Pressure Metalorganic Vapor Phase Research Articles

Enhancement of quantum cascade laser intersubband transitions via coupling to resonant discrete photonic modes of subwavelength gratings.

We present an optical spectroscopic study of InGaAs/AlInAs active region of quantum cascade lasers grown by low pressure metal organic vapor phase epitaxy combined with subwavelength gratings fabricated by reactive ion etching. Fourier-transformed photoluminescence measurements were used to compare the emission properties of structures before and after processing the gratings. Our results demonstrate a significant increase of the photoluminescence intensity related to intersubband transitions in the mid-infrared, which is attributed to coupling with the grating modes via so called photonic Fano resonances. Our findings demonstrate a promising method for enhancing the emission in optoelectronic devices operating in a broad range of application-relevant infrared.

Influence of As-N Interstitial Complexes on Strain Generated in GaAsN Epilayers Grown by AP-MOVPE

This work presents an investigation of the fully strained GaAsN/GaAs heterostructures obtained by atmospheric pressure metalorganic vapor phase epitaxy, focusing on the analysis of the strain generated in the GaAsN epilayers and its correlation with the formation of split interstitial complexes (N-As)As. We analyzed strained GaAsN epilayers with nitrogen contents and thicknesses varying from 0.93 to 1.81% and 65 to 130 nm, respectively. The composition and thickness were determined by high resolution X-ray diffraction, and the strain was determined by Raman spectroscopy, while the N-bonding configurations were determined by X-ray photoelectron spectroscopy. We found that the strain generated in the GaAsN epilayers is mainly caused by a lattice mismatch with the GaAs substrate. This macroscopic strain is independent of the amount of (N-As)As interstitial defects, while the local strain, induced by an alloying effect, tends to decrease with an increasing ratio of (N-As)As interstitial defects to substitutional nitrogen atoms incorporated into an arsenic sublattice—NAs. Here, we show experimentally, for the first time, a correlation between the strain in the GaAsN epilayers, caused by an alloying effect determined by Raman spectroscopy, and the (N-As)As/NAs ratio estimated by the XPS method. We found out that the (N-As)As interstitials compensate the local strain resulting from the presence of N in the GaAs matrix, if their amount does not exceed ~65% of the substitutional introduced nitrogen NAs.

Raman spectra of semi-polar (11-22) InGaN thick films

We report the semi-polar (11-22) InGaN thick films (∼1 μm) grown on m-plane sapphire by a low pressure metal-organic vapor phase epitaxy (MOVPE) system. The Raman spectra of semi-polar InGaN thick films have been studied. For Raman spectra excited by the laser with wavelength of 532 nm, the E2(h) modes of GaN around 569 cm−1 and longitudinal optical (LO) phonon-plasma coupled modes (LPP) were mainly discussed. The E2(h) mode of GaN in the InGaN thick film would correspond to the lower frequency with wavenumber of 569.1 cm−1, compared to that in the InGaN thinner film (569.6 cm-1) in our case. Meanwhile, LPP modes also exhibited red shift with the increase of InGaN film thickness, which would be attributed to the stress relaxation and the decrease of free carrier concentration. The E2(h) modes of InGaN around 560 to 565 cm-1 and A1(LO) modes of InGaN around 729 to 731 cm-1 were found in Raman spectra excited by the laser with wavelength of 325 nm, but not probed in Raman spectra excited by a 532 nm laser. With the increase of In component, both modes were shifted towards low frequency. In addition, it was found that the Frölich electron-phonon interaction would significantly affect the peak position (729.2 cm-1) and full width at half maximum (FWHM, 17.2 cm-1) of A1(LO) mode of InGaN in the thicker film. Moreover, we have compared the Raman spectra of polar and semi-polar InGaN thick films so that the difference of Raman spectra between the two counterparts could be indicated due to lattice mismatch, In component incorporation efficiency, crystal quality and phase separation. The stress distribution of semi-polar InGaN materials is not as uniform as that of polar materials.

