Organometallic Vapor Phase Epitaxy Research Articles

In Situ Smoothing of Facets on Spalled GaAs(100) Substrates during OMVPE Growth of III-V Epilayers, Solar Cells, and Other Devices: The Impact of Surface Impurities/Dopants.

One possible pathway toward reducing the cost of III-V solar cells is to remove them from their growth substrate by spalling fracture, and then reuse the substrate for the growth of multiple cells. Here we consider the growth of III-V cells on spalled GaAs(100) substrates, which typically have faceted surfaces after spalling. To facilitate the growth of high-quality cells, these faceted surfaces should be smoothed prior to cell growth. In this study, we show that these surfaces can be smoothed during organometallic vapor-phase epitaxy growth, but the choice of epilayer material and modification of the various surfaces by impurities/dopants greatly impacts whether or not the surface becomes smooth, and how rapidly the smoothing occurs. Representative examples are presented along with a discussion of the underlying growth processes. Although this work was motivated by solar cell growth, the methods are generally applicable to the growth of any III-V device on a nonplanar substrate.

Strategy for reliable growth of thin GaN Caps on AlGaN HEMT structures

This paper presents the growth of thin GaN capping layers on standard AlGaN HEMT structures. It has been found that the reliable growth of thin (≤5nm) GaN capping layers by organometallic vapour phase epitaxy is challenging as GaN is unstable at high growth temperatures even in atmospheres with high ammonia partial pressure. To overcome this challenge a growth strategy based on the controlled desorption of GaN has been adopted. By intentionally growing thicker than desired capping layers and controlling the desorption during the cool down after growth it is feasible to reliably grow high quality GaN capping layers with a specific target thickness. The development of the controlled desorption process has been simplified by predicting the desorption based on the computer controlled cooling ramp and the temperature dependent GaN desorption rate. The latter was obtained by analysing in-situ reflectance traces for relevant growth conditions. Moreover, examples on how to identify exposed AlGaN barriers, i.e. without intact GaN caps, by TEM and AFM are presented.

Demonstration of GaN:Eu/GaN nanowire light emitting diodes grown by selective-area organometallic vapor phase epitaxy

We report on the demonstration of GaN:Eu/GaN nanowire (NW) LEDs grown by organometallic vapor phase epitaxy (OMVPE). The GaN:Eu/GaN NW LED structures with a large aspect ratio (>8) are formed by selective-area OMVPE, and have pedestals that are free from lateral overgrowth of p-GaN in order to prevent short circuiting. The structures are embedded in polydimethylsiloxane followed by a wet-etching process to expose the tips of NW LEDs for contacting. Red luminescence with a narrow linewidth originating from Eu3+ ions is observed at room temperature under current injection in the GaN:Eu/GaN NW LEDs.

Physical origins of optical anisotropy in quantum-confined semiconductors: The roles of valence band mixing, transition broadening, and state filling

We investigate experimentally and theoretically the impact of valence band mixing and spectrum of confined states on the polarization of light emitted from or absorbed by GaAs/AlGaAs semiconductor quantum dots and quantum wires with tailored heterostructure potential. In particular, such nanostructures with parabolic-profile confinement potentials, realized by organometallic vapor phase epitaxy inside pyramidal pits, served as model systems for the study. Different degrees of linear polarization (DOLP) of emitted light, depending on the confinement potential profile, the specific excitonic transition, and the level of excitation, are observed. A theoretical model shows that, besides the impact of valence band mixing, the overlap of conduction and valence band wavefunctions as well as state occupation probability and broadening of transitions determine the DOLP. The conclusions are useful for the design of quantum light emitters with controlled polarization properties.

Formation and optical characteristics of GaN:Eu/GaN core–shell nanowires grown by organometallic vapor phase epitaxy

We report on the formation and optical characteristics of Eu-doped GaN (GaN:Eu)/GaN core–shell nanowires (NWs) grown by organometallic vapor phase epitaxy. A moderate V/III ratio and growth temperature enable the incorporation of Eu3+ ions into the GaN shells on GaN core NWs, resulting in red luminescence at room temperature whose peak intensity is more than 20 times larger than for a GaN:Eu film on planar GaN. The cathodoluminescence characterization corroborates that the observed enhanced Eu luminescence originates from the predominant formation of highly efficient Eu centers in the sidewalls of the GaN:Eu/GaN core–shell NWs.

