Epitaxial Growth Conditions Research Articles

InAlN/GaN and AlGaN/GaN HEMT technologies comparison for microwave applications

High electron mobility transistors (HEMTs) technologies based on AlGaN/GaN and InAlN/GaN heterostructures have been developed. The research focused on influence of epitaxial growth conditions and buffer doping profiles on electrical properties HEMTs. An output power density of 4W/mm at 17 GHz was demonstrated for InAlN/GaN HEMTs and 7W/mm at 10 GHz for AlGaN/GaN HEMTs.

High Efficiency Inverted GaAs and GaInP/GaAs Solar Cells With Strain‐Balanced GaInAs/GaAsP Quantum Wells

AbstractHigh‐efficiency solar cells are essential for high‐density terrestrial applications, as well as space and potentially vehicle applications. The optimum bandgap for the terrestrial spectrum lies beyond the absorption range of a traditional dual junction GaInP/GaAs cell, with the bottom GaAs cell having higher bandgap energy than necessary. Lower energy bandgaps can be achieved with multiple quantum wells (QWs), but such a pathway requires advanced management of the epitaxial growth conditions in order to be useful. Strain‐balanced GaAsP/GaInAs QWs are incorporated into a single junction GaAs solar cell and a dual junction GaInP/GaAs solar cell, leading to 27.2% efficiency in the single junction device and a one‐sun record 32.9% efficiency in the tandem device. Good carrier collection and low non‐radiative recombination are observed in the cells with up to 80 QWs. The GaAs cells employ a rear‐heterojunction architecture to boost the open‐circuit voltage to over 1.04 V in the quantum well device, despite the large number of QWs.

Absolute surface energies of oxygen-adsorbed GaN surfaces

Absolute surface energies of reconstructed polar, nonpolar, and semi-polar GaN surfaces formed under oxide vapor phase epitaxy (OVPE) growth conditions were investigated via first-principles calculations. The relationships of stable surface orientations and surface structures with the growth conditions of Ga pressure and temperature were examined in terms of absolute surface energies. The stability of the surface orientation was in the order (0001¯), (11¯01), (11¯00), (112¯0), (0001), and (11¯01¯). High O concentrations on OVPE-grown GaN could be explained in relation to the stability of the (11¯01) surface. The stable (11¯01) surface consisted of N vacancies and H and O adatoms. Therefore, it was deduced that its appearance results in increased O impurity incorporation.

Effect of molecular beam epitaxy growth conditions on phase separation in wide-bandgap InAlAsSb lattice-matched to InP

InxAl1-xAs1-ySby is the only III-V material lattice-matched to InP with a direct bandgap energy range as large as ~1.45–1.80 eV, making it interesting for multiple optoelectronic applications. However, inherent material challenges in this immature quaternary alloy have thus far precluded the growth of InxAl1-xAs1-ySby with device-quality properties at the InP lattice constant. We have investigated how molecular beam epitaxy growth conditions affect the photoluminescence intensity, spectrum, temperature- and power-dependence, as well as the surface morphology of In0.23Al0.77As0.75Sb0.25, which is lattice-matched to InP with bandgap energy of ~1.68 eV as measured by ellipsometry. We find that reducing the adatom mobility, via a lower substrate temperature and higher group-V overpressure, diminishes the tendency for phase separation, but also quenches the photoluminescence intensity. These findings provide a step toward identifying an optimal growth window for realizing high-quality, wide-bandgap InxAl1-xAs1-ySby lattice-matched to InP.

Tuning of structural and dielectric properties of Gd2O3 grown on Si(001)

The structural and dielectric properties of gadolinium oxide (Gd2O3) grown on Si(001) depending on the epitaxial growth conditions were investigated. Gd2O3 layers were grown at temperatures between 250°C and 400°C with an oxygen partial pressure between 2×10−7mbar and 5×10−7mbar. The crystal structure of the Gd2O3 turns out to be monoclinic with rotational domains as revealed by x-ray diffraction measurements and transmission electron microscopy (TEM) investigations. The dielectric properties can be tuned with growth temperature, forming gas annealing, and an increase in oxygen partial pressure. Furthermore, the dielectric constant was found to increase with the layer thickness. This can be interpreted in terms of the presence of a two layer stack consisting an interfacial quasi-amorphous and monoclinic Gd2O3 on top, as confirmed by TEM. The value of around 33 was extracted for the dielectric constant of monoclinic Gd2O3, which is much higher than for cubic Gd2O3. The best Gd2O3 layers grown at 400°C and pO2=5×10−7mbar exhibit also a characteristic leakage current value J(Vfb−1V) for a CET value of around 2 nm in the range of a few nA/cm2, which enable the applicability in electronic devices.