Optical characterization by photoreflectance of GaN after its partial thermal decomposition

In the current study, we investigated the partial decomposition of GaN layers grown on SiN-treated sapphire substrate in an atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor under N2 ambient at 1200 °C. Under these experimental conditions, we studied the decomposition of different coalescence degrees GaN layers and the early thermal decomposition stages of GaN films. By the help of the scanning electron microscope (SEM), the surface morphology of the decomposed GaN epilayers was examined. We demonstrated that the thermal decomposition leads to an irregular etching process, which was carried out step by step. The optical properties of the decomposed films were then studied by using the photoreflectance (PR) technique at room temperature and at 11 K. We determined a logarithmic relation between the built-in electric field in the surface of GaN layers and the stress value. A model is also proposed to extract the stress value from PR spectra. It should be noted also that by using the thermal decomposition process, the stress inside GaN layers can be reduced by a factor of 6.

Analysis of Current Transport Mechanism in AP-MOVPE Grown GaAsN p-i-n Solar Cell

Basic knowledge about the factors and mechanisms affecting the performance of solar cells and their identification is essential when thinking of future improvements to the device. Within this paper, we investigated the current transport mechanism in GaAsN p-i-n solar cells grown with atmospheric pressure metal organic vapour phase epitaxy (AP-MOVPE). We examined the electro-optical and structural properties of a GaAsN solar cell epitaxial structure and correlated the results with temperature-dependent current-voltage measurements and deep level transient spectroscopy findings. The analysis of J-V-T measurements carried out in a wide temperature range allows for the determination of the dominant current transport mechanism in a GaAsN-based solar cell device and assign it a nitrogen interstitial defect, the presence of which was confirmed by DLTFS investigation.

Correlation of structural and optical properties of AlGaN films grown on SiN-treated sapphire by MOVPE

We investigated the structural and optical properties of AlGaN films grown on SiN-treated sapphire substrates, without and with GaN-template, by atmospheric pressure metal organic vapor phase epitaxy. The samples were characterized using high-resolution X-ray diffraction (HR-XRD), time-resolved photoluminescence (TR-PL), and photoreflectance (PR) spectroscopies. Furthermore, the carrier mobility was determined from Hall-effect measurements. When the AlGaN (GaN-template) layer thickness increases up to 0.6 µm (1.3 µm), an increase in the PL decay times is observed and correlated with the transition from 3D to 2D growth mode resulting in a decrease in the dislocations density as obtained from the HR-XRD measurements. Beyond the aforementioned layer thicknesses, we observed a deterioration in the PL transient corresponds to an increase in the density of VAl-related complexes during the relaxation process, which act as non-radiative recombination centers. Our observations strongly suggest that this type of defects influences the carrier transport and carrier recombination process in the AlGaN layers. Furthermore, our results reveal a phenomenological linear relationship between the internal electric field, obtained from the PR measurements, and the dislocations density. This finding predicts an increase in the GaN internal electric field by about 147 KV/cm when the Al content is increased to 7% in the AlGaN layers. We attribute this increase to a rise in the polarization-induced electric field due to Al incorporation in the AlGaN layer. Based on the obtained correlation between the internal electric field and the dislocation density, we propose an experimental approach that can be utilized to determine the internal electric field, at zero dislocation density, which is very important for designing high-efficient electronic and optoelectronic devices.

The influence of quantum well and barrier thicknesses on photoluminescence spectra of InGaAs/AlInAs superlattices grown by LP-MOVPE

In the presented work, the influence of the quantum well and barrier thicknesses on optical characteristics of InGaAs/AlInAs superlattices was reported. Six different structures of In0.53Ga0.47As/Al0.48In0.52As superlattices lattice-matched to InP were grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE). Optical properties of the obtained structures were examined by means of photoluminescence spectroscopy. This technique allows quick, simple and non-destructive measurements of radiative optical transitions in different semiconductor heterostructures. The analysis of recorded photoluminescence spectra revealed the influence of the quantum well and barrier thicknesses on the emission line energy.