Burton-Cabrera-Frank theory for surfaces with alternating step types

Burton-Cabrera-Frank (BCF) theory has proven to be a versatile framework to relate surface morphology and dynamics during crystal growth to the underlying mechanisms of adatom diffusion and attachment at steps. For an important class of crystal surfaces, including the basal planes of hexagonal close-packed and related systems, the steps in a sequence on a vicinal surface can exhibit properties that alternate from step to step. Here we develop BCF theory for such surfaces, relating observables such as alternating terrace widths as a function of growth conditions to the kinetic coefficients for adatom attachment at steps. We include the effects of step transparency and step-step repulsion. A general solution is obtained for the dynamics of the terrace widths, assuming quasi-steady-state adatom distributions on the terraces. An explicit simplified analytical solution is obtained under widely applicable approximations. From this we obtain expressions for the full-steady-state terrace fraction as a function of growth rate. Fits of the theoretical predictions to recent experimental determinations of the steady state and dynamics of terrace fractions on GaN (0001) surfaces during organometallic vapor phase epitaxy give values of the kinetic coefficients for this system. In Appendixes, we also connect a model for diffusion between kinks on steps to the model for diffusion between steps on terraces, which quantitatively relates step transparency to the kinetics of atom attachment at kinks, and consider limiting cases of diffusion-limited, attachment-limited, and mixed kinetics.

Elucidation of the excitation mechanism of Tb ions doped in AlxGa1−xN grown by OMVPE toward a wavelength-stable green emitter

The trivalent terbium ion (Tb3+) emits ultra-stable visible light consisting of blue, green, yellow, and red. Tb-doped semiconductors are candidates for novel full-color light sources in next-generation displays. Particularly, Tb-doped AlxGa1−xN (AlxGa1−xN:Tb) has attracted much attention for device applications. We present the luminescence properties of AlxGa1−xN:Tb grown by the organometallic vapor phase epitaxy. At 15 K, emission related to the 5D4–7FJ (J = 3, 4, 5, 6) transitions is observed for AlxGa1−xN:Tb with x ≥ 0.03. Thermal quenching of emission originating from the 5D4–7FJ transition is suppressed for higher Al compositions, and the luminescence is clearly observed at room temperature for AlxGa1−xN:Tb with x ≥ 0.06. The small thermal quenching is attributed to the enhanced excitation to the 5D4 level of Tb3+ ions via the 4f–5d transition and not due to the suppression of energy back-transfer paths in excited Tb3+ ions. Although additional emission originating from the 5D3–7FJ transitions is observed at 15 K for AlxGa1−xN:Tb with x ≥ 0.15, it is not observed at room temperature because the excitation to the 5D3 level via the 4f–5d transition is less efficient at high temperature. For Al0.15Ga0.85N:Tb, monochromatic green light is demonstrated using a SiO2/ZrO2 distributed Bragg reflector.

Analysis of interface roughness in strained InGaAs/AlInAs quantum cascade laser structures (λ ∼ 4.6 μm) by atom probe tomography

In this study, in-plane interface roughness (IFR) values are extracted via atomic probe tomography (APT) for a few key interfaces within a quantum cascade laser (QCL) active region composed of compressively-strained InGaAs quantum wells (QWs) and tensilely-strained AlInAs barriers. The structure was grown by organometallic vapor phase epitaxy (OMVPE) at 605 °C, 100 rpm, 100 torr, with a 5 s interruption time between layers. The full-stage thickness measured via high-resolution x-ray diffraction (HR-XRD) is used to calibrate the reconstruction to within 5%. From the APT results, it is found that interfaces to two barriers of different tensile-strain value (i.e., of different Al concentration) have different RMS-roughness amplitude. Specifically, barriers of higher Al concentration have ~50% larger RMS-roughness amplitude, as the strain differential increases by a factor of 2.2. To the best of our knowledge, this is the first experimental data showing that roughness parameters within a QCL active region are different for different interfaces within the same grown structure.

