydrogen treatment decreases non-radiative recombination centers in active region, yet has little impact on carrier localization. Moreover, surface roughness and V-pit density are significantly reduced after hydrogen treatment as revealed by atomic force microscopy. Because the emission energy is quite stable after the hydrogen treatment, this method can also be promoted to improve the quality of green and yellow-green emission MQWs.

Performance Enhanced by Inserting an InGaN/GaN Shallower-Quantum Well Layer in InGaN Based Green Laser Diodes

A series of samples with a different InGaN/GaN shallower-quantum well (SQW) interlayer are grown, and their effects on optical properties of InGaN/GaN multi-quantum wells and optical confinement factor of laser diodes (LDs) are investigated. It is found that when an SQW interlayer is inserted below InGaN/GaN green multiquantum wells (GMQWs), the emission intensity of the GMQW active region increases. This is attributed to the reduction of quantum confined stark effect and the V-pit density due to the decreased strain in GMQWs. In addition, we also find that inserting an SQW interlayer theoretically results in a weak leakage of the optical mode for green LDs. Therefore, their performance will be improved.

Structural and optical investigations of AlGaN MQWs grown on a relaxed AlGaN buffer on AlN templates for emission at 280 nm

10-period Al0.57Ga0.43N/Al0.38Ga0.62N multi-quantum wells (MQWs) were grown on a relaxed Al0.58Ga0.42N buffer on AlN templates on sapphire. The threading dislocations and V-pits were characterized and their origin is discussed. The influence of V-pits on the structural quality of the MQWs and on optical emission at 280nm was analyzed. It was observed that near-surface V-pits were always associated with grain boundaries consisting of edge threading dislocations originating from the AlN/Al2O3 interface. Although the high density of V-pits disrupted MQWs growth, it did not affect the internal quantum efficiency which was measured to be ~1% at room temperature even when V-pit density was increased from 7×107cm−2 to 2×109cm−2. The results help to understand the origin, propagation and influences of the typical defects in AlGaN MQWs grown on AlN/Al2O3 templates which may lead to further improvement of the performance of DUV devices.

Growth and Characterization of High-Quality, Relaxed In y Ga1−y N Templates for Optoelectronic Applications

Thick, high-quality InGaN layers can be used as templates for quantum well strain reduction in light-emitting diodes and as optical absorption layers in solar cell structures. Current InGaN growth technology, however, is primarily limited by V-pit formation and non-uniform indium composition. We report the growth and characterization of thick, strain-relaxed In y Ga1− y N layers, with 0.08 ≤ y ≤ 0.11, by metal organic chemical vapor deposition using the semibulk approach, which consists in periodic insertion of 2-nm GaN interlayers into the bulk In y Ga1− y N structure; these are then spike-annealed at 1000°C. Photoluminescence, x-ray diffraction, and scanning electron and atomic force microscopy revealed that the semibulk In y Ga1− y N had optical and electrical properties superior to those of conventional bulk In y Ga1− y N grown at the same temperature. Homogeneous indium content and substantial reduction of V-pit density were observed for the semibulk In y Ga1− y N films, even when grown above the critical layer thickness. Double-crystal x-ray diffraction rocking curves also revealed a one order of magnitude reduction of screw dislocation density in the semibulk In y Ga1− y N film compared with the bulk In y Ga1− y N film.

MOCVD epitaxy of InAlN on different templates

InAlN epilayers were grown on high quality GaN and AlN templates with the same growth parameters. Measurement results showed that two samples had the same In content of ∼ 16%, while the crystal quality and surface topography of the InAlN epilayer grown on the AlN template, with 282.3″ (002) full width at half maximum (FWHM) of rocking curve, 313.5″ (102) FWHM, surface roughness of 0.39 nm and V-pit density of 2.8 × 108 cm−2, were better than that of the InAlN epilayer grown on the GaN template, 309.3″, 339.1″, 0.593 nm and 4.2 × 108 cm−2. A primary conclusion was proposed that both the crystal quality and the surface topography of the InAlN epilayer grown on the AlN template were better than that of the InAlN epilayer grown on the GaN template. Therefore, the AlN template was a better choice than the GaN template for getting high quality InAlN epilayers.

The influence of various MOCVD parameters on the growth of Al 1− xIn xN ternary alloy on GaN templates

We have studied the growth of AlInN lattice matched to GaN. We present the effect of reactor pressures, ammonia flux, total nitrogen gas flow, and the presence of hydrogen on the growth of Al-rich Al 1− x In x N epilayers ( x<0.2) on GaN templates on (0 0 0 1) sapphires by MOCVD. The reactor pressure is found to be a critical parameter not only for the surface morphology but also for indium incorporation. Our experimental result ascertains that the main controlling factor associated with the pressure is the chemical parasitic reaction. With an increase in NH 3 flux from 2.5 to 5 slm, the surface roughness has substantially improved from 8.3 to 2.3 nm and the V-pit density has reduced from 7.2×10 8 to 3.2×10 8 cm −2. This significant improvement is ascribed to the enhanced lateral growth rate induced by the raised V/III ratio. The effects of hydrogen with different flow rates during the growth of ∼200 nm-thick Al 1− x In x N epilayers have been investigated with respect to the surface morphology, bulk crystalline quality, indium incorporation and strain relaxation.

Influence of penetrating V-pits on leakage current of GaN based p-i-n UV detector

The leakage mechanism of GaN-based p-i-n （p-AlGaN/i-GaN/n-GaN） UV detector has been investigated. With the same dislocation density, devices made from material with higher density of V-pits on surface produce larger leakage current. SEM images show that some V-pits penetrate into i-GaN layer, sometimes even the n-GaN layer. If p-ohmic contact metal （Ni/Au） deposits in the V-pits, Schottky contact would be formed at the interface of metal and i-GaN, or form ohmic contact at the interface of metal and n-GaN. The existence of parallel Schottky junction and ohmic contact resistance enhances the leakage current greatly.

Influence of growth temperature on properties of AlGaInN quaternary epilayers

Aluminum gallium indium nitride（AlGaInN） quaternary epilayers were prepared by metalorganic chemical vapor deposition （MOCVD） on c-plane sapphire （α-Al2O3） substrates. Three samples were grown under different temperatures of 800 ℃，850 ℃ and 900 ℃. It is found that the In composition monotonically decreases with the increasing growth temperature，while the Al composition is nearly invariable. The V-pits appeared when the growth temperature increases to 850 ℃, and the size and density of V-pits drastically decrease and the nucleation of V-pits is passivated when the growth temperature rises to 900 ℃ due to the desorption enhancement of segregated In atoms.

Surface morphological studies of green InGaN/GaN multi-quantum wells grown by using MOCVD

Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) grown by using metal organic chemical vapor deposition has been analyzed by using atomic force microscopy, scanning electron microscopy, and cathodoluminescence (CL). Effects of barrier growth temperature, indium composition, and background threading dislocation (TD) density on the evolution of the MQW surface morphology have been studied. Low temperature GaN barrier growth generates a high density of V-pits and inclusions embedded within V-pits. Using trimethylindium flow prior to InGaN well growth, In segregation on GaN barrier is shown to be the prime cause for the formation of some inclusions which appear as bright spots in CL mapping while some inclusions are related to low temperature barrier growth. High temperature GaN barrier growth at 910°C completely suppresses these inclusion defects. In high indium containing InGaN/GaN MQWs, the large lattice mismatch induced strain in the MQW plays the key role in producing V-pits as well as inclusion defects rather than the background TD density of GaN templates.