Highly Reliable 4H-SiC Epitaxial Wafer with BPD Free Recombination-Enhancing Buffer Layer for High Current Applications

Epitaxial growth of 4H-SiC on 150 mm wafers with the recombination-enhancing buffer layer was studied. In order to accomplish the reduction of basal plane dislocations in the buffer layer to almost free level and assure its quality in production, non-destructive evaluation using photoluminescence method was investigated. Epitaxial wafers of which the buffer layer and the drift layer have more than 99% BPD free area in a 2.6 mm × 2.6 mm block evaluation were realized by optimizing the epitaxial growth conditions. Furthermore, very low surface defects density and excellent thickness and doping uniformity were achieved simultaneously.

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.

Properties of Si-doped a-plane AlGaN layers

The non-polar a-plane (112¯0) n-type AlGaN epi-layers with high electron concentration were successfully grown on r-plane (11¯02) sapphire substrates by metal organic chemical vapor deposition. High-resolution X-ray diffraction, room temperature photoluminescence spectrum, and Hall effect measurement were used to examine the influence of Si-doping on the crystalline quality, and the optical and electrical characteristics of the non-polar a-plane n-AlGaN epi-layer, respectively. The characterization results indicate that the electron concentration (EC), the electron mobility, and the crystalline quality for the non-polar a-plane n-AlGaN epi-layers could be improved significantly by optimizing the epitaxial growth conditions, especially the mole flow rate of SiH4 in the Si-doping process. In fact, at a SiH4 mole flow rate of 10.2 nmol/min, an EC as high as 1.25 × 1018 cm−3 and an electron mobility up to 3.86 cm2/V were achieved with the non-polar n-AlGaN epi-layer sample with the Al content of 41%.

Effect of Various Defects on 4H-SiC Schottky Diode Performance and Its Relation to Epitaxial Growth Conditions.

In this paper, the chemical vapor deposition (CVD) processing for 4H-SiC epilayer is investigated with particular emphasis on the defects and the noise properties. It is experimentally found that the process parameters of C/Si ratio strongly affect the surface roughness of epilayers and the density of triangular defects (TDs), while no direct correlation between the C/Si ratio and the deep level defect Z1/2 could be confirmed. By adjusting the C/Si ratio, a decrease of several orders of magnitudes in the noise level for the 4H-SiC Schottky barrier diodes (SBDs) could be achieved attributing to the improved epilayer quality with low TD density and low surface roughness. The work should provide a helpful clue for further improving the device performance of both the 4H-SiC SBDs and the Schottky barrier ultraviolet photodetectors fabricated on commercial 4H-SiC wafers.

Alloy Stability of Ge1\u2212xSnx with Sn Concentrations up to 17% Utilizing Low-Temperature Molecular Beam Epitaxy

The binary alloy germanium tin has already been presented as a direct group IV semiconductor at high tin concentrations and specific strain. Therefore, it offers a promising approach for the monolithic integrated light source towards the optical on-chip communication on silicon. However, the main challenge faced by many researchers is the achievement of high tin concentrations and good crystal quality. The key issues are the lattice mismatch to silicon and germanium, as well as the limited solid solubility of tin in germanium of less than 1%. Therefore, this paper presents a systematic investigation of the epitaxial growth conditions of germanium tin with tin concentrations up to 17%. For this, we performed two growth experiments utilizing molecular beam epitaxy. In one experiment, we varied the growth temperature for the epitaxy of germanium tin with 8% tin to investigate the inter-growth temperature stability. In the second experiment, we focused on the strain-relaxation of germanium tin, depending on different tin concentrations and doping types. The results of subsequent material analysis with x-ray diffraction and scanning electron microscopy allow us to narrow the epitaxial window of germanium tin. Furthermore, we present a possible explanation for the unique relaxation mechanism of germanium tin, which is significantly different from the well-known relaxation mechanism of silicon germanium.

Aluminum metallization of III–V semiconductors for the study of proximity superconductivity

Managing the interaction of materials with insertion layers and nonconventional molecular beam epitaxy growth conditions allows for interfaces that are more precise but requires judicious examination of the multiple possible design variables. Here, we show a comparison between As- and Sb-containing insertion layers between Al and two binaries with different group V elements and demonstrate that antimonide layers greatly improve the interface. In addition to depositing Al at extremely slow growth rates onto cold (below 0 °C) substrates, the reactivity is particularly minimized with AlSb insertion layers, which improves interface abruptness, preserves the underlying semiconductor layer’s crystalline properties, and produces flatter superconductor surfaces.