Characterization of Hazy Morphology on AlInP/GaAs Epitaxial Wafers Grown by Organometallic Vapor-Phase Epitaxy

Six-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor-phase epitaxy (OMVPE) are being developed for light-emitting diodes (LEDs). The surface morphology of the epilayer changes under different growth conditions, where hazy features arise under high growth pressure, preventing their use in device fabricatiion. Applying optical microscopy, atomic force microscopy (AFM), and synchrotron x-ray topography, it is observed that the hazy region is rougher than the clear region and additional lattice distortion exists in the hazy region. The tilt and strain are quantitatively analyzed using 004 reciprocal-space mapping (RSM), where the hazy regions are associated with weakened and broadened peaks surrounding the sharp peak from the clear regions of the epilayer. More importantly, by employing 002 RSM to lower the x-ray penetration depth, it is revealed that the hazy features are formed when a certain thickness is exceeded, indicating that the epilayers do not suffer from lattice distortion at the beginning of the epitaxial growth. The characterization results suggest that changes in the growth conditions at the epilayer–substrate interface may not help to avoid the formation of the hazy features, but that other growth parameters at the bulk epilayer are worth investigation.

In situ microbeam surface X-ray scattering reveals alternating step kinetics during crystal growth

The stacking sequence of hexagonal close-packed and related crystals typically results in steps on vicinal {0001} surfaces that have alternating A and B structures with different growth kinetics. However, because it is difficult to experimentally identify which step has the A or B structure, it has not been possible to determine which has faster adatom attachment kinetics. Here we show that in situ microbeam surface X-ray scattering can determine whether A or B steps have faster kinetics under specific growth conditions. We demonstrate this for organo-metallic vapor phase epitaxy of (0001) GaN. X-ray measurements performed during growth find that the average width of terraces above A steps increases with growth rate, indicating that attachment rate constants are higher for A steps, in contrast to most predictions. Our results have direct implications for understanding the atomic-scale mechanisms of GaN growth and can be applied to a wide variety of related crystals.

Dependence of the properties of GaAs (111)A and Ga<sub>1-x</sub>Al<sub>x</sub>As (111)A epitaxial layers on the conditions of their growth by organometallic vapor phase epitaxy

Dependence of the properties of GaAs (111)A and Ga<sub>1-x</sub>Al<sub>x</sub>As (111)A epitaxial layers on the conditions of their growth by organometallic vapor phase epitaxy

Characterization of Hazy Morphology on AlInP/GaAs Epitaxial Wafers Grown by Organometallic Vapor Phase Epitaxy

6-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor phase epitaxy (OMVPE) technique are being developed for light emitting diodes (LEDs) applications [1]. Recently, dislocation configurations in the GaAs substrates and AlInP epilayers have been characterized and conditions for relaxation and formation of misfit dislocations were analyzed [2,3]. Apart from defects such as dislocations, epilayer surface reveals a hazy morphology formed when the epitaxial growth pressure is increased. It is observed that the epilayer surface is rougher compared to regular clear regions as revealed by both optical microscopy and atomic force microscopy. Additionally, the size of the hazy regions increases with growth pressure but other growth parameters such as growth temperature, V/III ratio, likely influence the onset of hazy morphology. Previous research on the formation of hazy features reports that these can be caused by the deficiency of phosphorus [4], lowered V/III ratio [5] or stacking faults [6]. However, the hazy features in our case, are different from the ones reported. Synchrotron X-ray topographs of the hazy regions showed blurred contrast, indicating disordered lattice arrangement. Moreover, internal structure of the hazy regions was studied by reciprocal space mapping (RSM), where weakened and broadened peaks around the clear epilayer peak were observed using 004 reflection, indicating continuously varying strain and tilt between the hazy region and the clear region. The thickness of the hazy regions was also analyzed by employing the 002 RSM, where the X-ray penetration depth is lower. Comparison of the 002 and 004 RSMs reveals that at the beginning of epitaxial growth, the epilayers do not suffer from lattice distortion and the formation of the hazy morphology occurs when a critical thickness is exceeded.Reference:[1] C. H. Chen, S. A. Stockman, M. J. Peanasky, C. P. Kuo. Brightness Light Emitting Diodes Semiconductors and Semimetals, SEMICONDUCT SEMIMET, 48 (1997) pp. 97–144[2] Hongyu Peng, Tuerxun Ailihumaer, Yafei Liu, Balaji Raghothamachar and Michael Dudley, J. Cryst. Growth, 533 (2020) 125458[3] Hongyu Peng, Tuerxun Ailihumaer, Balaji Raghothamachar and Michael Dudley, J. Electron. Mater. 49, 3472–3480 (2020)[4] H. H. Ryu, M. H. Jeon, J. Y. Leem, H. J. Song, L. P. Sadwick, G. B. Stringfellow, J Mater Sci (2006) 41:8265–8270[5] D. S. Cao and G. B. Stringfellow, Journal of Electronic Materials, Vol. 20, No. 1, 1991[6] Yoshihiro Hiraya, Fumiya Ishizaka, Katsuhiro Tomioka and Takashi Fukui, Applied Physics Express 9, 035502 (2016) Figure 1

