Growth process studies by reflectance anisotropy spectroscopy on MOVPE ZnSe

The growth of ZnSe on GaAs by metal organic vapour phase epitaxy (MOVPE) has been studied using reflectance anisotropy spectroscopy (RAS). The RA spectra of ZnSe are significantly different for growth on initially Se- or Zn-exposed GaAs surfaces. The Se-terminated GaAs (001) RA spectrum has Se-dimer-related features at 3.3 and 5.1 eV, and the large, high energy peak dominates during ZnSe growth on this surface. Transmission electron microscopy (TEM) analysis has been used to show that these large RA signals arise from anisotropic surface corrugation of the growing ZnSe epilayer. Under initially Zn-stabilised growth conditions, the ZnSe epilayer RA spectrum is largely featureless, showing only a weak peak at 4.7 eV and a dip at 5.1 eV. The corresponding surface anisotropy is greatly reduced in comparison with growth from the initially Se-terminated surface. These observations reveal RAS to the an important technique for ensuring the desired initial GaAs surface since the grown ZnSe surface morphology is critically dependent on the pre-growth substrate surface treatment. However, as the characteristic ZnSe RA spectra are relatively insensitive to changes in substrate temperature and VI–II ratio, RAS is of more limited use as an in-growth surface probe for MOVPE-grown ZnSe.     

Growth monitoring by reflectance anisotropy spectroscopy in MOVPE reactors for device fabrication

Reflectance anisotropy spectroscopy (RAS) has proved its capability to study surface processes during metalorganic vapour phase epitaxy (MOVPE) growth of a variety of III–V compounds. However, these investigations up to now have been mostly restricted to specialized research reactors. Therefore, we studied the feasibility of in-situ monitoring by RAS during growth on two production-type MOVPE reactors: horizontal 2 inch single wafer reactor AIX 200 and Planetary Reactor™ AIX 2000 for 5 × 3 inch. The slight modifications of the reactors necessary to gain normal incidence optical access to the sample do not alter the properties of the grown materials. While in the horizontal reactor the strain-free optical window allows one to obtain well-resolved RAS spectra the signals in the multiwafer reactor are affected by the anisotropy of the ceiling plate. Even in this case RAS spectra can be extracted. First measurements on rotating samples in the horizontal reactor demonstrate the possibility to obtain RAS spectra by multitransient spectroscopy. As an application monitoring of the growth of p-type layers for the base of GaInP/GaAs hetero-bipolar-transistors (HBTs) is discussed. The linear electro-optic effect (LEO) gives information on doping type and doping level. Time-resolved transients at specific energies are used to study the impact of different switching schemes on the properties of the base-emitter interface.     

GaN的MOVPE生长中台阶表面吸附的 量子化学计算

利用量子化学的密度泛函理论,计算GaN的MOVPE生长中主要的表面反应前体NH3、GaCH3(简写为MMG)在GaN(0001)面台阶处的吸附特性,并与理想平台表面对比.结果表明,在台阶吸附时,NH3有分子吸附和分解吸附两种结构,MMG只有两种分子吸附结构.NH3分子吸附时,吸附能在台阶处大于在平台表面;NH3分解吸附时,吸附能在平台表面大于在台阶处.说明NH3在台阶处容易发生分子吸附,而在平台表面容易发生分解吸附.MMG在台阶处的吸附能均大于在平台表面,说明它们在台阶处吸附比理想表面更容易.     

The growth of antimonides by MOVPE

There are three main reasons for the study of antimonides, they are the optical [mainly infrared], electrical [mainly Insb; the Gasb/Inas heterojunction] and structural [mainly ordering and spinodal decomposition] properties. These properties, together with the various techniques used to measure them, are discussed in the context of several difficulties from which the growth of antimonides suffer compared to the growth of nitrides, arsenides or phosphides. These difficulties include the vapour pressure of antimony over the growing surface, the lack of a stable group V hydride, the kinetically controlled nature of the growth and the lack of an insulating antimonide substrate. The effect of these difficulties on the growth of the binary materials, and hence, the various antimonide based devices such as lasers, LEDs, photodetectors, and Hall probes; will be discussed.     

In situ surface passivation of III–V semiconductors in MOVPE by amorphous As and P layers

A new process for chemical passivation of III–V semiconductor surfaces in metalorganic vapour phase epitaxy (MOVPE) is developed. A passivation layer is deposited directly after growth in the reactor. It consists of amorphous arsenic or a double-layer package of amorphous phosphorus and arsenic, which are grown by photo-decomposition of the group-V hydrides. These layers (caps) serve to protect the surfaces against contamination in air after removing the samples from the MOVPE growth reactor. Such passivation is applicable e.g. for a two-step epitaxy or for further surface characterizations.     

Growth of InN films and nanostructures by MOVPE

Since a few years, a lot of research efforts have been devoted to InN, the least known of the semiconducting group-III nitrides. Most of the samples available today have been grown using the molecular beam epitaxy technique, and fewer using the metal organic vapor phase epitaxy (MOVPE) method. Whatever the method, the growth of InN is extremely challenging, in particular due to the fact that no lattice matched substrate is available.     