Influence of strain on the InAs1 – xSbx composition

The composition of III–V semiconductor alloys with multiple group V elements results from a complex interaction of each group V species with each other and with group IIIs. Molecular beam epitaxy growth conditions, such as the group V absolute fluxes and flux ratios, substrate temperature, group III growth rate, the presence of surfactants, and the alloy's strain state, all affect the composition of InAs1 – xSbx. These factors are codependent in a manner that is far from completely established. In this work, the authors examine how the sign and degree of strain affects the film’s composition. In this study, the authors show that InAs1 – xSbx alloys have some ability to resist the creation of strain by self-adjusting the incorporation of the group V elements that would otherwise be changed in an unfavorable way (by, for example, a substrate temperature change). This self-latching to the substrate lattice constant is beneficial, since it means that the fluxes may not have to be adjusted as precisely as otherwise needed.

Interfacial hydrogen incorporation in epitaxial silicon for layer transfer

Recently, epitaxial Si layers have attracted strong attention, particularly in photovoltaics. This successful application depends mainly on the easiness of their transfer to a foreign carrier substrate. Therefore, developing a simple and efficient method to realize the transfer is a key issue. A most delicate point is the lift-off of the epitaxial layer from its parent substrate. In this work, we present a method to weaken the interface based on hydrogen incorporation. We have been able to control the hydrogen content at the interface between the crystalline silicon substrate and the epitaxial films by changing the epitaxial growth conditions. Several bonding techniques have been tested and epitaxial Si films have been transferred successfully via anodic bonding. A hydrogen-assisted transferring mechanism is presented.

Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in a-axis-oriented BaSi2 films

We grow 0.5 μm thick a-axis oriented BaSi2 films on Si(111) substrates by a conventional two-step growth method and compare their properties with those grown by a three-step growth method. Both grow methods consist of the initial growth of a BaSi2 template layer and the following molecular beam epitaxy (MBE) to form BaSi2 films. In the two-step growth method, Ba-to-Si deposition rate ratios (RBa/RSi) were varied in the range 0.4–4.7 during MBE. On the other hand, in the three-step growth method, MBE-grown BaSi2 layers are composed of BaSi2 films grown first under Ba-rich conditions (RBa/RSi = 4.0), followed by those grown under Si-rich conditions (RBa/RSi = 1.2). The grain size of BaSi2 films by the three-step growth method was much smaller than those grown by the two-step growth method. To our surprise, however, much higher photoresponsivity was obtained for BaSi2 films grown by the three-step growth method.

Low-density patterned InAs quantum dot arrays

The authors present work on patterned InAs quantum dot (QD) arrays using periodicities between 0.25 and 10 μm, with tailored molecular beam epitaxy growth conditions to promote high QD occupancy in the 10 μm array for further device implementation. The gallium-deoxidation method was used to maintain the pattern integrity. At high growth temperatures and reduced arsenic overpressure conditions, the authors have shown increasing QD occupancy as the patterned spacing increases. The 10 μm array was found to have 89% single QD occupancy. A low density of QDs in known locations enables the QDs to be patterned singly into quantum device applications.

On quantum-dot lasing at gain peak with linewidth enhancement factor αH = 0

This paper describes an investigation of the linewidth enhancement factor αH in a semiconductor quantum-dot laser. Results are presented for active region parameters and laser configurations important for minimizing αH. In particular, the feasibility of lasing at the gain peak with αH = 0 is explored. The study uses a many-body theory with dephasing effects from carrier scattering treated at the level of quantum-kinetic equations. InAs quantum-dot lasers with different p-modulation doping densities are fabricated and measured to verify the calculated criteria on laser cavity design and epitaxial growth conditions.This paper describes an investigation of the linewidth enhancement factor αH in a semiconductor quantum-dot laser. Results are presented for active region parameters and laser configurations important for minimizing αH. In particular, the feasibility of lasing at the gain peak with αH = 0 is explored. The study uses a many-body theory with dephasing effects from carrier scattering treated at the level of quantum-kinetic equations. InAs quantum-dot lasers with different p-modulation doping densities are fabricated and measured to verify the calculated criteria on laser cavity design and epitaxial growth conditions.

