Effects of real-time monitored growth interrupt on crystalline quality of AlN epilayer grown on sapphire by molecular beam epitaxy

Crack-free aluminum nitride (AlN) epilayers were grown on sapphire using growth-interrupt technique by radio-frequency assisted molecular beam epitaxy. In-situ reflectance spectroscopy was introduced for real-time monitoring of the growth of AlN epilayers. X-ray diffraction and atomic force microscopy measurements reveal that the threading dislocation density decreases considerably by using the growth-interrupt technique. Raman spectroscopy is used to characterize the residual stress of AlN epilayers. The optical transmittance and absorption spectra of AlN epilayers show a high transmittance and a sharp absorption edge.     

Behaviour of vicinal InP surfaces grown by MOVPE: exploitation of AFM images

InP substrates and epilayers grown by MOVPE have been studied by AFM. For different misorientation angles, we observed the surface of the substrate after annealing under phosphine (PH₃) and of the epilayers under different growth conditions such as growth temperature T_g and trimethylindium (TMI) partial pressure. After annealing the terrace width corresponds to the nominal value of misorientation angle measured by X-ray diffraction. We observed different topographies and roughnesses for the grown layers corresponding to different growth modes. We propose, taking into account the roughness of the surface, a calculation of the step height and terrace width. For 2D nucleation (θ ≤ 0.2° and T_g = 500°C) and step flow mode, the roughness is low while it is increased by step bunching (θ ≥ 0.5° and T_g ≥ 580°C). Moreover we have examined the surface morphology for different misorientation angles and determined the influence of growth conditions (growth temperature, indium partial pressure) on the growth mechanism. At T_g = 580°C the increase of the TMI partial pressure in the reactor enhances the step bunching and leads to larger terraces.     

Structural and optical properties of quaternary AlInGaN epilayers grown by MOCVD with various TMGa flows

AlInGaN quaternary epilayers have been grown with various TMGa flows by metalorganic chemical vapor deposition to investigate the influence of growth rate on the structural and optical properties. Triple-axis X-ray diffraction measurements show AlInGaN epilayers have good crystalline quality. Photoluminescence (PL) measurements show that the emission intensity of AlInGaN epilayers is twenty times stronger than that of AlGaN epilayer with comparable Al content. V-shaped pits are observed at the surface of AlInGaN epilayers by atomic force microscopy (AFM) and transmission electron microscopy (TEM). High growth rate leads to increased density and size of V-shaped pits, but crystalline quality is not degraded.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

AFM analysis of sidewall formation in low angle incidence microchannel epitaxy of GaAs

In this paper we show a detailed AFM observation of GaAs epilayers grown on GaAs (0 0 1) substrate patterned with SiO₂ mask by low angle incidence microchannel epitaxy (LAIMCE). We have found that low index facets are formed as sidewalls of the epilayers grown in open window aligned along singular directions. We have also found that low index facets are formed as sidewalls for epilayers showing a zigzagging edge. In the case of the epilayers grown in an open window aligned 10° off [0 1 0], for which lateral growth is maximum, we have found round shape sidewalls and (0 0 1) terraces at the boundary of the sidewall and the top surface. We concluded that the lateral growth proceeds when the sidewall is rough, while it stops when low index facets are formed as sidewalls.     

On the isothermal closed space sublimation growth of CdSe using a mixed source for selenium

The use of a selenium–tellurium (SeTe) mixed source in the isothermal close space sublimation growth of CdSe epilayers is considered. The epitaxial growth was performed in flowing helium by sequential exposures of the substrate to vapors of the mixed SeTe source and elemental cadmium at temperatures within 350–410 °C. In spite of the mixed source (proposed to decrease the partial pressure of Se), tellurium incorporation was small and CdSe_xTe_(1−x) (x∼0.98) epilayers were obtained. X-ray diffraction reciprocal space mapping shows the existence of hexagonal inclusions mainly on the (1 1 1) facets of the cubic phase. Material deposition on areas of the graphite crucible exposed to the sources, contamination of the Cd source and large growth rates suggest the existence of a selenium transport process via graphite. This transport might be the result of the combination of selenium deposition on graphite with a subsequent activated desorption of selenium under cadmium exposure. It affects Cd source purity and growth kinetic bringing on a modification of the usual atomic layer deposition regimen; however, a reproducible growth rate of the epilayers was obtained.     

