Recent developments in the MOVPE growth of low H content ZnSe-based compounds and heterostructures

The origin of unintentional hydrogen (H) incorporation during metalorganic vapour phase epitaxy (MOVPE) of ZnSe-based compounds is reviewed and discussed. Hydrogen enters in MOVPE-grown ZnSe as a result of alkyls surface reactions, effectively passivating intentional nitrogen (N) acceptors in p-doped ZnSe during the fabrication of blue-light emitting diodes and laser diodes. The existence of a marked trade-off between the proclivity of common Se alkyls to incorporate H and their thermal stability is pointed out. Current strategies to overcome this process limitation are then described along with results achieved and technological drawbacks. The use of a novel class of VI-group alkyl precursors of the form R₂X₂ [where X=Se, S and R is an ethyl (Et) or methyl (Me) radical] is proposed as an alternative solution. These alkyls allow a reduction of H incorporation in ZnSe-based materials, whilst retaining the low temperatures required for the growth of device quality wide band-gap II–VI compounds. Dimethyldiselenide (Me₂Se₂) and diethyldisulphide (Et₂S₂) allow the pyrolytic MOVPE growth of Zn(S)Se compounds below 400 °C. Mass spectrometry fragmentation experiments performed on the alkyl molecular ions allowed to investigate their relative bond strengths and likely decomposition paths. The reduced thermal stability of these alkyls is attributed to a weakening of the XC bonds in the R₂X₂ molecule induced by the stronger XX bond. Secondary ion mass spectrometry (SIMS) analysis showed that as-grown ZnSe have [H]≈(1–3)×10¹⁷ cm⁻³, i.e. among the lowest ever reported for MOVPE-grown layers. The functional validation of the new S and Se alkyls is completed by the structural and optical characterisation of Zn(S)Se-based heterostructures grown on (100)GaAs. High-resolution X-ray diffraction studies are presented along with cathodoluminescence (CL) measurements and compared to what reported in the literature. The epilayer structural properties compare well with that of molecular beam epitaxy and MOVPE grown Zn(S)Se heterostructures. CL spectra of ZnSe epilayers appear of good quality, with pronounced band-edge emissions and reduced deep level contributions. Specific emissions in the spectra of ZnS and ZnSe confirm the occurrence of several impurities in the layers, whose origin can be in part attributed to the yet insufficient purity of the novel alkyls.     

Metalorganic chemical vapor epitaxy and doping of ZnMgSSe heterostructures for blue emitting devices

Blue-green semiconductor laser diodes operating at room temperature are still the domain of wide bandgap II–VI compound semiconductors. CW operation at room temperature and hours of lifetime were reported. However, the conductivity control, defect generation and the ohmic contacts still need improvement. Therefore we focused our work on the MOVPE growth and the optimization of ZnMgSSe/ZnSSe/ZnSe heterostructures as well as on nitrogen doping of ZnSe. To verify the layer quality characterization was carried out by X-ray diffraction, electron probe microanalysis, electrical measurements and photoluminescence. ZnMgSSe/ZnSSe/ZnSe and ZnSSe/ZnSe quantum wells and superlattices were grown to investigate structural as well as interface properties. Electron beam and optical pumping was used to clarify the laser mechanism and to clarify the suitability of a MOVPE process to grow laser quality material. The electrical compensation of ZnSe doped with nitrogen is still controversially discussed whereas high n-type doping with chlorine was reproducible achieved. ZnSe: N doped at different growth conditions (II/VI ratio, growth temperature, nitrogen supply) using N₂ excited in a plasma source or by the use of nitrogen containing precursors was investigated to study the compensation mechanisms.     

Interfacial reactions in As---Se/Zn

As-Se-based chalcogenide glasses are IR transparent, and are an ideal braze for the ZnSe and ZnS. Interfacial reactions between As-Se and Zn were investigated using reaction couples As-50 at.% Se/Zn at 350°C, As-50 at.% Se/Zn at 370°C and As-60 at.% Se/Zn at 350°C. Cross-sections of reaction couples were analyzed by using optical microscopy and electron probe micro-analysis. Two phases were formed at the interface, and diffusion paths were glass/ZnSe/As₂Zn₃/Zn. Growth rates of the intermetallic layers were determined and at 350°C follow a parabolic law which indicates a diffusion-controlled mechanism.     