Impact of gallium concentration in the gas phase on composition of InGaAsN alloys grown by AP-MOVPE correlated with their structural and optical properties

This work presents the epitaxial growth and material properties of InGaAsN epilayers obtained by atmospheric pressure metal organic vapour phase epitaxy. The main goal was to obtain InGaAsN quaternary alloys lattice-matched to GaAs in order to apply them as an intrinsic thick absorber in p-i-n solar cells. It allows improvement of their photovoltaic parameters (e.g. short circuit current, open circuit voltage) by reducing the density of misfit dislocations. To overcome the main difficulties connected with achieving InGaAsN composition with In/N ratio of ~ 3, which guarantees a lattice matching to GaAs, epitaxial processes were carried out with different concentration of gallium source in the gas phase. Diffraction curves, measured using HRXRD, indicated that the main aim of this work was achieved for the gas molar ratio Ga/(Ga + In) = 0.935. The optical quality and surface morphology of the investigated structures examined by PL, CER and AFM methods are also presented and discussed.

AlxGa1−xN GROWTH BY Si/N TREATMENT OF SAPPHIRE SUBSTRATE: COMPARISON WITH OTHERS GROWTH TECHNIQUES

AlxGa[Formula: see text]N films were grown on Si/N-treated sapphire substrate by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE) in a home-made vertical reactor. This process can be considered as randomly in situ epitaxial lateral overgrowth (ELO) technology. The growth firstly begins by three-dimensional (3D) mode and is completed in two-dimensional (2D) growth mode as shown by real time in situ laser [Formula: see text][Formula: see text]nm[Formula: see text] reflectometry measurements and confirmed by scanning electron microscopy (SEM) images. Secondary ion mass spectroscopy (SIMS) measurements evidence Al composition pulling effect in the AlxGa[Formula: see text]N layer. The Si/N treatment technique is compared to conventional AlxGa[Formula: see text]N growth techniques. The results of high-resolution X-ray diffraction (HRXRD), photoluminescence (PL) measurements and SEM images agree well on the fact that the Si/N treatment produces AlxGa[Formula: see text]N layers with comparable qualities of AlxGa[Formula: see text]N layers grown on high temperature GaN template but with much higher qualities than AlxGa[Formula: see text]N layers grown on low temperature AlN nucleation layer. Moreover, the Si/N treatment technique permits the growth of high quality AlxGa[Formula: see text]N layers with appreciable thicknesses with respect to the others techniques.

Photoelectrochemical Water Splitting Using Multiple Quantum Well Photovoltaic Devices

