Substrate-epilayer Interface Research Articles

Enhancing the electron mobility in Si-doped (010) β-Ga2O3 films with low-temperature buffer layers

We demonstrate a new substrate cleaning and buffer growth scheme in β-Ga2O3 epitaxial thin films using metal–organic vapor phase epitaxy (MOVPE). For the channel structure, a low-temperature (LT, 600 °C) un-doped Ga2O3 buffer was grown, followed by a transition layer to a high-temperature (HT, 810 °C) Si-doped Ga2O3 channel layers without growth interruption. The (010) Ga2O3 Fe-doped substrate cleaning uses solvent cleaning, followed by additional hydrofluoric acid (49% in water) treatment for 30 min before the epilayer growth. This step is shown to compensate the parasitic Si channel at the epilayer–substrate interface that originates from the substrate polishing process or contamination from the ambient. From secondary ion mass spectroscopy (SIMS) analysis, the Si peak atomic density at the substrate interface is found to be several times lower than the Fe atomic density in the substrate—indicating full compensation. The elimination of the parasitic electron channel at the epi–substrate interface was also verified by electrical (capacitance–voltage profiling) measurements. In the LT-grown (600 °C) buffer layers, it is seen that the Fe forward decay tail from the substrate is very sharp, with a decay rate of ∼9 nm/dec. X-ray off-axis rocking curve ω-scans show very narrow full width at half maximum (FWHM) values, similar to the as-received substrates. These channels show record high electron mobility in the range of 196–85 cm2/V⋅s in unintentionally doped and Si-doped films in the doping range of 2 × 1016–1 × 1020 cm−3. Si delta-doped channels were also grown utilizing this substrate cleaning and the hybrid LT buffers. Record high electron Hall mobility of 110 cm2/V⋅s was measured for sheet charge density of 9.2 × 1012 cm−2. This substrate cleaning, combined with the LT buffer scheme, shows the potential of designing Si-doped β-Ga2O3 channels with exceptional transport properties for high-performance Ga2O3-based electron devices.

Long-Range Orbital Hybridization in Remote Epitaxy: The Nucleation Mechanism of GaN on Different Substrates via Single-Layer Graphene.

Remote epitaxy is a very promising technique for the preparation of single-crystal thin films of flexibly transferred III-V group semiconductors. However, the epilayer nucleation mechanism of remote epitaxy and the epilayer-substrate interface interactions on both sides of graphene are not well-understood. In this study, remote homo- and heteroepitaxy of GaN nucleation layers (NLs) were performed by metal organic chemical vapor deposition on GaN, sapphire (Al2O3), and AlN substrates with transferred single-layer graphene, respectively. The results show that the interface damage of SLG/GaN at high temperature is difficult for us to achieve the remote homoepitaxy of GaN. Therefore, we explored the nucleation mechanism of remote heteroepitaxy of GaN on SLG/Al2O3 and SLG/AlN substrates. Nucleation density, surface coverage, diffusion coefficient, and scaled nucleation density were used to quantify the differences in nucleation information of GaN grown on different polar substrates. Using high-resolution X-ray diffraction and high-resolution transmission electron microscopy analysis, we revealed the interfacial orientation relationship and atomic arrangement distribution between the GaN NLs and substrates on both sides of the SLG. The electrostatic potential effect and adsorption ability of the substrates were further investigated by first-principles calculations based on density functional theory, revealing the principle that the substrate polarity affects the atomic nucleation density. The partial density of states shows that there is long-range orbital hybridization of the electronic states of the substrate and adsorbed atoms in remote epitaxy, and the crystal properties of the substrate play an important role in the in-plane orientation relationship of the NL and substrate across the SLG. The abovementioned results reveal the nature of remote epitaxy and broaden the perspective for the rapid and large-area preparation of single-crystal GaN films.

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.