Growth of zincblende MgS/ZnSe superlattices and their heterointerface properties

MgS naturally forms the rocksalt structure, but zincblende MgS/ZnSe superlattices were grown with atmospheric-pressure MOVPE. The lattice constant of zincblende MgS was determined to be 5.59 Å from a series of X-ray measurements. The purging effect during the growth of MgS/ZnSe heterointerfaces was studied. While purging with a Se precursor resulted in the formation of an extra MgSe-related interface layer, purging with a Zn precursor improved the abruptness of the heterointerfaces.     

Highly reflective AlGaAsSb/InP Bragg reflector at 1.55 μm grown by MOVPE

The metalorganic vapor-phase epitaxy growth of a highly reflective 24-pair AlGaAsSb/InP-distributed Bragg reflector (DBR) is reported for the first time. The influence of the growth parameters such as the V/III input ratio, the growth temperature and the pressure, the total H₂ flow, the gas velocity and the switching sequence of the source gases at the interfaces has been deeply investigated and optimized to achieve stable growth conditions. The DBR achieves a reflectivity as high as 99.5% around 1.55 μm, a uniform stable composition, and an excellent crystal quality over the 2 inch wafer, with a surface free of crosshatch and a defect density below 1/cm². For the optical characterizations, measurements of linear and nonlinear reflectivity, transmission, pump-probe and photoluminescence were done. The interfaces and bulk layers of InP/AlGaAsSb/InP heterostructures were analyzed by transmission electron microscopy. High resolution X-ray diffraction measurements were used to determine the composition shift in the growth plane of the DBR. The measurements show the high quality of the growth and demonstrate that thick AlGaAsSb/InP heterostructures can be grown by metalorganic vapor-phase epitaxy (MOVPE), and in particular DBRs above 1.31 μm.     

Characterization of p-type ZnSe grown by MOVPE; excitonic emission lifetime measurements

Time-resolved photoluminescence (TRPL) measurements are made on p-type nitrogen-doped ZnSe grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) together with post-growth thermal annealing, in order to investigate optical quality of the layers. It is suggested that the annealing degrades the layer quality and the MOVPE samples have more non-radiative recombination centers compared with MBE samples. A key issue for high quality p-ZnSe by MOVPE seems to be optimization of annealing conditions.     

MOVPE of GaN using a specially designed two-flow horizontal reactor

GaN epilayers have been grown on (0001) sapphire substrates with a specially designed two-flow horizontal metalorganic vapor phase epitaxy (MOVPE) reactor. Epilayers with flat and smooth surfaces were obtained at the growth temperature of 950°C with relatively low source supply rates. This indicates a relatively high growth efficiency of the reactor. Characterization by photoluminescence, X-ray diffraction and Hall measurements reveal that the epilayers are of reasonably high quality.     

Studies in the growth of ZnSe on GaAs(001)

This paper reports a study of the molecular beam epitaxial (MBE) growth of ZnSe on GaAs substrates using elemental sources. Growth rates of ZnSe as a function of Se:Zn flux ratio for constant Zn flux were determined over a wider range of values than previously reported. Careful measurements of atomic fluxes and sample thickness lead to a determination of the sticking coefficients of Zn and Se which are at variance with many previously reported values. The temperature dependence of the sticking coefficients of Zn and Se has been measured carefully and provides evidence for a greater desorption of Zn from the growing surface than previously thought, an effect which persists at low growth temperatures. Measurements at high flux ratios support the use of a precursor model to describe MBE growth of ZnSe on GaAs substrates.     

Temperature dependences of the optical characteristics of ZnMgSe single crystals

The temperature dependences of the refractive index of wide-gap Zn_{1 − x} Mg _x Se (x ∼ 0.5) single crystals are measured. It is established that, in the temperature range 300–530 K, the value of dn/dT for such crystals grown by the vertical Bridgman method is 9.31 × 10⁻⁵ at λ = 0.63 μm and 5.29 × 10⁻⁵ at λ = 10.6 μm. The obtained values of the coefficients dn/dT for hexagonal Zn_{1 − x} Mg _x Se single crystals are close to the corresponding values for cubic ZnSe single crystals and are much lower than those for hexagonal CdS. Examples of practical application of single crystals of the ZnMgSe substitutional solid solution as a thermostable material for polarization optical elements in the IR range are given.     