Improved morphology for epitaxial growth on 4° off-axis 4H-SiC substrates

A process optimization of the growth of silicon carbide (SiC) epilayers on 4° off-axis 4H-SiC substrates is reported. Process parameters such as growth temperature, C/Si ratio and temperature ramp-up conditions are optimized for the standard non-chlorinated growth in order to grow smooth epilayers without step bunching and triangular defects. The growth of 6-μm-thick n-type-doped epitaxial layers on 75-mm-diameter wafers is demonstrated as well as that of 20-μm-thick layer. The optimized process was then transferred to a chloride-based process and a growth rate of 28 μm/h was achieved without morphology degradation. A low growth temperature and a low C/Si ratio are the key parameters to reduce both the step bunching and the formation of triangular defects.     

Gettering Properties of Praseodymium in InGaP LPE Growth

Praseodymium (Pr) is added to the InGaP growth melt during Liquid Phase Epitaxy (LEP) for the first time. The epilayers are grown by using a supercoling method, on (100) Cr-doped Semi-Insulating (SI) GaAs substrates at a growth temperature of 790°C. An examination of the electrical properties reveals that, depending on the amount of Pr in the growth melt, n-tpye InGaP epilayers with room temperature electron concentrations in the range of 3.4 × 10¹⁶ cm⁻³ to 5.3 × 10¹⁵ cm⁻³ and electron mobilities from 730 to 1310 cm²/Vs can be prepared. The photoluminescence spectral results show that by increasing the amount of Pr in the growth melt, smaller Full Width at Half Maximum (FWHM) values and better band edge (BE) recombination intensities result.     

Iso-epitaxial growth of n-octacosane crystals

Iso-epitaxial(epilayer)growth of n-octacosane crystals is reported. The morphology of the epilayers is in the form of triangular growth islands with or without truncature. Epilayer growth precedes spiral growth mechanism which occurs at medium and low supersaturations, respectively. The occurrence of triangular and truncated rhombic platelets is found to be a manifestation of growth at medium supersaturations.     

Editorial Board

The employment of more than 10 μm thick AlN epilayers on SiC substrates for AlGaN/GaN high-electron-mobility transistors (HEMTs) substantially raises their performance in high-power energy-efficient amplifiers for 4G wireless mobile stations. In this paper, structural properties and surface morphology of thick AlN epilayers deposited by hydride vapor phase epitaxy (HVPE) on off-axis conductive 6H-SiC substrates are reported. The epilayers were examined in detail by high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), Nomarski differential interference contrast (DIC), scanning electron microscopy (SEM), and selective wet chemical etching. At optimal substrate preparation and growth conditions, a full width at half-maximum (FWHM) of the XRD rocking curve (RC) for the symmetric (00.2) reflex was very close to that of the substrate (less than 40 arcsec) suggesting low screw dislocation density in the epilayer (～10⁶ cm^?2) and small in-plane tilt misorientation. Reciprocal space mapping around asymmetric reflexes and measured lattice parameters indicated a fully relaxed state of the epilayers. The unit-cell-high stepped areas of the epilayers with 0.5 nm root mean square (RMS) roughness over 1×1 μm² scan were alternated with step-bunching instabilities up to 350 nm in height. Low warp of the substrates makes them suitable for precise epitaxy of HEMT structures.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Epitaxy and structural characterization of ZnSeTe layers grown on InP substrates

ZnSeTe epilayers were grown on InP substrates with and without ZnCdSe buffer layers. Structural X-ray and Raman characterization results indicate a large range of Se composition in the epilayers. The introduction of a buffer layer and the control of the growth conditions improve the structure by limiting compositional variations close to the stoichiometric value where lattice matching conditions are met. Effects of plastic strain relaxation are thereby minimized.     

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity ^tBuSH, ^tBu₂Se and the adduct Me₂Zn:Et₃N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm².     

Characterization of the carrot defect in 4H-SiC epitaxial layers

Characterization of the epitaxial defect known as the carrot defect was performed in thick 4H-SiC epilayers. A large number of carrot defects have been studied using different experimental techniques such as Nomarski optical microscopy, KOH etching, cathodoluminescence and synchrotron white beam X-ray topography. This has revealed that carrot defects appear in many different shapes and structures in the epilayers. Our results support the previous assignment of the carrot defect as related to a prismatic stacking fault. However, we have observed carrot defects with and without a visible threading dislocation related etch pit in the head region, after KOH etching. Polishing of epilayers in a few μm steps in combination with etching in molten KOH and imaging using Nomarski optical microscope has been used to find the geometry and origin of the carrot defects in different epilayers. The defects were found to originate both at the epi-substrate interface and during the epitaxial growth. Different sources of the carrot defect have been observed at the epi-substrate interface, which result in different structures and surfaces appearance of the defect in the epilayer. Furthermore, termination of the carrot defect inside the epilayer and the influence of substrate surface damage and growth conditions on the density of carrot defects are studied.     