Large scale manufacturing of compound semiconductors by MOVPE

As more compound semiconductor devices reach large volume manufacturing levels, a trend toward the use of the MOVPE technique is clear. In this paper we examine the criteria needed for MOVPE equipment suitable for large scale manufacturing. We find that although uniformity and device performance are necessary, reproducibility is also critical, along with high throughput and low operating costs. These points are illustrated by actual examples including MMIC power amplifiers, HB-LEDs, and solar cells. A realistic COO model provides a tool for evaluating MOVPE systems of different capacities. In situ control of key parameters during growth is now feasible, and will become an important method for increasing reproducibility and throughput. Lastly we look at the prospects for automation, for decreasing labor costs as well as wafer handling. This is likely to first have an impact on systems for the growth of electronic device structures on large (100 and 150 mm) wafers.     

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al_0.3Ga_0.7As/In_1−xGa_xP structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In_1−xGa_xP layers as well as reverse ones with top Al_0,3Ga_0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al_0.3Ga_0.7As/In_1−xGa_xP interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al_0.3Ga_0.7As/In_0.485Ga_0.515P lattice-matched structure and a linear dependence of the conduction band offset on In_1−xGa_xP composition, with a zero offset in the Al_0.3Ga_0.7As/In_0.315Ga_0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.     

Surface morphology of carbon-doped GaAs grown by MOVPE

The development of the surface structures of carbon-doped epitaxial GaAs layers grown by metalorganic vapor phase epitaxy was investigated by atomic force microscopy (AFM). Carbon-doped GaAs layers were grown using trimethyl gallium and a mixture of AsH₃/TMAs. The AFM micrographs were quantitatively analyzed through the determination of the height-height and height-difference correlation functions, which yields both the short and long range surface structures. The incorporation of carbon leads to the progressive roughening of the GaAs surface as well as an increase in surface correlation length. The high concentration of surface-adsorbed methyl radicals are suggested to lead to the diminution of growth rate and change in surface structure.     

Correlation of ordering formation and surface structure in (GaIn)P using modulated MOVPE

Modulated metalorganic vapour phase epitaxial growth (MOVPE) is used to clarify the role of the surface conditions on the ordering behaviour in ternary (GaIn)P layers. The alternating deposition of GaP and InP layers with individual thicknesses of up to one monolayer is successfully used for the growth of (GaIn)P bulk layers lattice matched to (100) GaAs substrates with various off-orientations. The layer quality and the degree of ordering are investigated using high-resolution X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence spectroscopy (PL), respectively. The application of modulated growth conditions for the deposition of (GaIn)P bulk layers has a strong influence on the degree of ordering achieved in the intermediate growth temperature regime where the highest degree of ordering occurs under continuous MOVPE. Beside a new boundary structure observed in layers grown under modulated flux conditions, the successful growth of highly ordered (GaIn)P layers grown using the modulated MOVPE technique support the model that up to 2 monolayers of the (GaIn)P growth surface are involved in the ordering formation process.     

Photoluminescence characteristics of nitrogen atomic-layer-doped GaAs grown by MOVPE

Nitrogen atomic-layer-doped and uniformly doped GaAs were grown by MOVPE using dimethylhydrazine on a (001) plane. They showed several sharp photoluminescence lines with a full width at half maximum less than 1 meV at 8 K. Compared with uniformly doped GaAs, the photoluminescence intensity of the nitrogen-related line at the longest wavelength is enhanced in nitrogen atomic-layer-doped GaAs, suggesting that it is easier to form nitrogen pairs during atomic layer doping. To investigate the sharp nitrogen-related lines, we also grew GaAs with double atomic-layer-doped planes and varied the distance between the two planes. When the two planes are brought close to 1 nm, two new lines, NN_C and NN_D, appear between the two nitrogen-related lines, NN_A and NN_B, observed in a single nitrogen atomic-layer-doped GaAs. The NN_C and NN_D lines are also observed in uniformly doped GaAs. Therefore, NN_A and NN_B originate from excitons bound to pairs of nitrogen atoms, both of which are in the (001) plane, while NN_C and NN_D originate from those bound to pairs of nitrogen atoms, of which pairing directions are not included in the (001) plane. From the photoluminescence characteristics, distances between nitrogen atoms of a pair are estimated for each line.     

Progress in Modeling of III-Nitride MOVPE

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.     

