III-Nitride semiconductor growth by MBE: Recent issues

Semiconductor III-Nitrides, such as GaN, AlN, InN and their ternaries, have recently gained considerable attention after an uneven effort around the first half of the 1970s which paved the way to intense activity in the preceding decade. This is in part due to early obstacles achieving high quality layers, particularly those with p-type conductivity. With marketing of blue LEDs, the interest and consequently the effort grew to the point that CW lasers, high power amplifiers, and UV detectors have been added to the list of devices made in this material system. GaN and its allied semiconductors are grown with a variety of techniques. Generally, thick GaN layers are grown with hydride vapor phase epitaxy whereas the thin ones and heterojunctions formed by the above mentioned binaries and their ternaries are grown by organometallic vapor phase epitaxy and molecular beam epitaxy. OMVPE uses ammonia and metalorganics for group V and III elements whereas MBE uses either ammonia or RF activated , and metal for group III elements. Among the above mentioned thin-film growth methods, only the issues pertaining to MBE will be reviewed in this paper.     

Acoustic charge transport in GaN nanowires

We present acoustic charge transport in GaN nanowires (GaN NWs). The GaN NWs were grown by molecular beam epitaxy (MBE) on silicon(111) substrates. The nanowires were removed from the silicon substrate, aligned using surface acoustic waves (SAWs) on the piezoelectric substrate LiNbO(3) and finally contacted by electron beam lithography. Then, a SAW was used to create an acoustoelectric current in the GaN NWs which was detected as a function of radio-frequency (RF) wave frequency and its power. The presented method and our experimental findings open up a route towards new acoustic charge transport nanostructure devices in a wide bandgap material such as GaN.     

Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching

Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K₂S₂O₈. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process.     

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

MBE of InGaN/GaN heterostructures using ammonia as a source of nitrogen

Technical Physics Letters - InGaN/GaN heterostructures have been grown by molecular beam epitaxy (MBE) using ammonia as a source of nitrogen. The influence of the growth temperature and rate on the...     

Growth of A3N whiskers and plate-shaped crystals by molecular-beam epitaxy with the participation of the liquid phase

It is shown that InN and GaN whiskers and plate-shaped crystals can be grown by molecular-beam epitaxy (MBE), and the growth mechanism on gallium arsenide and sapphire substrates is investigated. A comparison is made with the theory. It is proved that the growth mechanism corresponds to the vapor-liquid-solid (VLS) mechanism, and the parameters of the crystallization process are determined. The nanometer sizes of the crystals grown give hope that the crystals and the VLS growth method itself can be used to obtain quantum-size objects (quantum dots and wires) by MBE in the promising system of elements A³B⁵-AlGaInN. Pis’ma Zh. Tekh. Fiz. 25, 55–63 (September 26, 1999)     

Effect of the nitrogen ion energy on the MBE growth of thin gallium nitride films

The influence of the energy of bombarding nitrogen ions on the growth of thin gallium nitride (GaN) films under molecular beam epitaxy (MBE) conditions has been simulated using the method of balance kinetic equations. The dependence of the GaN film growth rate on the ion energy is determined and changes in the structure of films grown at different ion energies are explained. Theoretical estimates satisfactorily agree with the available experimental data.     

III-Nitrides growth and AlGaN/GaN heterostructures on ferroelectric materials

The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb₃O₈) along with lithium niobate (LiNbO₃), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.     

Spontaneous Formation of Indium Clusters in InN Epilayers Grown by Molecular-Beam Epitaxy

We have studied the influence of growth conditions on the number of metallic indium clusters formed spontaneously in indium nitride (InN) layers grown by nitrogen plasma-assisted molecular-beam epitaxy (PAMBE). InN epilayers of N-and In-polarity were grown on c-sapphire substrates and GaN and AlN templates, respectively. N-polar layers were obtained in the standard PAMBE regime, while In-polar layers were grown using a three-stage regime including the stages of epitaxy with enhanced atomic migration and interruption of growth under nitrogen flow. A series of samples were prepared at various growth temperatures and relative In/N flow rates. Measurement of the magnetic-field dependences of the Hall-effect coefficient and its model approximation were used to determine the percentage content of In clusters in various InN layers and the minimum amount of such inclusions that can be achieved by varying the conditions of MBE growth.     

