Inter-surface Diffusion Research Articles

Formation mechanism and suppression of Ga-rich streaks at macro-step edges in the growth of AlGaN on an AlN/sapphire-template

Abstract The formation mechanism of Ga-rich streaks in the AlGaN layers formed at macro-step edges was investigated by analyzing AlGaN layers grown by metalorganic vapor phase epitaxy on AlN/sapphire-templates. Energy dispersive X-ray spectroscopy in the cross-sectional scanning transmission electron microscopy revealed that the macro-steps formed during AlN growth were maintained through the upper AlGaN layer. This can be explained by the less interaction between adjacent macro-steps due to the sufficiently smaller surface diffusion lengths of Ga and Al species than the inter-macro-step distance of submicron to a few microns. Moreover, atomic force microscopy indicated that the apparent smooth region between macro-steps involved many atomic steps and terrace width of a few ten nanometers which may be the actual scale of the diffusion lengths. At the macro-step edge, facets that are different from a c-plane are formed. The higher incorporation efficiency of Ga atoms on these facets compared with that of the c-plane likely led to the formation of Ga-rich streaks due to the less inter-surface diffusion between the facets and c-plane regions. We demonstrated AlGaN growth without Ga-rich streaks using the flattened AlN/sapphire-template by polishing; this resulted in the achievement of homogeneous emission energies in AlGaN-based quantum wells.

Effect of Supply Direction of Precursors on a-Plane GaN Low Angle Incidence Microchannel Epitaxy by Ammonia-Based Metal–Organic Molecular Beam Epitaxy

The effects of varying the supply direction of precursors on a-plane GaN low angle incidence microchannel epitaxy by ammonia-based metal–organic molecular beam epitaxy were studied. Lateral growth was found to be wider when precursors were supplied from the [0001] direction than from the [0001̄] direction. The (0001̄) face formed on the supply side suppressing lateral growth when the precursors were supplied from the [0001̄] direction, while lateral growth was enlarged by the intersurface diffusion of adatoms from the side to the top when supplied from the [0001] direction. A smaller cross-section of lateral growth was experimentally observed when the (0001̄) face appeared on the supply side, which suggests that the chemical character of the side also affects lateral growth. The offset angle of the opening is another important factor for determining the lateral growth, which largely affects the formation of facets on the sides and, consequently, the grown shape.

Coalescence of a-plane GaN stripes in low angle incidence microchannel epitaxy by ammonia-based metal-organic molecular beam epitaxy

The coalescence of adjacent a-plane GaN stripes grown by low angle incidence microchannel epitaxy is studied to produce wide flat GaN layers with a low dislocation density. Coalescence was successfully achieved ,resulting in an extremely flat and smooth GaN layer being obtained using NH3-based metal-organic molecular beam epitaxy. Transmission electron microscopy observations revealed that lateral grown areas above SiO2 masks had a very low dislocation density, although several dislocations were observed where two adjacent stripes intersect. In contrast, there is an extremely high dislocation density in areas grown above openings, which were directly transferred from the template. Voids are occasionally formed where adjacent stripes intersect. The coalescence mechanism (including void formation) is studied. Inter-surface diffusion of Ga adatoms plays an important role in coalescence and flattening the surface.

Growth optimization toward low angle incidence microchannel epitaxy of GaN using ammonia-based metal-organic molecular beam epitaxy

Growth optimization toward low angle incidence microchannel epitaxy (LAIMCE) of GaN was accomplished using ammonia-based metal-organic molecular beam epitaxy (NH3-based MOMBE). Firstly, the [NH3]/[trimethylgallium (TMG)] ratio (R) dependence of selective GaN growth was studied. The growth temperature was set at 860°C while R was varied from 5 to 200 with precursors being supplied parallel to the openings cut in the SiO2 mask. The selectivity of the growth was superior for all R, because TMG and NH3 preferably decompose on the GaN film. The formation of {112̄0}GaN or {112̄2}GaN sidewalls and (0001)GaN surface were observed by the change in R. The intersurface diffusion of Ga adatoms was also changed by a change in R. Ga adatoms migrate from the sidewalls to the top at R lower than 50, whereas the migration weakened with R greater than 100. Secondly, LAIMCE was optimized by changing the growth temperature. Consequently, 6μm wide lateral overgrowth in the direction of precursor incidence was achieved with no pit after etching by H3PO4, which was six times wider than that in the opposite direction.