Elucidation of factors obstructing quality improvement of MOVPE-grown InN

A systematic study of the crystallographic and electrical/optical properties of MOVPE-grown InN was performed, and the factors that restrict the quality of MOVPE InN were elucidated. The quality of grown InN is highly dependant on the thermal decomposition of NH₃ as a nitrogen source. At a lower growth temperature (~550 °C) a shortage of active nitrogen, due to a lower decomposition rate of NH₃, causes the formation of N vacancies in the grown InN. With increasing growth temperature, a more stoichiometric crystal is grown and the electrical/optical properties improve. At temperatures above 600 °C, however, deterioration occurs at the N-face of In-polar InN near the substrate interface. This deterioration results in the formation of a porous layer during high temperature (~650 °C) growth. There are a few evidences that show that the hydrogen produced by NH₃ decomposition causes this degradation. Thus, improving the quality of MOVPE-grown InN by changing the growth temperature can be difficult. However, a short growth time at a high growth rate and a relatively high temperature is one effective way to solve this dilemma, and one can achieve carrier concentrations as low as 4×10¹⁸ cm⁻³ by growth at 650 °C for 30 min.     

Intersubband transition at 1.55 μm in AlN/GaN multiple quantum wells by metal organic vapor phase epitaxy using the pulse injection method at 770 °C

Intersubband transition (ISBT) at 1.55 μm in AlN/GaN multi quantum wells (MQWs) was realized by metal organic vapor phase epitaxy (MOVPE) using the pulse injection (PI) method to grow GaN well layers at 770 °C. It was shown that a main factor for shifting ISBT wavelength to shorter region to cover 1.55 μm and improving ISBT properties of MQWs is the growth temperature of MQWs. Best structural and ISBT properties are observed at low growth temperature of 770 °C in this study. Carbon incorporation level in GaN layer grown by the PI method (PI-GaN) showed one order smaller value compared with that by the conventional continuous method. Moreover, further decrease in growth temperature to 770 °C did not show significant increase in carbon incorporation in PI-GaN layer. It clearly indicates that the PI method is very effective in reducing carbon concentration in GaN layer, especially at low temperature region. The low carbon concentration of 4×10¹⁸ cm^?3 released by the PI method was indispensable for realizing enough carrier concentration of 1.6×10¹⁹ cm^?3 to achieve strong ISBT at 1.55 μm.     

Nitrogen as carrier gas for the growth of ZnSe and ZnSe:N in plasma enhanced metalorganic vapor phase epitaxy

For the precursor combination dimethylzinc-triethylamine and ditertiarybutylselenide the use of nitrogen carrier gas was investigated for the metalorganic vapor phase epitaxy (MOVPE) of ZnSe and ZnSe:N. The nitrogen doping was carried out with a separate nitrogen flow which was activated by a dc-plasma. In the photoluminescence spectra of undoped layers grown at 340°C with a VI/II ratio of 0.18 only excitonic emissions, separated into free and donor bound excitons, can be observed. The background carrier concentration was of the order of 1 × 10¹⁶ cm⁻³. The growth rate of epilayers grown in nitrogen is reduced by about 75% in comparison to the value obtained by using hydrogen as carrier gas. The doping can be regulated by the dopant flow and by the total pressure in the reactor. With increasing flow of plasma activated nitrogen and a reduced total pressure, the PL spectra showed broadened DAP emission without excitonic emissions. The electrical and optical properties obtained with nitrogen carrier gas are comparable with the results obtained with hydrogen carrier gas. So far, no p-type conductivity was measured. Therefore, the problem of compensation of p-type conductivity of ZnSe : N doped by dc-plasma enhanced N₂ was not solved by the use of N₂ carrier gas.     

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four‐fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH^‐ and S^2‐ in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S‐Zn₄]⁶⁺, [Zn‐S₄]^6‐) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechansim can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.     

热等静压处理对CVD ZnS/ZnSe复合材料性质的影响

对利用化学气相沉积(CVD)制备的ZnS/ZnSe复合材料进行了显微结构和光学性质的测试和分析.研究表明,热等静压过程使得CVD ZnS/ZnSe晶体内部晶粒尺寸明显增大,减少或消除了内部缺陷,提高了材料的光学透过率.     