Characterization of Hazy Morphology on AlInP/GaAs Epitaxial Wafers Grown by Organometallic Vapor Phase Epitaxy

6-inch AlInP/GaAs epitaxial wafers grown by organometallic vapor phase epitaxy (OMVPE) technique are being developed for light emitting diodes (LEDs) applications. Defects such as dislocations and inclusions have been characterized by synchrotron X-ray topography. Burgers vector and line direction of the dislocations were determined by ray-tracing simulation. Moreover, epilayer surface of the samples grown under higher pressure reveals a hazy morphology, which is rougher compared to regular clear regions. Synchrotron X-ray topographs of the hazy regions showed blurred contrast, indicating disordered lattice arrangement. Internal structure of the hazy regions was studied by reciprocal space mapping (RSM), where weakened and broadened peaks around the clear epilayer peak were observed using 004 reflection, indicating continuously varying strain and tilt between the hazy region and the clear region.

Features of the Vapor-Phase Epitaxy of GaAs on Nonplanar Substrates

The features of the surface shape of GaAs epitaxial layers grown on grooves several micrometers wide with vertical walls and an aspect ratio close to unity are investigated. The grooves are formed on the surface of a GaAs wafer in a plasma-chemical etching installation and overgrown by organometallic vapor-phase epitaxy under reduced pressure in a reactor.

Effect of hydride vapor phase epitaxy growth conditions on the degree of atomic ordering in GaInP

We report a systematic study of CuPt-type ordering in hydride vapor phase epitaxy (HVPE)-grown Ga0.5In0.5P. Selected-area electron diffraction reveals ordering in samples grown on (001) GaAs substrates offcut toward (11¯1)B. The ordering is single-variant, occurring only on (11¯1)B planes and not on (1¯11)B. Quantitative analysis of the order parameter by high-resolution x-ray diffraction (HRXRD) indicates that ordering increases with deposition temperature in samples grown at 600–700 °C with a constant gas-phase V/III ratio ∼3. Ordering increases with V/III ratio in the range of 1.3–6.7 at a constant deposition temperature of 650 °C. Photoluminescence measurements correlate the order parameter with Ga0.5In0.5P bandgap contraction, though the contraction is larger than expected based on the magnitude of order parameters measured by HRXRD. A possible reason for this discrepancy is that the photoluminescence emission occurs in the lower bandgap ordered domains, which are small and evenly dispersed throughout the material. We also show that the degree of ordering decreases with growth rate, disappearing at ∼60 μm/h. The HVPE-grown material exhibits a generally weaker ordering than organometallic vapor phase epitaxy (OMVPE)-grown material, likely due to the moderate V/III ratios employed, in contrast to the V/III ratios in the 100s typical of OMVPE. However, the tendency for ordering to increase with V/III ratio suggests that the same dimer-induced stress mechanism used to explain the occurrence of ordering in OMVPE-grown material also applies to HVPE. The tendencies for ordering to increase with deposition temperature and decrease with growth rate show that kinetics limit the degree of ordering, also in agreement with OMVPE trends.