Low angle incidence microchannel epitaxy of GaN grown by ammonia-based metal–organic molecular beam epitaxy

GaN was grown by low angle incidence microchannel epitaxy (LAIMCE) using NH3-based metal–organic molecular beam epitaxy (NH3-based MOMBE). The growth mechanism was studied by varying the growth temperature and time. The effect of the incidence direction of precursors on lateral growth was also investigated by comparing the results obtained when precursors were supplied perpendicular and parallel to the openings in the mask. The thickness and width of lateral growth were largely influenced by the formation of a facet on the surface, which frequently terminates further growth. For example, a sample grown at 700°C with a perpendicular supply of precursors stopped growing both vertically and laterally after a certain time despite continuous supply of the precursors. On the other hand, a sample grown at 820°C with a parallel supply of precursors exhibited stable growth, and its width increased continuously with time. This is because inter-surface diffusion of adatoms occurred from the top to the sides, which enhanced the width of lateral growth. In contrast, low angle incidence supply of molecular beams perpendicular to the openings resulted in a Ga-rich condition on the side and formed the side facet, which terminated further LAIMCE growth.

Effect of crystallographic orientation of microchannel on low-angle incidence microchannel epitaxy on (0 0 1) GaAs substrate

Low-angle incidence microchannel epitaxy (LAIMCE) is a technique to obtain a lateral overgrowth using molecular beam epitaxy. In order to produce a wide and flat lateral overgrowth of GaAs, the effect of the crystallographic orientation of microchannel on GaAs LIMCE was investigated. The cross-sectional shape of GaAs LAIMCE was studied by scanning electron microscope and found to largely change by the choice of microchannel orientation. For example, the layer grown from a microchannel with the orientation of −10° off from [0 1 0] showed a trapezoidal shape, while the layer grown from a microchannel with that of +10° off from [0 1 0] had an inverted-triangular shape with a sharp peak on the growth front. The layer grown from that of +20° off from [0 1¯ 1] had a vertical side. Detailed observation of the layers revealed that several kinds of facets, such as (1 0 1), (1 1¯ 2)B, (1 1 n¯), (3 1 0), formed on the side and shaped the layers. As adatoms on the surface diffuse from these facets to the top of the layers, the lateral growth would be suppressed. Therefore, it is important to control the growth condition not to form facets on the side, at least to suppress the formation of the facets with strong nature of inter-surface diffusion of adatoms form side to top.

Growth mechanism of beam-induced lateral epitaxy on (0 0 1) GaAs substrate in molecular beam epitaxy

The mechanism of GaAs beam-induced lateral epitaxy (BILE) on (0 0 1) GaAs substrate by molecular beam epitaxy (MBE) was investigated by systematically varying the crystal orientation of ridges on the surface. In the growth sequence of BILE, a certain number of facets were first formed on the growth front in connection with the crystal orientation of the ridges. These facets competed with each other as they grew. As a result, the formation of the dominant facet determined the main shape. The crystal orientation of the ridge largely affected the formation of facets and their development. The cross-sectional images of the grown layers observed by a scanning electron microscope (SEM) indicates that the facets formed in the following order from first to last: (1 1 1)A facets, (1 1 0) facets, (1 1 1)B facets, and (0 0 1) facets. During the growth, facet formation was largely controlled by the intersurface diffusion of adatoms, although shadowing effects also influenced the grown shapes.

Understanding of crystal growth mechanisms through experimental studies of semiconductor epitaxy

Elementary crystal growth mechanisms in solution and vapor growths are discussed taking semiconductor as a model material. Solution growth mechanism is argued at first taking liquid phase epitaxy (LPE) as a model system. On the surface of the LPE layer, macrosteps are often generated as a result of step bunching. By studying the behavior of the macrosteps using photoluminescence image and chemical etching of the cleaved cross section, it was found that the shape of the macrostep is governed by bulk diffusion and growth kinetics. This has been confirmed by space experiments. It is concluded that the growth on the macrostep terrace is conducted by steps generated at screw dislocations. A technique, which is called microchannel epitaxy, was developed to realize macrostep-free and dislocation-free layers. Vapor growth mechanism is discussed next taking molecular beam epitaxy of III–V compound as a model system. Here, the mechanism of inter-surface diffusion is investigated. The surface diffusion of group-III elements between facets was studied by changing group-V pressure. It was found that the direction of the inter-surface diffusion is reversed twice as the group-V pressure is increased. This has been attributed to the different group-V pressure dependence of group-III ad-atom lifetime on different facet. It was concluded that in MBE the growth on the facet is conducted by birth and spread of two-dimensional nuclei and by mass transports from next facet and from effusion cell.

MBE growth of GaAs/AlGaAs quantum well on a patterned GaAs (0 0 1) substrate

A GaAs/AlGaAs quantum well (QW) was grown on a stripe-patterned (0 0 1) GaAs substrate with conventional molecular beam epitaxy. A mesa structure was formed with (0 0 1) facet on the top and (1 1 1) facet on the sides. It was found that the thickness of the QW layer on the (0 0 1) facet is controlled by the inter-surface diffusion of Ga adatoms from the (1 1 1) side facets. The behavior was investigated as a function of V/III ratio and the growth temperature.