Strain analysis of InGaN/GaN multi quantum well LED structures

Five period InGaN/GaN multi quantum well (MQW) light emitting diode (LED) structures were grown by a metalorganic chemical vapor deposition (MOCVD) system on c‐plane sapphire. The structural characteristics as a strain‐stress analysis of hexagonal epilayers MQWs were determined by using nondestructive high resolution x‐ray diffraction (HRXRD) in detail. The strain/stress analysis in AlN, GaN, and InGaN thin films with a variation of the In molar fraction in the InGaN well layers was conducted based on the precise measurement of the lattice parameters. The a‐ and c‐lattice parameters of the structures were calculated from the peak positions obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. The biaxial and hydrostatic components of the strain were extracted from the obtained a‐ and c‐direction strains values.     

Effect of growth parameters on the properties of GaN : Zn epilayers

The vapor-phase HCl/Ga/NH₃ method for deposition of GaN : Zn epilayers on sapphire substrates has been investigated to determine the dependence of epilayer resistivity, cathodoluminescence, and surface quality on the growth parameters. Both nucleation and control of the epilayer properties have been significantly improved by introducing HCl directly into the deposition zone in addition to the HCl that passes over the Ga source to produce GaCl. The effect of annealing on the stability of undoped layers and on the cathololuminescence of Zn-doped layers has also been investigated. Electroluminescent devices with reproducible properties have been obtained by growing structures consisting of an undoped n⁺ layer, a Zn-doped n-type layer, and a very thin, Zn-doped, high-resistivity layer whose growth parameters determine the emission wavelength and electroluminescence efficiency.     

Improved quality of InGaN/GaN multiple quantum wells by a strain relief layer

Different InGaN/GaN multi quantum wells (MQWs) structures were grown by metalorganic chemical vapor deposition (MOCVD). Samples were investigated by photoluminescence (PL), atom force microscopy (AFM) and double crystal X-ray diffractometry (DCXRD) to character their optical, morphological and crystal properties. By inserting the strain relief layer, the PL intensity was increased more than two times. The surface morphology was improved and the density of V-pits was reduced from 16–18×10⁸ to 6–7×10⁸/cm². Further, the interface abruptness was also improved. We attributed the improvements of the quality of InGaN/GaN MQWs to the relief of strain in the InGaN/GaN MQWs.     

Studies on the vapour growth of ZnS,ZnSe and ZnTe single crystals

Large, highly perfect single crystals (up to 10 × 10 × 5 mm³) of ZnS, ZnSe and ZnTe have been grown from the vapour phase by dissociative sublimation and chemical transport with iodine. Good quality crystals were obtained when the growth rate was limited by diffusion of the vapours rather than by thermal convection or by reaction rates at the charge and or the growing surface. The presence of an inert gas and defined iodine concentrations increase the growth stability, especially for charges with slight deviations from stoichiometry. Electronmicrographs, X-ray topographs and etching experiments revealed low dislocation densities and relatively large inclusion-free regions.     

Effect of hetero-interfaces on in situ wafer curvature behavior in InGaAs/GaAsP strain-balanced MQWs

Using high-accuracy in situ curvature measurement during growth of InGaAs/GaAsP strain-compensated multiple quantum wells (MQWs) by metal organic vapor phase epitaxy (MOVPE), we have successfully clarified the effect of hetero-interfaces on strain control in InGaAs/GaAsP strain-balanced MQWs. By analyzing curvature transients and X-ray diffraction (XRD) fringe patterns, we found that an inadequate gas-switching sequence induces unintended atomic content at the interfaces between InGaAs and GaAsP and then influences the average strain of the structure. Through considering the atomic characteristics and measuring the reflectance anisotropy transient during growth, it has been revealed that the optimized stabilization time for arsenic and phosphorus mixture before GaAsP barrier growth should be longer than 3 s at 610 °C.     

Thermally stimulated relaxation of misfit strains in Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(100) heterostructures with different buffer layers

The regularities of the defect formation in Si_1−x Ge_x/Si heterostructures (x = 0.15 and 0.30), consisting of a low-temperature Si buffer layer and a SiGe solid solution, during their growth and subsequent annealings at temperatures 550–650°C are investigated by the methods of optical and transmission electron microscopy and X-ray diffraction. It is shown that the misfit-strain relaxation by plastic deformation under the conditions studied occurs most intensively in heterostructures with low-temperature SiGe buffer layers. The maximum degree of misfit-strain relaxation (no higher than 45%) is observed in the heterostructures with x = 0.30 after annealing at 650°C. The results obtained are explained by the effect of the nature and concentration of dislocation-nucleation centers, existing in low-temperature buffer layers, on the characteristics of the formation of a dislocation structure in the heterostructures under consideration.