Plasma assisted molecular beam epitaxy growth of GaN

The growth of GaN on sapphire by plasma assisted molecular beam epitaxy (MBE) is investigated with the object of optimizing the material quality. The V/III flux ratio as well as the growth temperature are discussed in relation to their impact on electrical, optical and structural layer properties. Samples grown under nearly stoichiometric conditions exhibit both the highest mobilities and the highest photoluminescence efficiencies. Growth temperatures above 800°C were found to result in narrow reflections in X-ray diffraction. However, the chemical decomposition of GaN at temperatures above 850°C limits the suitable temperature range for the growth under high vacuum conditions.     

Influence of double buffer layers on properties of Ga-polarity GaN films grown by rf-plasma assisted molecular-beam epitaxy

Ga-polarity GaN thin films were grown on sapphire (0001) substrates by rf-plasma assisted molecular beam epitaxy (MBE) using a double buffer layer, which consisted of an intermediate-temperature GaN buffer layer (ITBL) grown at 690 °C and a conventional AlN buffer layer deposited at 740 °C. Raman scattering spectra showed that the E₂ (high) mode of GaN film grown on conventional AlN buffer layer is at about 570 cm⁻¹, and shifts to 568 cm⁻¹ when an ITBL was used. This indicates that the ITBL leads to the relaxation of residual strain in GaN film caused by mismatches in the lattice constants and coefficients of thermal expansion between the GaN epilayer and the sapphire substrate. Compared to the GaN film grown on the conventional AlN buffer layer, the GaN film grown on an ITBL shows higher Hall mobility and substantial reduction in the flicker noise levels with a Hooge parameter of 3.87×10⁻⁴, which is believed to be, to date, the lowest reported for GaN material. These results imply that the quality of Ga-polarity GaN films grown by MBE can be significantly improved by using an ITBL in addition to the conventional low-temperature AlN buffer layer.     

GaN:Eu electroluminescent devices grown by interrupted growth epitaxy

In this paper we report on electroluminescent devices fabricated using Eu-doped GaN films grown by interrupted growth epitaxy (IGE). IGE is a combination of conventional molecular beam epitaxy and migration enhanced epitaxy. It consists of a sequence of ON/OFF cycles of the Ga and Eu beams, while the N₂ plasma is kept constant during the entire growth time. IGE growth of GaN:Eu resulted in significant enhancement in the Eu emission intensity at 620.5 nm. The nitridation of the surface that occurs during the OFF cycle appears to be the dominant process producing the enhancement. Thick dielectric devices fabricated on glass substrates using IGE-grown GaN:Eu have resulted in luminance of ∼1000 cd/m² and luminous efficiency of ∼0.15 lm/W.     

The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

Interlayer relaxation in group-III-nitride-based heterostructures according to X-Ray diffraction data

The structure perfection in two samples of the InN-GaN bilayer heterosystem, grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, has been studied by the X-Ray diffraction techniques. Components of the microdistortion tensor were determined from an analysis of the broadening of diffraction peaks measured in various geometries. These data were used to evaluate the densities of various dislocation families in each layer of the heterosystem and to trace a change in the dislocated structure from the lower (GaN) to upper (InN) layer. A difference in the behavior of dislocations in the two samples grown by different methods (MBE versus MOVPE) suggests that different mechanisms of relaxation of the elastic stresses between InN and GaN layers are operative in these cases.     

Properties and applications of MBE grown AlGaN

AlGaN epitaxial films have been grown on sapphire by plasma-induced molecular beam epitaxy (MBE) over the entire composition range from GaN to AlN. Structural and optical properties of the alloys have been investigated by X-ray diffraction (XRD), transmission electron and atomic force microscopy, Raman scattering, ellipsometry, optical transmission, and subgap absorption spectroscopy. Electron spin resonance has been used to study the dependence of intrinsic paramagnetic defects on Al mole fraction. N- and p-type doping with Si and Mg, respectively is found to become increasingly difficult with increasing Al content because of a continuous shift of the donor and acceptor levels deeper into the bandgap. Apart from the use of AlGaN as cladding layers in light emitting diodes, applications in MODFET transistors, solar blind photodetectors, surface acoustic wave devices and Bragg reflectors appear interesting and will be discussed briefly.     