Quantitative evaluation of enhanced Er luminescence in GaAs-based two-dimensional photonic crystal nanocavities

We evaluate the enhancement of Er luminescence coupled to two-dimensional (2D) photonic crystal (PC) nanocavities by means of photoluminescence measurements and numerical simulations. L3 PC nanocavities are utilized for characterization and evaluation, with GaAs:Er,O grown by low-pressure organometallic vapor phase epitaxy as the active layer. Optical characterization at room temperature demonstrates a 5.8-fold enhancement of Er luminescence due to coupling to the cavity mode of the 2D-PC nanocavities. This enhancement of Er luminescence is supported by a finite-difference time domain simulation where an enhancement of 4.1 times is found, which is in reasonable agreement with the observed experimental results. These results pave the way toward understanding the interaction between cavity modes in PC nanocavities and luminescence from rare-earth elements.

Room-temperature operation of near-infrared light-emitting diode based on Tm-doped GaN with ultra-stable emission wavelength

Near-infrared (NIR) light with a wavelength of 650–950 nm is used for various biomedical applications. Although NIR emitters are typically based on GaAs-related materials, they contain toxic elements, and the emission wavelength can easily shift during the device operation due to temperature changes and current injection levels. On the other hand, Tm3+, which is one of the rare-earth ions, can generate ultra-stable NIR luminescence with a wavelength of ∼800nm, based on 3H4–3H6 transitions in a 4f shell, and we have recently focused on Tm-doped GaN (GaN:Tm) based light-emitting diodes (LEDs) as novel NIR emitters. In this paper, we present a demonstration of a NIR-LED based on GaN:Tm grown by the organometallic vapor phase epitaxy method with optimized growth conditions and structures, where the parasitic reaction is well suppressed. NIR luminescence from the GaN:Tm-based LED is derived from 3H4–3H6 transitions of Tm3+ ions and consists of three dominant peaks at 795, 806, and 814 nm. The turn-on voltage of the NIR-LED is ∼6.9 V, and it is significantly lower than the previously reported electroluminescent devices based on GaN:Tm with impact ionization processes. From a current dependence of the electroluminescence spectra and temperature-dependent photoluminescence for the NIR-LED, the peak shifts are determined to be <7.6 pm/mA and ∼1.3 pm/K, respectively.

Layer-thickness dependence of the compositions in strained III–V superlattices by atom probe tomography

Abstract In order to conduction-band engineer quantum cascade lasers (QCLs) for emission wavelength, specifically QCLs emitting at 4.6 µm, precise control over layer thicknesses and compositions is required. In this study, Al/Ga incorporation in a strain-balanced InAlAs/InGaAs superlattice (SL) on InP is characterized as a function of layer thickness, keeping the composition constant, using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and Atom Probe Tomography (APT). A SL structure was grown on an InP substrate by organometallic vapor phase epitaxy (OMVPE) at a temperature of 605 °C and a reactor pressure of 100 torr, with 5 sec interruption time between layers. The full growth thickness obtained from STEM images was used to calibrate the reconstruction of the atom probe data. From the APT results it was found that the Al and Ga incorporation in thin layers (

Особенности газофазной эпитаксии GaAs на непланарных подложках

Abstract The features of the surface shape of GaAs epitaxial layers grown on grooves several micrometers wide with vertical walls and an aspect ratio close to unity are investigated. The grooves are formed on the surface of a GaAs wafer in a plasma-chemical etching installation and overgrown by organometallic vapor-phase epitaxy under reduced pressure in a reactor.

Optically pumped room temperature low threshold deep UV lasers grown on native AlN substrates

We report here an optically pumped deep UV edge emitting laser with AlGaN multiple quantum wells (MQWs) active region grown on AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a high-temperature reactor. The 21 period Al_0.53Ga_0.47N/Al_0.7Ga_0.3N MQWs laser structure was optically pumped using 193 nm deep UV excimer laser source. A laser peak was achieved from the cleaved facets at 280.3 nm with linewidth of 0.08 nm at room temperature with threshold power density of 320 kW/cm². The emission is completely TE polarized and the side mode suppression ratio (SMSR) is measured to be around 14 dB at 450 kW/cm².