AMMONIA MOLECULAR BEAM EPITAXY OF AlGaN HETEROSTRUCTURES ON SAPPHIRE SUBSTRATES

In order to develop a technology for the growth of Al(Ga)N-based heterostructures, the effect of different molecular beam epitaxy growth conditions on the properties of AlN and AlGaN layers was studied. The optimal conditions for the growth of AlN buffer layers were established, which made it possible to achieve a root mean square roughness as small as 0.7 nm. It was shown that an increase of AlN layer thickness leads to a decrease of density of edge dislocations, while no explicit dependence of the screw dislocation density on the layer thickness was observed. The minimal obtained dislocations density values for 1.25μm-thick AlN layer were n edges = 5.9×10 9 cm -2 and n screw = 2.2×10 7 cm -2 for edge and screw dislocations respectively. As a result of optimization of the AlGaN growth temperature, a series of 0.15μm-thick layers was grown, which showed stimulated emission at wavelengths λ = 330 nm, 323 nm, 303 nm, and 297 nm with threshold power densities of 0.7 MW/cm 2 , 1.1 MW/cm 2 , 1.4 MW/cm 2 and 1.4 MW/cm 2 , respectively. The determined optimal epitaxy conditions for AlN and AlGaN layers were used to grow the AlGaN/GaN high electron mobility transistor structure on a sapphire substrate with two-dimensional electron gas, which had a mobility of 1950 cm 2 /(Vs) at a concentration of 1.15×10 13 cm -2 . The obtained results are important for creating of nitride-based UV-emitting optoelectronic semiconductor devices, as well as high-power and high-frequency electronic devices.

Tailoring commensurability of hBN/graphene heterostructures using substrate morphology and epitaxial growth conditions

Hexagonal boron nitride (hBN) thin films were grown by plasma-enhanced chemical beam epitaxy (PE-CBE) on epitaxial graphene (EG) on macrostepped 4°-offcut 4H-SiC(0001) substrates. The choice of growth conditions in this system allowed for two prominent in-plane hBN/EG rotational alignments: a direct alignment of the hBN and EG lattices or a 30° in-plane rotational twist such that the ⟨112¯0⟩hBN and ⟨101¯0⟩EG directions are parallel. The use of nitrogen plasma in conjunction with borazine at growth temperatures of 1450 °C increased the crystallinity of the few-monolayer-thick films relative to films grown by CBE without plasma exposure. In vacuo x-ray photoelectron spectroscopy showed that films grown with nitrogen plasma exposure were stoichiometric to nitrogen-rich, depending on growth conditions, and exhibited no bonding indicative of additional phase formation. This PE-CBE process was shown to produce films with atomically abrupt interfaces between the hBN and EG lattices, as determined by cross-sectional transmission electron microscopy (TEM). Annular dark field and bright field scanning TEM paired with energy dispersive x-ray spectroscopy confirmed that the EG persisted throughout this deposition and no intercalative growth of hBN under the EG was detected. Higher PE-CBE growth rates produced hBN domains that nucleated uniformly across the substrate with little preferred orientation of their edges. In comparison, lower growth rates appeared to cause preferential nucleation on the macrostep edges with a 30° in-plane rotation relative to the EG, as confirmed by cross-sectional TEM. By correlating the hBN nuclei shape in AFM to the atomic registry of the hBN to the substrate, it was found that the triangular, macrostep-edge nuclei were arm-chair edge terminated. The ability to select different rotational alignments by changing epitaxial growth conditions may be used in future wafer-scale growth of hBN/graphene heterostructures to achieve varying degrees of graphene band structure modulation.

Extensive 99% Killer Defect Free 4H-SiC Epitaxial Layer toward High Current Large Chip Devices

Epitaxial growth of 4H-SiC on 150 mm wafers using 3 x 150 mm multi-wafer CVD has been investigated to realize extremely low defect density on the epitaxial layer in order to achieve stable fabrication of high current devices with large die size. By optimizing the epitaxial growth conditions as well as the improved procedures for the inside the furnace to remain cleaned stably for cumulative growth processes, we have demonstrated an extensive 99% defect free epitaxial inlayer in a 5 mm x 5 mm block evaluation which is having excellent doping and thickness uniformity simultaneously.

1080 nm InGaAs laser power converters grown by MOCVD using InAlGaAs metamorphic buffer layers

Metamorphic InGaAs laser power converters have a great potential to be used for a delivery of electrical energy over distances of centimeters up to kilometers. The direct band gap InGaAs material covers monochromatic wavelengths around 1100 nm, which can be supplied by widespread commercial lasers. In this study, the InGaAs-based laser power converter have been designed to absorb a laser at 1080 nm and grown by metalorganic chemical vapor deposition. Step graded InAlGaAs buffer layers were employed to overcome the lattice mismatch problem between InGaAs and GaAs. Various epitaxial growth conditions such as overshoot layer which contains higher indium composition than the final buffer layer, back surface field barrier, growth temperature and V/III ratio were studied to obtain high quality InAlGaAs metamorphic buffer. The fabricated InGaAs-based laser power converter, which had an enhanced metamorphic buffer and 5 μm thick grid metal, showed a high external quantum efficiency of 82.16% at 1080 nm wavelength. As a result, a conversion efficiency of 37.87% was achieved under 1080 nm monochromatic light at a laser power density of 538 mW/cm2. Further improvements are expected when the device structure is modified to reduce the series resistance and the growth condition is optimized to increase the minority carrier life time.