Mg acceptor doping in MOCVD (010) β -Ga2O3

In this Letter, in situ Mg doping in β-Ga2O3 was demonstrated via metalorganic chemical vapor deposition (MOCVD) epitaxy. The electrical insulating property of the Mg acceptors in β-Ga2O3 was found to be intrinsically activated in the as-grown Mg-doped β-Ga2O3 thin films. Growth conditions for MOCVD β-Ga2O3 were further explored and optimized at a lower growth temperature regime, leading to a better confinement of the Mg-doping profile. Detailed analysis of Mg diffusion characteristics revealed a diffusion barrier energy Ebarrier ∼ 0.9 eV for Mg in MOCVD β-Ga2O3, which is likely related to an interstitial-assisted process. Surface morphologies and electron transport were characterized on samples grown with different growth temperatures and Mg doping levels. The MOCVD growth method demonstrated its feasibility to grow semi-insulating Mg-doped β-Ga2O3 epilayers with controllable Mg incorporation while maintaining good material quality and smooth surface morphology. From capacitance-voltage charge profiling, it is verified that the Mg-doped buffer layer grown at the substrate-epilayer interface could effectively compensate the charge accumulation at the interface. The in situ acceptor doping of Mg in MOCVD β-Ga2O3 will provide versatility for designing β-Ga2O3 power devices.

Structure of Straight-Line Defect and its Effect on the Electrical Properties of Schottky Barrier Diodes

We have investigated the “straight-line defect,” which has not been classified separately and is quite similar to the carrot defect. We found that the straight-line defect differed structurally from the carrot defect. The presence of a particle on the substrate-epi layer interface seemed to be the cause of the defect; a layer of poly-type (3C-SiC) extended from the particle to the epi-layer surface. The straight-line defect likely resulted from shape change from the 3C-SiC triangular defect. This change in shape from triangular to straight-line defects depended on the C/Si ratio. To investigate the electrical characteristics, we fabricated a Schottky barrier diode (SBD) structure on a silicon carbide (SiC) epi wafer. With application of a high voltage, destruction occurred on both the upstream and the downstream side of the step flow of straight-line defects in the reverse voltage test. This reverse direction characteristic differed from that observed with triangular defects.

Doping induced lattice misfit in 4H–SiC homoepitaxy

The impact of doping on the lattice constants of 4H–silicon carbide (4H–SiC) is an important material aspect influencing several steps of material and device production. Dopant incorporation in 4H–SiC causes misfit between the highly N-doped substrate and differently doped epilayers and hence, wafer bowing and the existence of a critical epilayer thickness. In this paper, the wafer bow is determined by geometrical measurements of the substrate prior to and after the epitaxial growth of single epilayers with different epilayer thicknesses and doping states, i.e. with nitrogen (N) or aluminum (Al) doping and different dopant concentrations. The misfit between substrate and epilayer is deduced from these bow measurements based on a model by Stoney. For highly Al-doped epilayers grown on highly N-doped substrates, the misfit is determined additionally by HRXRD measurements. The doping-dependent misfit, obtained from bow and HRXRD measurements, is compared to theoretical values calculated based on a model by Jacobson. For Al-doped epilayers, the experimental and theoretical values of misfit agree well. For N-doped epilayers, an effective covalent radius of N in 4H–SiC of 66 pm has to be introduced to match the theoretical misfit to the experimental one. The critical epilayer thickness is calculated based on the models by Matthews and Blakeslee and by People and Bean. The comparison of calculated values and experimental findings with respect to dislocation behavior at the substrate–epilayer interface and the dislocation densities of epilayers proves that the model by Matthews and Blakeslee underestimates the critical epilayer thickness for 4H–SiC homoepitaxy.

MOCVD growth of GaN layer on InN interlayer and relaxation of residual strain

100 nm InN layer was grown on sapphire c-plane using a metal-organic chemical vapor deposition (MOCVD) system. Low temperature (LT) GaN layer was grown on InN layer to protect InN layer from direct exposure to hydrogen flow during high temperature (HT) GaN growth and/or abrupt decomposition. Subsequently, thick HT GaN layer (2.5 μm thick) was grown at 1000 °C on LT GaN/InN/sapphire template. Microstructure of epilayer–substrate interface was investigated by transmission electron microscopy (TEM). From the high angle annular dark field TEM image, the growth of columnar structured LT GaN and HT GaN with good crystallinity was observed. Though thickness of InN interlayer is assumed to be about 100 nm based on growth rate, it was not clearly shown in TEM image due to the InN decomposition. The lattice parameters of GaN layers were measured by XRD measurement, which shows that InN interlayer reduces the compressive strain in GaN layer. The relaxation of compressive strain in GaN layer was also confirmed by photoluminescence (PL) measurement. As shown in the PL spectra, red shift of GaN band edge peak was observed, which indicates the reduction of compressive strain in GaN epilayer.