26aE01 Inter-surface Diffusion and its application : For fabrication of micro/nano-structures(NCCG-34)

26aE01 Inter-surface Diffusion and its application : For fabrication of micro/nano-structures(NCCG-34)

Start of 2D nucleation by accumulation of Ga adatoms on GaAs (1 1 1)B facet

The effect of increasing the width of the (1 1 1)B facet on the molecular beam epitaxy (MBE) growth of a GaAs mesa structure consisting of (0 0 1) top and (1 1 1)B side surfaces was studied. The growth occurred only on the (0 0 1) surface in the beginning of the MBE deposition, while growth was observed on both surfaces as the (1 1 1)B facet width was intentionally increased by depositing a very thick epitaxial layer of more than 3 μm thickness. It is indicated that growth on (1 1 1)B starts when the Ga adatom concentration on (1 1 1)B is raised to the critical value for 2D nucleation by increasing the facet width, which reduces the efficiency of adatom flow from (1 1 1)B to (0 0 1) by intersurface diffusion. We show that a simple rate equation gives the critical value of ∼0.37 ML for the Ga coverage.

Effect of As molecular species on inter-surface diffusion in GaAs MBE for ridge structure fabrication

The faceting and the shrinkage processes of ridge structure fabricated along [1 1 0] on GaAs (0 0 1) substrate with As 2 flux and As 4 flux were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in the MBE chamber. It was found that the shrinkage of top size with As 4 flux was faster than that with As 2 flux. To analyze the result of the experiments, the calculation based on two-face inter-surface diffusion model was conducted. From this calculation, we show that the shrinkage of the top size is slower when the arsenic pressure is higher, simply because the lateral flow of Ga adatoms towards the top surface becomes smaller. Under the As 2 flux, the amount of Ga adatoms which migrate to (0 0 1) surfaces from (1 1 1)B surface becomes smaller compared with that under As 4 flux due to the higher reactivity of As 2 to Ga adatoms.

2D-nucleation on (1 1 1)B micro-facet studied by microprobe-RHEED in GaAs MBE for mesa-structure fabrication

2D-nucleation on GaAs (111)B micro-facet was studied by using microprobe-RHEED/ SEM MBE. The RHEED intensity oscillation on the (111)B facet was observed only when incident Ga flux was relatively large, and the oscillation disappeared by reducing Ga flux. It is indicated that, 2D-nucleation on (111)B surface occurs only when incident Ga flux is large enough so that surface Ga adatoms accumulate sufficiently to start 2D-nucleation, otherwise all of the Ga adatoms flow away by inter-surface diffusion to (001) where they are incorporated into the crystal. By using simple rate equations, we show that it is possible to estimate the critical value of Ga adatom concentration for nucleation and Ga incorporation lifetime on GaAs (111)B surface.

(1 1 1)B growth elimination in GaAs MBE of (0 0 1)-(1 1 1)B mesa structure by suppressing 2D-nucleation

MBE growth of GaAs mesa structure consisting of (0 0 1) top and (1 1 1)B side surfaces was studied and the ratio of growth rate on (1 1 1)B facet to that on (0 0 1) was measured in various growth conditions. The value changed drastically, from 0 to 0.8, depending on the growth conditions. It was found that complete growth elimination occurred on (1 1 1)B facet by decreasing incident Ga flux and suppressing 2D-nucleation. We obtained various information from these experiments about elementary process of GaAs MBE, including critical Ga adatom concentration for 2D-nucleation on (1 1 1)B surface and (1 1 1)B-(0 0 1) inter-surface diffusion.

Effect of substrate rotation on inter-surface diffusion in MBE for mesa-structure fabrication

Effect of substrate rotation on inter-surface diffusion between (0 0 1) and (1 1 1)B of GaAs(0 0 1) patterned substrates in MBE was studied. Growth on the (1 1 1)B facet was eliminated when rotation speed was high, while the growth occurred at lower rotation speed. These phenomena indicate that, the onset of the growth on (1 1 1)B is determined by whether the amount of Ga adatoms on the surface is sufficient to start 2D-nucleation. We show that 2D-nucleation can be suppressed by rotating the substrate since the adatom concentration on (1 1 1)B is decreased to below the critical value for the nucleation.