Photoelectrochemical (PEC) water splitting is a sustainable process for generating hydrogen gas that can be stored efficiently and converted back to usable electricity in a fuel cell. Solar-to-hydrogen (STH) efficiencies in the range of 10-12% have been demonstrated using GaInP/GaAs dual junction photovoltaic-based devices immersed in electrolyte. With bandgaps of ~1.82 and 1.42 eV, respectively, these baseline devices generate an oversupply of photovoltage, even accounting for the kinetic overpotentials at both the HER and OER electrodes. Progress toward higher efficiency was demonstrated at NREL by growing a GaInP/GaInAs tandem with bandgaps of 1.82 and ~1.2eV, increasing the lattice constant of the lower bandgap material in order to access a lower bandgap alloy. PEC devices based on the “inverted metamorphic multijunction” (IMM) architecture demonstrated STH efficiencies >16%. However, because the very thin epitaxial films of the IMM are removed from the growth substrate and bonded to a secondary, mechanical handle, the IMM is susceptible to growth-related defects that lead to long-term stability problems. The specifics of the secondary bonding have also restricted the post-growth deposition temperature of protective coatings and co-catalysts to <140°C, which limits the available strategies for improving durability and efficiency. Notwithstanding the lower STH efficiency, the on-substrate GaInP/GaAs architecture may nonetheless have advantages in terms of stability, durability and processability that are potentially more important for a working PEC product than STH efficiency. Here we show that the baseline device can be improved by extending the wavelength range of photon absorption by including quantum wells (QWs) in the current-limiting bottom cell. By adding GaAsP/GaInAs QWs in the depleted region of the GaAs bottom junction, the absorption range can be extended from ~870 nm to ~930 nm, leading to an overall increase of >2.5 mA/cm2that is split between the two cells, or equivalently a >1.25 mA/cm2gain in the light-limiting photocurrent of a current-balanced tandem. This represents a >10% increase in the relative STH efficiency compared to a baseline GaInP/GaAs device. Since the baseline device has a voltage oversupply, the ~100 mV drop in voltage because of the QWs is unimportant. Quantum wells are composed of alternating barrier and well layers. Though the design is flexible and may be tuned to a different absorption edge, the design shown here is composed of GaAs0.9P0.1with a bandgap of ~1.55 eV, and Ga0.9In0.1As with a bandgap of ~1.27 eV. The thickness of the well layer is ~85 Å and the two-dimensional quantum confinement pushes up the effective band edge to 1.34 eV (930 nm). Although neither of these alloys alone are lattice-matched to the GaAs substrate, the entire QW region is carefully strain-balanced so that the net strain is nearly zero. The devices reported here were grown by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Growth, fabrication and characterization of the devices will be discussed in greater detail at the conference. Figure 1a shows a not-to-scale schematic of the device with the main layers and QWs indicated. The two cells are separated by a tunnel junction interconnect. Figure 1b shows an (004) x-ray rocking curve of a test structure with 80 quantum wells. The narrow, high intensity peaks indicate good quality interfaces between the barrier and well layers. Strain-balancing these two alloys requires different thicknesses in the two layers, which gives rise to the asymmetry in the satellite positions. Figures 1c and 1d show the incident photon-to-current efficiency (IPCE) and current-voltage curves for a baseline GaInP/GaAs device and an 80-QW device. The increased absorption edge of the bottom cell is clearly visible in Figure 1c. Though not optimized, the increased light-limited photocurrent is visible at 0 V in Figure 1d. With a thicker top cell and improvements to the device isolation, an STH efficiency >13% is achievable with this architecture, even without an anti-reflection coating. Figure 1

Исследование однородности состава по толщине слоев GaInAsP, полученных на подложках InP методом газофазной эпитаксии

GaInPAs/InP heterostructures grown by low pressure (0.1 bar, 600 oC) metal-organic chemical vapor phase deposition were investigated. The thicknesses of grown GaInAsP layers were about 1 µm. For the epitaxial layers Ga&lt;sub&gt;1-x&lt;/sub&gt;In&lt;sub&gt;x&lt;/sub&gt;P&lt;sub&gt;1-y&lt;/sub&gt;As&lt;sub&gt;y)&lt;/sub&gt; with average compositions of x = 0.77 – 0.87 and y = 0.07 – 0.42 the variation of V group elements content y with the epilayer depth were revealed, weher the compositions of V-group elements were changed up to Δy = 0.1 atomic fractions in V group elements sublattice. In most cases, y change occurs in a GaInAsP region up to 200 nm thick adjacent to the InP. In some cases, y changes throughout the whole GaInPAs layer thickness. Fo the epitaxial layers with a satisfactory crystal perfection the less was the mismatch between the substrate and the GaInPAs epitaxial layer, the smaller was the value of Δy. For GaInPAs layers characterized by a low degree of crystal perfection and a high lattice mismatch between GaInAsP and InP layers, the value of Δy was about zero. These data let us suggest that the incorporation of atoms of the V group in the epitaxial layer strongly depends on elastic deformation of the growing monolayer, that is mismatched with the underlying crystal surface.