Study of the correlation between GaN material properties and the growth conditions of radio frequency plasma-assisted molecular beam epitaxy

The material properties of GaN thin films grown by radio frequency (RF) nitrogen plasma source molecular beam epitaxy (MBE) on (0001) Al₂O₃ substrates have been correlated to the V/III flux ratio during GaN growth and to the type and thickness of the buffer layer. The most remarkable observation is the change in the sign of the residual strain, from tensile to compressive as the V/III ratio alters from N-rich to stoichiometric (or slightly Ga-rich) conditions for GaN layers with a 17 nm AlN buffer layer. The residual strain was significantly reduced for a thinner 5 nm AlN buffer and it was zero for a 20 nm GaN buffer. A reduction of the rms surface roughness from 20 to 3 nm was achieved by decreasing the V/III ratio. Finally, stacking faults were observed only for significantly N-rich growth conditions.     

Growth and characterization of ZnSe based blue green VCSELs

Blue green vertical cavity surface emitting lasers (VCSELs) have been produced in which the active region comprised molecular beam epitaxy (MBE) grown ZnSe layers containing one or more ZnCdSe quantum wells. As a first step towards the production of a workable VCSEL we have produced optically pumped structures which have allowed us to address a number of issues relating to the design of the devices and the reproducibility of the grown active layers. In this paper we discuss those factors which are specific to II–VI compounds relating to the design, growth and fabrication of optically pumped blue green VCSELs, with the emphasis on overcoming problems in producing practical devices. We have demonstrated that with careful control of the growth conditions we have been able to grow structures in which the predicted and measured spontaneous emission peaks differed by only 1%, leading to the growth of VCSEL structures with less than 3% misalignment in the gain and cavity resonances. © 1998 Chapman & Hall     

Preparation of Porous GaN Buffer and Its Influence on the Residual Stress of GaN Epilayers Grown by Hydride Vapor Phase Epitaxy

The preparation of porous structure on the molecular beam epitaxy （MBE）-grown mixed-polarity GaN epilayers was reported by using the wet chemical etching method. The effect of this porous structure on the residual stress of subsequent-growth GaN epilayers was studied by Raman and photoluminescence （PL） spectrum. Substantial decrease in the biaxial stresse can be achieved by employing the porous buffers in the hydride vapour phase epitaxy （HVPE） epilayer growth.     

Multilayer AlN/AlGaN/GaN/AlGaN heterostructures for high-power field-effect transistors grown by ammonia MBE on silicon substrates

We report preliminary results on the transfer of the ammonia MBE technology of AlN/AlGaN/GaN/AlGaN heterostructures to silicon substrates. Optimization of the growth conditions allowed the number of macroscopic cracks in the epilayers to be reduced and ensured the growth of heterostructures with two-dimensional electron gas, which are suitable for the creation of field-effect transistors. The saturation current of prototype devices based on the heterostructures grown on silicon substrates are comparable with the analogous parameter of devices grown on sapphire and exhibits no decrease related to thermal scattering at high bias voltages.     

Investigations of selectively grown GaN/InGaN epitaxial layers

We have studied GaN/InGaN heterostructures grown by selective area low pressure metalorganic vapor phase epitaxy (LP-MOVPE). A GaN layer already grown on the c-face of sapphire has been used as substrate, partly masked by SiO₂. In a second epitaxial step a GaN/InGaN single heterostructure and GaN/InGaN/GaN double heterostructures were grown on the unmasked rectangular fields. We obtained good selectivity for GaN and for InGaN. A larger growth rate as compared to planar epitaxy and strong growth enhancement at the edges was observed. Spatially resolved measurements of the luminescence show an increase in indium incorporation of about 80% at the edges. Besides the larger indium offering at the edges, this is due to an enhanced growth rate. Very smooth facets are obtained. The influence of pressure on the surface morphology and growth enhancement was investigated.