固态源MBE系统生长高质量的调制掺杂GaAs结构材料和InP/InP外延材料的兼容性研究

通过固态源的分子束外延系统生长了调制掺杂AlGaAs/GaAs结构材料和InP/InP外延材料.在生长含磷材料之后,生长条件(真空状态)变差;我们通过采取合理的工艺方法和生长工艺条件的优化,获得了电子迁移率为1.86×105cm2/Vs(77K)调制掺杂AlGaAs/GaAs结构材料和电子迁移率为2.09×105cm2/Vs(77K)δ-Si掺杂AlGaAs/GaAs结构材料.InP/InP材料的电子迁移率为4.57×104 cm2/Vs(77K),该数值是目前国际报道最高迁移率值和最低的电子浓度的InP外延材料.成功地实现了在一个固态源分子束外延设备交替生长高质量的调制掺杂AlGaAs/GaAs结构材料和含磷材料.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

On the use of remote RF plasma source to enhance III–V MOCVD technology

A remote RF (13.56 MHz) plasma source, assembled on a metalorganic chemical vapour deposition (MOCVD) system, was used to investigate the processes of (a) cleaning and passivation of InP substrates with H atoms (H₂ plasmas), (b) deposition of InP epilayers from In(CH₃)₃ and PH_x radicals (PH₃/H₂ plasmas), and (c) deposition of InN on sapphire from In(CH₃)₃ and N atoms (N₂---H₂---Ar plasmas). From kinetic and spectroscopic ellipsometric in situ analysis, the removal of native oxide from InP surface was found to be complete, without surface damage (phosphorus depletion), at 230°C and 7 min of H atoms exposure. The growth of InP epilayers with PH₃ plasma pre-activation was successful (stoichiometric InP) even at low V/III ratio. During InN growth, the use of optical emission spectroscopy (OES) and of in situ ellipsometry (SE) was determinant for the process control.     

Acceptor incorporation in SiC epilayers grown at high growth rate with chloride-based CVD

A systematic p-type doping study has been performed on 4H- and 6H-silicon carbide (SiC) epilayers grown at high growth rate using chloride-based chemical vapor deposition. The effect of temperature, pressure, growth rate, C/Si-, Cl/Si-ratios and dopant flow on the incorporation of the acceptor atoms aluminum and boron has been studied. The C/Si-ratio on the aluminum incorporation has similar behavior to what has been reported for the standard non-chlorinated low growth rate process, while no clear effect of C/Si-ratio was observed for the boron incorporation. A higher Cl/Si-ratio seems to lead to lower the aluminum and boron incorporation either due to more effective silicon supply at high Cl/Si-ratio or due to removal of dopant atoms from the surface by chlorine. The doping concentration is stable to the variations in silicon molar fraction, growth pressure and growth temperature for the aluminum-doped layers. Also p-type doping with gallium was tested.     

Effect of defects on strain state in nonpolar a-plane GaN

We have investigated the influence of basal stacking fault (BSF) and impurity related defect on the strain state of a-plane GaN epilayers. Four a-plane GaN epilayers were grown on r-plane sapphire using different growth strategies by metalorganic chemical vapor deposition. It is found that with a growing number of stacking fault, both the anisotropic in-plane strain and compressive out-plane strain along c-axis are relieved. Epitaxial lateral overgrowth with a TiN interlayer is an effective way to relieve in-plane strain and reduce BSF density. The extrapolated lattice parameters free of biaxial strain increase with the normalized yellow luminescence intensity. Hydrostatic strain induced by impurity-related defects is the possible cause of this phenomenon.     

Effects of ultraviolet irradiation during growth of ZnSe epilayers by atmospheric pressure metalorganic vapor-phase epitaxy using dimethyl zinc and H2Se

This work has investigated the effects of ultraviolet irradiation on the epitaxial growth process of undoped ZnSe by atmospheric-pressure metalorganic vapor phase epitaxy. Dimethyl zinc and H₂Se at a [VI]/[II] mole ratio of 20 were the source gases used for growth onto (100)-just oriented semi-insulating GaAs substrates. Hydrogen was used as the carrier gas. A 500 W Hg-Xe lamp irradiated the substrate at 300 nm wavelength during growth. Growth temperature was varied from 210 to 450°C. Epilayers grown in the presence of irradiation experienced a prominent decrease in growth rate, which occurred even at high temperatures. Through a combination of surface and vapor-phase reactions, UV irradiation also affected the photoluminescence properties, crystalline quality, and surface morphology of the epilayers.