Dependence of optimum V/III ratio on substrate orientation, and influence of buffer layer on MOVPE grown InSb/GaSb quantum dots

The dependence of optimum V/III ratio on substrate orientation and the influence of buffer layer on the growth of InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this study. The misorientation on the (1 0 0) substrate is suggested to enhance the formation of surface steps, which provides low energy growth sites and hence promotes island nucleation. An increase in V/III ratio from 1.0 to 3.0 resulted in a significant reduction in dot density on both (1 0 0) and (1 1 1) substrates, a phenomenon attributed to the reduced number of indium adatoms available on the growth surface, which simultaneously stimulates a decrease in the migration length of indium species. The shape and size of the dots were observed to be sensitive to the thickness of the buffer layer. Likewise, an increase in buffer layer thickness from 50 nm to ∼300 nm shows the onset of surface corrugation for buffer layer thickness of ∼150 nm, which persisted up to a buffer thickness of ∼250 nm. The corrugated buffer surfaces were observed to introduce order in the arrangement of the dots, which formed preferentially inside the troughs. A reduction in growth rate (from 0.21 to 0.17 nm/s) is presumed to induce sufficient adatom migration and strong bonding between the adsorbate atoms which results in an even buffer layer surface due to an enhancement in surface coverage of adatoms.

Long wavelength stacking induced shift of the near-infrared photoluminescence from unintentional MOVPE grown InGaSb/GaSb quantum wells

The effect of stacking on the near-infrared photoluminescence (NIR-PL) of InGaSb/GaSb quantum wells (QWs) which were inadvertently formed during an attempt to fabricate stacked InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this work. The morphology of uncapped dots was studied by means of scanning probe microscopy (SPM) which shows a significant deviation in the shape and density of dots grown directly on the buffer compared to those that terminated an “embedded-dot” sample. Cross-sectional scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) of the capped structures clearly revealed the formation of QWs in the capped structures. An increase in the number of InSb QD-layers, which metamorphosed into QWs, was observed to cause an increase in the luminescence spectral line width and a long-wavelength shift of the QW PL lines, together with an enhancement in the strength of PL emission. Variations in layer thicknesses and alloy composition introduced as a result of inter-diffusion of Ga and In which is enhanced by the prolonged annealing time of the QDs (during spacer/cap layer deposition) and In adatom migration is suggested to alter the morphology of the capped dots and induce a change in PL peak positions and the spectral linewidth of the NIR low energy lines.

DLTS study of InGaAs and GaAsN structures with different indium and nitrogen compositions

In this article Deep Level Transient Fourier Spectroscopy experiments and various evaluation procedures were used to study emission and capture processes of deep energy levels in intentionally undoped InGaAs and GaAsN semiconductor structures. The examined samples, grown by Atmospheric Pressure Metal Organic Vapour Phase Epitaxy on GaAs substrates, were analyzed at various indium and nitrogen concentrations. Main attention was focused on differences in defect distributions, relations in possible composition and growth condition sensitive defect states. Valuable characteristics of particular In/N contents capable to eliminate or reduce specific impurities are discussed. A possible indium dependent dual state InGaAs complex and a nitrogen and growth condition dependent dual type GaAsN complex was introduced/confirmed. The most balanced samples for further utilizations were achieved for In = 8.9% and N = 1%.

Observation of the early stages of GaN thermal decomposition at 1200 °C under N2

We investigated the partial decomposition of GaN layers grown in an atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor at 1200 °C under N2 ambient. In these conditions, the early stages of GaN thermal decomposition were studied. The GaN decomposition was monitored by in-situ laser reflectometry (LR). The properties of the as grown and decomposed GaN samples were analyzed by scanning electron microscope (SEM). We show that GaN decomposition starts by the formation of GaN nano-grains at the early stages. Then, due to the anisotropy of the decomposition, the GaN nano-grains are more easily etched than the GaN (00.2) surface. The lateral etching may results in local smooth GaN surface formation. After that the depth etching starts again on the etched pits. Room temperature cathodoluminescence (CL) study revealed better optical properties of the GaN grains when compared to the whole GaN surface.

Atomic configurations in AP-MOVPE grown lattice-mismatched InGaAsN films unravelled by X-ray photoelectron spectroscopy combined with bulk and surface characterization techniques

Abstract This work presents the results of X-ray photoelectron spectroscopy studies on the bonding N configuration in InGaAsN epilayers grown by atmospheric pressure metal organic vapour phase epitaxy. Growth temperature has been tuned in order to obtain both, relaxed and strained layers. The studies were concentrated on analysing the influence of the growth temperature, post growth thermal annealing process and surface quality on the formation of Ga-N and In-N bonds as well as N-related defects. The contamination of InGaAsN films by growth precursor residues and oxides has also been addressed. The growth temperature stands out as a decisive factor boosting In-N bonds formation, while the thermal annealing seems to affect the N-related defects density in the layers.