Sources of Epitaxial Growth-Induced Stacking Faults in 4H-SiC

Successive reactive-ion etching and microphotoluminescence (PL) intensity mapping have been performed in order to investigate the sources of epitaxial growth-induced stacking faults (SFs) in thick 4H-SiC epilayers. Three kinds of SFs, i.e., 4SSFs, 3SSFs, and 2SSFs, have been identified in the samples. Two of these (3SSFs and 2SSFs) show similar nucleation behaviors, and their formation may be due to stress within the epitaxial layer. In contrast, 4SSFs nucleate at the epilayer–substrate interface and might be related to an unknown dislocation, which shows a rounded-shape etch pit in the substrate.

The proximity effect of the regrowth interface on two-dimensional electron density in strained Si

We studied the density of two-dimensional electron gas (2DEG) in Si∕SiGe heterostructures as a function of the distance between the substrate-epilayer interface and the 2DEG layer. The 2DEG sheet density was observed to change from 2.2×1011to3.5×1011cm−2. Theoretical simulations are shown to be consistent with the experimental results within experimental errors. The slight deviations of the experimental results possibly come from temperature variation of the Sb dopant source during the growth of the Sb doping layer.

High-temperature interstitial oxygen diffusion retardation in epitaxial-layered heavily arsenic- or boron-doped Czochralski silicon wafers

Interstitial oxygen (Oi) diffusivity was examined in epitaxial-layered silicon substrates of various types (heavily doped arsenic and heavily doped boron) in varying annealing conditions. When a heavily doped arsenic substrate was annealed at up to 1125°C, the Oi diffusion slowed by 28% compared to that in the epilayer or a lightly doped boron substrate. When a heavily doped arsenic substrate is annealed at 1200°C in N2, Oi accumulates in the epilayer subsurface. A heavily doped boron substrate annealed at 1200°C shows both significantly reduced Oi outdiffusion in the substrate and Oi segregation at the epilayer-substrate interface due to misfit dislocation.

Microstructural and morphological properties of homoepitaxial (001)ZnTe layers investigated by x-ray diffuse scattering

The microstructural and morphological properties of homoepitaxial (001)ZnTe layers grown by metalorganic vapor phase epitaxy at a temperature (TG) between 325°C and 400°C are investigated by x-ray diffuse scattering. High resolution reciprocal space maps (RSMs) recorded close to the ZnTe (004) Bragg peak show different diffuse scattering features that can be ascribed to (i) the specific surface morphology of the sample, and (ii) the presence of extended lattice defects in the epilayers. One kind of cross-shaped diffuse scattering streaks, appearing for TG⩾350°C, extend along ⟨111⟩ directions and can be attributed to stacking faults (SFs) occurring at the epilayer–substrate interface, within the epilayers. The SF diameter was estimated around 200–300nm, while their density increases with TG. Another kind of cross-shaped diffuse streaks, inclined at an angle β≈80° with respect to the ⟨110⟩ in-plane direction, arises from the morphology of epilayers grown above 360°C, their surfaces being covered by pairs of pyramidal hillocks up to a density of 106–107cm−2. Atomic force microscopy (AFM) measurements showed that the apex angles of the pyramids compare well with the value of 2β. The hillock formation is ascribed to Te adatoms experiencing a Schwoebel potential barrier at the step edges around pairs of partial dislocations (dipoles) bounding the SFs. In a quite narrow growth temperature interval around 350°C no β-crossed diffuse streaks are instead observed in the RSMs, indicating a smooth ZnTe surface. Finally, at a lower growth temperature (TG=325°C) a diffuse scattering intensity distribution defined by an angle γ≈63° with respect to the ⟨110⟩ in-plane direction is observed, corresponding to a dense ridging of the epilayer surface along the perpendicular direction. Both RSM analysis and AFM measurements indicate that the ridge sidewalls are {113} planes.

