Pressure MOVPE Research Articles

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.

Fast and efficient approach for multi-component quantum wells analysis based on FFT

Elaboration of the technology of the multicomponent Quantum Wells (QWs) based devices requires repeated time consuming diffraction curves analysis proceeded for each of the grown structures to determine their basis structural parameters. Optimization of the diagnostic procedures and making them more efficient is the key point from the manufacturer point of view. In this paper we present the use of Fast Fourier Transform (FFT) employed to fast determination of the thickness of epitaxial layers. The presented method combines the approach using the Rocking Curves (RC) measured by X-ray Diffraction (XRD) together with the analysis of the FFT spectrum. In this work GaInNAs/GaAs MQWs structures grown by atmospheric pressure-metalorganic vapour phase epitaxy (AP-MOVPE) were investigated. In the case of the series of the defined type samples, this method allows replacement of the time consuming simulation method used for diagnostic of the thickness of deposited layers in favor of FFT spectrum analysis.

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.

Impact of Carrier Gas on the GaN Layers Properties Grown on (001) and (11n) GaAs Substrates by AP-MOVPE: Comparative Study

The impact of carrier gas on the GaN layers properties grown by atmospheric pressure metal-organic vapor-phase epitaxy (AP-MOVPE) on (001) and (11n) GaAs substrates were investigated. The Arrhenius plots of growth rate deduced from laser reflectometry measurements give an activation energy of Ea1 = 0.045 eV when the H2 was used as the carrier gas. In the case of using N2 as the carrier gas, the results give Ea2 = 0.081 eV as a value of activation energy, which is approximately 2 times greater than Ea1. Scanning electron microscopy results show that when N2 is used, the resulting material quality is low, but the use of H2 is successful to prevent the cracking of GaN layers and results in improvement of crystalline properties. From the X-ray diffraction result, we conclude that both (001) and (113) GaAs substrate orientations as well as the use of H2 as the carrier gas favors the GaN growth with cubic structure, whereas the GaN hexagonal structure is favored for growth on (112) and (111) GaAs substrates orientations with N2. Cathodoluminescence measurements show that a mechanism of phase transformation occurs when the growth temperature rise from 800 to 900°C.

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

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.

MOVPE growth and transport characterization of Bi2−xSbxTe3−ySey films

We present a first study of films of the quaternary Bi2−xSbxTe3−ySey solid solutions on (0001) sapphire substrates grown by atmospheric pressure MOVPE. Trimethylbismuth, trimethylantimony, diisopropylselenide and diethyltelluride were used as precursors. To passivate the free bonds of the substrate and to improve the epitaxy, a thin (15 nm) ZnTe buffer layer was first grown. EDX analysis of the films grown at a temperature of 445 °C and about 10-fold excess of chalcogen in the vapor phase indicates on their compliance with V2VI3 stoichiometry. AFM and SEM investigations showed that at the initial stage of deposition the Stranski-Krastanov growth mode is dominant. Complete coalescence of nanoislands occurs at a thickness about 60 nm and further film formation is in the 2D layer-by-layer growth mode. A high mole fraction of antimony in the vapor phase leads to bad crystalline quality of the films and even to their discontinuity. Transport properties of the Bi2−xSbxTe3−ySey films were evaluated using Van der Pauw Hall effect measurements in the range of temperatures of 10–300 K. Some films are always n- or p-type; in other samples the change of conductivity from p- to n-type was observed when the temperature decreases.

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.

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.

MOVPE growth of InAsBi/InAs/GaAs heterostructure analyzed by in situ spectral reflectance

InAsBi/InAs heterostructures were elaborated on semi-insulating GaAs substrate by atmospheric pressure metalorganic vapor phase epitaxy. Spectral reflectance in the range of 400–800 nm was employed to in situ monitor epitaxy. In order to determine optical constants of InAsBi films, an optical model incorporating two parameters: time-dependent surface roughness and time-dependent growth rate was established to simulate the in situ reflectance. Since bismuth has a crucial effect on surface kinetic mechanisms during growth of III–V semiconductors, a theoretical motivation introducing these two parameters instead of a standard single rms roughness and growth rate was provided. In order to evaluate the structural and optical properties of this material, reflectivity responses were analyzed for several wavelengths. Results were ex situ correlated with atomic force microscopy measurements.

Characterization of deep-level defects in GaNAs/GaAs heterostructures grown by APMOVPE

AbstractConventional deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS techniques were used to study electrical properties of deep-level defects in dilute GaNAs epitaxial layers grown by atmospheric-pressure metalorganic vapourphase epitaxy (APMOVPE) on the GaAs substrate. Three samples with nitrogen concentrations of 1.2 %, 1.6 % and 2.7 % were investigated. In DLTS and LDLTS spectra of the samples, four predominant electron traps were observed. On the basis of the obtained electrical parameters and previously published results, one of the traps was associated with N-related complex defects, while the other traps with common GaAs-like native defects and impurities, called EL6, EL3 and EL2.

An investigation of near-infrared photoluminescence from AP-MOVPE grown InSb/GaSb quantum dot structures

In this work, the near-infrared photoluminescence (PL) of InSb/GaSb QD structures grown on GaSb substrate (2° off (100)) using atmospheric pressure Metalorganic Vapor Phase Epitaxy is investigated. The structures are analyzed before capping and after capping using scanning probe microscopy and high resolution transmission electron microscopy (HRTEM), respectively. At 10K, with an excitation power of 2mW, a PL peak at ∼ 732meV is observed. Upon an increase in laser power to 120mW, a blue shift of ∼ 8meV is noticed. This emission typically persists up to 60–70K, after which it becomes weak. An SPM analysis of the size distribution of uncapped dots reveals a mono-modal distribution with an average density of ∼ 5×1010cm−2. However, a HRTEM investigation of the capped dots reveals the formation of an InGaSb quantum well-like structure, ∼ 10nm thick, which gives rise to the PL signal mentioned above.

Growth of Bi2Te3 films and other phases of Bi-Te system by MOVPE

We have deposited films of Bi-Te system by atmospheric pressure MOVPE on (0001) Al2O3 substrates with thin ZnTe or thick GaN buffer layers at different temperatures and Te/Bi ratio in the vapor phase. As-grown films were studied by X-ray diffractometry, SEM microscopy and Raman spectroscopy. To determine the elemental composition of the films, an energy dispersive spectrometer was used. Single-phase films of Bi2Te3, Bi4Te5, BiTe, Bi10Te9, Bi4Te3, Bi3Te2 have been grown and growth parameter ranges for obtaining different phases were defined. It was found that under the same growth condition different phases of the Bi-Te system realize depending on the film's thickness. Thus, when growing of Bi2Te3 films by MOCVD method the careful control of the phase composition is required.