Time-resolved photoluminescence and photoreflectance spectroscopy of GaN layers grown on SiN-treated sapphire substrate: Optical properties evolution at different growth stages

In this paper, we present a systematic study of the optical properties evolution of GaN films during the complete growth process on SiN-treated sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy. The growth process was monitored using in-situ laser reflectometry and was interrupted at different stages to obtain the studied samples. The obtained samples were ex-situ characterized by means of photoluminescence (PL), photoreflectance (PR) and time-resolved PL (TRPL) spectroscopies. The PL emission from the samples of the initial growth stages originates from nano-crystallite and defect states due to the 3D growth mode. However, with increasing layer thickness, the 2D growth mode is established, and the PL spectrum is dominated by free-exciton emission. The electric field extracted by applying the Franz-Keldysh oscillation (FKO) theory on the PR spectra shows a trend to decrease as the GaN layer thickness is increased. For fully coalesced layers, the FKO totally disappears, and the PR spectrum is dominated by free-exciton transitions. TRPL measurements demonstrate the contribution of two processes to the PL decay, i.e., fast and slow components. While the slow decay time reveals the same sensitivity to different types of dislocations (twist and tilt mosaics), the fast decay time is more affected by the twist mosaic than by the tilt one.

Photoluminescence and structural properties of unintentional single and double InGaSb/GaSb quantum wells grown by MOVPE

The structural and photoluminescence (PL) characterization of InGaSb quantum well (QW) structures grown on GaSb substrate (100) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) is presented. Both structures (single and double-InGaSb QWs) were inadvertently formed during an attempt to grow capped InSb/GaSb quantum dots (QDs). In this work, 10K PL peak energies at ~735meV and ~740meV are suggested to be emissions from the single and double QWs, respectively. These lines exhibit red shifts, accompanied by a reduction in their full-widths at half-maximum (FWHM) as the excitation power decreases. The presence of a GaSb spacer in the double QW was found to increase the strength of the PL emission, which consequently gives rise to a reduced blue-shift and broadening of the PL emission line observed for the double QW with an increase in laser power, while the low thermal activation energy for the quenching of the PL from the double QW is attributed to the existence of threading dislocations, as seen in the bright field TEM image for this sample.

A cross‐sectional scanning capacitance microscopy characterization of GaAs based solar cell structures

Scanning Capacitance Microscopy was applied in investigation of individual components of tandem solar cell. The InGaAs tunnel junction, GaAs top subcell and GaAsN bottom subcell devices were grown by atmospheric pressure metalorganic vapour phase epitaxy. It was shown that accuracy of p‐n junction position determination by SCM method depends on doping concentration of layers in structure and measurement parameters.

Effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN grown by MOVPE

In this work, the effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN are systematically investigated. For this reason, a number of GaN epilayers with different impurity/defect concentrations have been grown on c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Variable-temperature photoreflectance (PR) spectroscopy is used, in the temperature range from 10K to 300K, to study the thermal broadening of excitonic transitions. A comprehensive comparison between the experimental data and the available theoretical models is presented. The results show that the theoretical models, in which the exciton scattering with thermal-ionized impurity is ignored, fail to account for the experimental data. However, when the exciton-ionized impurity interaction is considered, the simulation results become in good agreement with the experimental data. Based on the last approach, the parameters that describe the thermal broadening of the exciton linewidth are accurately evaluated and discussed. It is found that the exciton-LO phonon coupling strength, which suffers from scattering in the literature, is almost constant for the different samples (ΓLO≈255±15meV), and it is independent to impurity concentration. Furthermore, it is observed that the free exciton transitions have not the same sensitivity to the tilt and twist mosaic interaction, and the C-exciton undergoes a stronger dislocation scattering than FXB and FXA.