Studies of oxide desorption from GaAs substrates via Ga2O3 to Ga2O conversion by exposure to Ga flux

Removal of the native oxide from GaAs wafers in the process of thermal desorption in the presence of As flux causes very significant surface degradation. We examine the merits of an alternative oxide desorption method that consists in a standard thermal oxide removal procedure preceded by a partial oxide removal at lower temperatures via conversion of the stable Ga2O3 surface oxide into a volatile Ga2O oxide by a pulsed supply of Ga in the absence of As flux. We find that a reduction of the substrate roughness by more than one order of magnitude is obtained on most epi-ready GaAs substrates regardless of their age, even if only 70% of the original gallium oxide is removed with Ga pulses, and that nearly atomically smooth surfaces are obtained with 90% oxide removal. In addition, we demonstrate that the Ga-induced oxide removal process is laterally inhomogeneous, making this method vulnerable to the accumulation of Ga droplets on the GaAs surface when more than about 90% of the surface oxide is removed. The Ga2O desorption pattern monitored with quadrupole mass spectrometry indicates that other nonvolatile species initially present on the top of the gallium oxide are also converted to a volatile species by Ga exposure before large areas of Ga2O3 become exposed. We examine the residual surface contamination left at the substrate–epilayer interface and find no significant influence of the age of the substrate, or the oxide removal method, on the measured levels of silicon, oxygen, and carbon.

X-ray studies of As-doped GaN grown by plasma-assisted molecular beam epitaxy

We have used a range of X-ray diffraction techniques to study the structural properties of As-doped GaN samples grown on sapphire substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The X-ray methods employed include simple ω−2 θ scans, 2D reciprocal space maps and pole figures. We have estimated the ratio of cubic to hexagonal GaN as a function of As flux and sample thickness. We show that the cubic content is only weakly dependent on the arsenic flux in contrast to the intensity of the photoluminescence peak at 3.2 eV, previously associated with cubic-phase GaN material. This suggests that the photoluminescence at 3.2 eV does not arise mainly from direct recombination in cubic phase material, but is probably associated with a donor–acceptor pair transition. By studying As-doped GaN films of different thicknesses, we are able to show that the cubic material is predominantly at the substrate–epilayer interface and that in thicker samples, the growth of hexagonal phase material is dominant. As a result of this study, we can conclude that in As-doped GaN samples grown at 800°C on sapphire by PA-MBE, arsenic does not appear to stimulate the growth of pure cubic phase material.

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.

Investigation of surface recombination and carrier lifetime in 4H/6H-SiC

A spatially and time-resolved free carrier absorption method is applied to quantify surface recombination losses as compared to the bulk in 4H- and 6H-SiC structures. The observed carrier lifetime variation is discussed in terms of crystalline quality, top-surface properties and junction effects at the epilayer-substrate interface. Surface recombination parameters in epilayers with differently processed surfaces are extracted from fitting experimental data with numerical simulations. The as-grown bare epilayer is characterized by 10 4 cm s −1 surface recombination velocity. Mechanical polishing increases this parameter to 5×10 5 cm s −1. No noticeable passivation of 6H-SiC surface by an oxide film is observed, whereas an increase of the surface recombination velocity up to 10 5 cm s −1 has been detected after dry oxidation of 4H-SiC.

Room-temperature InAsSb strained-layer superlattice light-emitting diodes at λ=4.2 μm with AlSb barriers for improved carrier confinement

Room-temperature InAs/InAs1−xSbx strained-layer superlattice light-emitting diodes (x∼8%) are reported that emit at λ∼4.2 μm with an internal efficiency of 2.8%. The structures are grown by molecular beam epitaxy on slightly mismatched InAs substrates and include a strained AlSb barrier layer to prevent electron migration to the dislocated substrate–epilayer interface region. Comparison with a near identical structure grown without the barrier layer indicates a factor of four improvement in device efficiency at room temperature.