MOVPE生长m面GaN薄膜的表面吸附研究

采用基于密度泛函理论的Materials Studio中的CASTEP模块,对金属有机物气相外延MOVPE生长m面GaN薄膜的表面反应前体的吸附过程进行研究.针对吸附粒子GaCH3和NH3在m面GaN表面不同的初始吸附位,优化计算了GaCH3和NH3在表面的吸附能、与近邻原子的距离、态密度、电荷密度分布、电子布居.计算结果表明,GaCH3在表面Ga brg2位优化之后的位置最稳定,吸附能最低,GaCH3中的Ga原子与表面邻近的N原子、Ga原子分别形成Ga-N、Ga-Ga共价键.NH3在表面N brg2位最稳定,吸附能最低,NH3中的N原子与表面邻近的Ga原子形成N-Ga共价键.通过对比在最佳吸附位的MMG中的Ga原子和NH3中的N原子与表面原子的电荷分布情况和布居数,证明上述吸附粒子与表面确实存在共价作用,形成共价键.     

Chemical attack of optical glass surface by formic acid vapor

The surface deterioration of glass by the attack of formic acid vapor was observed by an optical microscope, an ellipsometer, an X-ray diffractometer and an electron microprobe X-ray analyzer. The refractive index and thickness of the anomalous surface film formed on the glass were measured by means of ellipsometry. In the weathering of SF-3 and KzFS-2 glasses, the presence of water vapor in the atmosphere is necessary to produce stains on the glass surface. In the SF-3 glass with a SiO₂ network, the diffusion of Pb²⁺ and the formation of fine crystalline lead formate play important roles. In the KzFS-2 glass with a B₂O₃ network, both the destruction of the B₂O₃ network and the formation of fine crystals of boric acid play important roles. The diffusion of Pb²⁺ is not so important.     

Nitrogen content of GaAsN quantum wells by in situ monitoring during MOVPE growth

Metal–organic vapor phase epitaxial growth of GaAsN quantum wells is monitored by in situ reflectance measurements. Correlation between the change in the reflectance intensity and nitrogen content of the quantum well is established. The reflectance as a function of time also reveals if there is deterioration of the crystalline quality during growth. This method together with X-ray diffraction and photoluminescence characterization is applied to analyze GaAsN growth using various reactor pressures and TBAs/III molar flow ratios.     

Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.     

Anisotropy in InGaAs/GaAs heterostructures grown by low-pressure MOVPE and CBE

InGaAs/GaAs heterostructures grown on (001) substrates by low-pressure MOVPE exhibit a measurable anisotropy in their structural, optical and electrical properties. This anisotropy occurs in structures which have undergone partial or complete strain relaxation and it can be strongly reduced by using slightly misoriented substrates. A comparison with similar structures grown by CBE indicates that this anisotropy is less important. This study suggests that strain relaxation is achieved by a combination of several mechanisms whose relative importance depends on the orientation of the substrate and on growth temperature which varies with the growth technique.     

MOVPE生长GaN薄膜的表面吸附和扩散研究

利用量子化学计算方法,对MOVPE生长GaN薄膜的表面反应进行研究.特别针对反应前体GaCH3(简称MMG)在理想、H覆盖和NH2覆盖GaN(0001)面的吸附和扩散进行计算分析.通过建立3×3 超晶胞模型,优化计算了MMG在三种不同覆盖表面的稳定吸附位、吸附能和电子布居,搜寻了MMG在稳定吸附位之间的扩散能垒.计算结果表明:对于三种表面,MMG的稳定吸附位均为T4位和H3位,H3位比T4位略微稳定.MMG在NH2覆盖表面吸附能最大,在H覆盖表面吸附能最小,在理想表面吸附能居中.MMG中的Ga与不同的表面原子形成的化学键的键强的大小顺序为:Ga-N>Ga-Ga>Ga-H.相比于理想表面和H覆盖表面,MMG在NH2覆盖表面的扩散能垒最大,因此表面过量的NH2会抑制MMG的扩散.     

The kinetics of parasitic growth in GaAs MOVPE

Gallium arsenide (GaAs) deposition was carried out in a horizontal quartz reactor tube with trimethylgallium (TMGa) and arsine (AsH₃) as precursors, using a hydrogen (H₂) carrier gas. Temperatures were in the range 400–500 °C, where surface reactions limit deposition rate. Nucleation time and deposition rate were monitored using laser interferometry, optimum reflectance was gained by aligning a quartz wafer to back reflect the incident beam. The 980 nm infrared laser beam was sufficiently long in wavelength to be able to penetrate the wall deposit. Results showing the effect of temperature and V/III ratio on the nucleation time and deposition rate are presented, where with temperature the nucleation delay was observed to reduce and the growth rate to increase. The nucleation delay is consistent with a thermally activated surface nucleation for the parasitic GaAs. A theoretical growth rate model, based on a restricted set of reaction steps was used to compare with the experimental growth rates. Without any free parameters, the growth rates from theoretical calculation and experiment agreed within a factor of two and showed the same trends with V/III ratio and temperature. The non-linearity of the theoretical growth rates on an Arrhenius plot indicates that there is more than one dominant reaction step over the temperature range investigated. The range of experimental activation energies, calculated from Arrhenius plots, was 17.56–23.59 kJ mol⁻¹. A comparison of these activation energies and minimum deposition temperature with the literature indicates that the wall temperature measurement on an Aixtron reactor is over 100 °C higher than previously reported.     

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.