A new way to achieve both selective and lateral growth by molecular beam epitaxy: low angle incidence microchannel epitaxy

In this paper we describe a completely new technique that has been called low angle incidence microchannel epitaxy (LAIMCE). By LAIMCE, selective area growth and enhancement of lateral growth on the mask can be achieved in molecular beam epitaxy (MBE). In LAIMCE, the molecular beams are sent with a small angle with the substrate of about 10–30°. By using LAIMCE we have successfully grown GaAs selective epilayers on GaAs (0 0 1) substrates patterned with a SiO 2 mask. These epilayers have been grown laterally on the mask material with a good ratio of width to thickness such as 8 in the present experiments. Moreover, we discuss the differences between conventional MBE and LAIMCE for what concerns the degree of selectivity and the growth mechanisms. It was found that a good degree of selectivity can be obtained at growth rates as high as 1.0 μm/h and at substrate temperatures as low as 610°C. In LAIMCE, inter-surface diffusion of adatoms plays a fundamental role in determining the shape of the growing epilayers. The morphology of the side surface and the lateral growth width are influenced by the crystallographic orientation of the open windows, the angles between the beams and the substrate and by the growth conditions.

Recent understandings of elementary growth processes in MBE of GaAs

Elementary growth processes involved in MBE of GaAs is discussed basing on the experiments done by a microprobe RHEED/SEM MBE system. The Incorporation diffusion length of Ga, λ inc during MBE growth was measured in situ on (001) and it was shown that λ inc depends on the arsenic pressure in different manner for the different ranges of arsenic pressure, which suggests different order of arsenic reaction is involved in the growth process. The direction of intersurface diffusion between two facets is studied and it is concluded that the key parameter which control the direction is the incorporation lifetime, τ inc of Ga. Its arsenic pressure and the surface reconstruction dependencies are found as the major factors which determine the direction of the intersurface diffusion.

Arsenic pressure dependence of incorporation diffusion length on (0 0 1) and (1 1 0) surfaces and inter-surface diffusion in MBE of GaAs

The arsenic pressure dependence of the inter-surface diffusion on the GaAs(0 0 1) and (1 1 0) surfaces was studied by using microprobe-RHEED/SEM MBE. Firstly, the arsenic pressure dependence of the incorporation diffusion length was studied on the (0 0 1) and the (1 1 0) surfaces. The incorporation diffusion length on both (0 0 1) and (1 1 0) was found inversely proportional to the square root of the arsenic pressure at low arsenic pressure, while it becomes proportional to inverse of the arsenic pressure at high arsenic pressure. From these results, it was deduced that the growth reaction with As 4 is of the first order at low arsenic pressure, while it becomes of the second order at high arsenic pressure. Next, the arsenic pressure dependence of the Ga lateral flux between (0 0 1) and (1 1 0) was studied. It was shown that the direction of the inter-surface diffusion is reversed with changing the arsenic pressure. At low arsenic pressure, the Ga inter-surface diffusion occurs from the (1 1 0) side surface to the (0 0 1) top surface, while it occurs in the opposite direction at high arsenic pressure.

Real-time observations of mesa shrinkage process in MBE of GaAs on (1 1 1)B patterned substrates and theoretical analysis

The faceting and shrinkage processes of mesa structures on GaAs (1 1 1)B substrates were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in MBE chamber. To understand the experimental results, the calculation based on inter-surface diffusion model was conducted and the results are compared with the experiments. From the comparison we evaluated diffusion coefficient and incorporation lifetime of Ga adatoms. The lifetime and the diffusion coefficient of (1 1 1)B substrate were estimated, respectively, as 1/70 as small and 50 times as large as those of {1 1 0} side wall. The mesa shrinks faster when arsenic pressure is higher, because Ga adatoms on {1 1 0} side wall flow more towards (1 1 1)B top and less towards the bottom.

Theoretical investigation of inter-surface diffusion on non-planar GaAs surfaces

We investigate the Ga adatom inter-surface diffusion between GaAs(001)-(2×4) top and GaAs(111)B-(2×2) side surfaces theoretically, based on microscopic calculations and a statistical mechanical discussion. The calculated migration potential based on the microscopic calculations imply that Ga adatoms are predominantly adsorbed on (001) surfaces. The results indicate that Ga adatoms basically diffuse from the (111)B side surface to the (001) top surface, so long as both top and side surfaces are single-domain structures. However, around the critical As pressure of surface transition, where (2×2) and ( 19 × 19 ) structures coexist on the (111)B side surface, boundaries exist that are covered with neither (2×2) nor ( 19 × 19 ) units. Since these non-covered boundaries can contain very reactive Ga adsorption sites, Ga adatoms tend to diffuse from the (001) top to the (111)B side surface, so long as non-covered boundaries appear on the (111)B side surfaces. Our theoretical investigation implies that the direction of the inter-surface diffusion between the GaAs(001) top and the (111)B side surfaces is reversed twice with increasing As pressure, which is in good agreement with the recent experimental reports.