The quality of 200 mm diameter epitaxial Si wafers for advanced CMOS technology monitored using synchrotron X-ray topography

The control and characterisation of wafer defect and strain distributions is of crucial importance for the development of advanced Ultra Large Scale Integration (ULSI) circuits. Within the IC manufacturing sector 0.35 μm linewidth-based advanced Complementary Metal Oxide Semiconductor (CMOS) logic has recently emerged at a high level of maturity, to be closely followed by an even more demanding 0.25 μm process. One very important issue is the need to ensure a uniform, high quality Si substrate, i.e. minimise defect densities and eliminate strain distributions in the starting wafer material. Synchrotron section and back-reflection topographic techniques were applied to 200 mm diameter p-Si wafers, upon which, boron and arsenic doped epitaxial silicon layers had been deposited. These wafers were supplied from manufacturers around the globe and revealed substantial differences in the overall quality of the epilayers and substrates. In all wafers the substrate quality varied significantly with position across the wafer, as measured by the presence of oxygen-related defects and dopant strain homogeneity. The strain field uniformity, induced by the growth of lightly doped Si epilayers, was also observed to vary qualitatively with location on a wafer. Back-reflection topographs verify that the quality of the epilayer-substrate interface improved as the thickness of the epilayer, or the gradient of dopant density across the interface, is reduced. Cellular strain-related structures, of the order of a few hundred μm in circumference, have been observed in the more stressed p on p + samples. Topographic results are in agreement with those obtained from X-ray diffraction measurements. Finally, an examination was carried out into the quality of commercially supplied 200 mm diameter Si wafers, revealing differences in the overall quality of the wafers.

Fabrication of heterojunction bipolar transistors with buried subcollector layers for reduction of base-collector capacitance by molecular beam epitaxy regrowth

A process for fabrication of heterojunction bipolar transistors (HBTs) with selectively buried subcollectors by molecular beam epitaxy (MBE) regrowth is described. This process can be used to reduce parasitic base-collector capacitance of HBTs and improve the speed of these devices. In situ etching by iodine prior to the regrowth was used for the first time to improve the quality of the substrate epilayer interface in a semiconductor device grown by MBE. The secondary ion mass spectroscopy depth profiles of regrown HBT structures suggest that the in situ surface cleaning by molecular iodine was not sufficient to remove all contamination from the substrate epilayer interface and that the microwave performance of HBTs fabricated by this process may have been affected by that contamination. The dc performance of devices which were fabricated by our process was not affected, however, by the contamination and was comparable to the dc performance of conventional HBTs. Our results suggest that the described process for fabrication of HBTs with selectively buried subcollectors by MBE regrowth would be feasible in conjunction with a more effective surface treatment.

Characterization of a liquid-phase-epitaxy-grown Al 0.2Ga 0.8As film on a GaAs (1 1 1) substrate by RBS, channeling and PIGE analyses

An Al 0.2Ga 0.8As thick film (∼3 μm) on a GaAs (1 1 1) substrate, grown by liquid phase epitaxy (LPE), has been analyzed by Rutherford Backscattering Spectrometry (RBS) and channeling experiments with He + ions. The channeling minimum yield, χ min, value of 5% indicates a good quality of the LPE-grown film. An RBS measurement with 1.62 MeV H + ions was carried out to locate the substrate–epilayer interface and to measure the thickness of the epilayer. A comparison on the measurements with He + and H + beams for characterizing a thick epitaxial layer is presented and discussed. Since the Al signal was not directly identifiable in the RBS spectra, proton induced γ-ray emission (PIGE) measurements were also taken up as a direct method to determine the Al content in the epilayer.

Deep levels in Er-doped liquid phase epitaxy grown silicon

The optical activity of Er-doped silicon is related to the presence of defects which enhance the typical radiative transition at 0.8 eV but whose nature and properties are still largely unknown. We have investigated the deep levels present in Er-doped liquid phase epitaxy silicon to identify the traps possibly related to the luminescence. Among the observed levels, two minority carrier traps labeled E2 (EC−0.20 eV) and E3 (EC−0.39 eV), and one majority carrier trap labeled H1 (Ev+0.32 eV) are good candidates for the material optical activity. The onset of luminescence occurs after a thermal treatment and is accompanied by a high space charge density localization at the epilayer-substrate interface.