Real-Time monitoring of III-V molecular beam epitaxial growth using spectroscopic ellipsometry

We report the real-time monitoring of monolayer thickness changes in AlAs and GaAs layer growth on rotating GaAs substrates using spectroscopic ellipsometry (SE). A phase-modulated spectroscopic ellipsometer was integrated with a III-V MBE system by triggering spectral acquisition synchronously with substrate rotation. Absolute thickness accuracy was verified using ex situ SE measurement. Reasonable agreement was also obtained between in situ growth rate measurements by SE and reflection high energy electron diffraction. The precision and speed of this method appears suitable for real-time control of quantum devices, such as resonant-tunneling diodes.     

Integrated multi-sensor system for real-time monitoring and control of HgCdTe MBE

We describe an integrated real-time sensing and control system for monitoring and controlling substrate temperature, layer composition, and effusion cell flux during molecular beam epitaxial growth of HgCdTe epilayers for advanced IR detectors. Substrate temperature is monitored in real-time using absorption-edge spectroscopy, allowing the temperature to be controlled within 1.5°C of the desired setpoint. In situ spectroscopic ellipsometry (SE) is used for monitoring HgCdTe layer composition in real-time. A comprehensive temperature- and composition-dependent dielectric function database has been recorded which allows the accurate and precise determination of Hg_1−xCd_xTe layer composition over a wide range of x-values, from 0.2 to 0.42. The composition changes inferred from real-time SE measurements obtained during growth of a two-layer structure are in excellent agreement with composition profiles obtained using post-growth secondary ion mass spectroscopy analysis. The accuracy and precision of SE measurements conducted over multiple growth runs are shown to be suitable for robust SE-based composition control. Changes in the Cd flux produced by a CdTe effusion cell are detected using an atomic optical absorption method. This method allows changes in HgCdTe layer composition to be correlated directly with variations in Cd flux. All of the in situ sensors are linked using a custom software framework to provide the foundation for real-time monitoring and control of HgCdTe MBE growth of high performance infrared detector structures over a wide range of compositions, layer thicknesses, and substrate temperatures.     

A composition-dependent model for the complex dielectric function of In1 xGaxAsyP1− y lattice-matched to InP

Accurate and numerically efficient models for the complex dielectric function vs wavelength and material characteristics are essential for the use of nondestructive optical techniques such as spectroscopic ellipsometry or reflectometry. These optical techniques are commonly used for real-time and run-to-run monitoring and control of growth and etch processes to determine a material's composition and thickness. In this work, we discuss an improved composition-dependent model for the complex dielectric function for lattice-matched In_1-xGa_xAs_yP_1-y/InP systems valid over the entire composition range 0 ≤ y ≤ 1. We describe our model, which is an extension of the critical point parabolic band method and is based on the model proposed by Charles Kim et al. for the A_xGa_1-x/GaAs system. We demonstrate the quality of the model in fitting optical data for individual compositions and compare our results to other established models including the harmonic oscillator approximation and the model of Adachi. Using results obtained from the individual fits, we generate a composition-dependent model that is valid for the entire range of lattice-matched compositions. Also, we show how this model can be used to accurately determine the composition (±0.01) of an unknown material whose dielectric response has been obtained using spectroscopic ellipsometry or a similar technique.     

Integrated multi-sensor control of II–VI MBE for growth of complex IR detector structures

Next-generation HgCdTe infrared detectors and detector arrays require the growth of multilayer heterojunction structures with precisely controlled alloy composition and doping levels and minimal defect densities. Molecular beam epitaxy (MBE) provides the ability to produce such structures. However, in the absence of a real-time, in situ control methodology the extreme sensitivity of HgCdTe layer quality and doping efficiency on fundamental MBE variable such a substrate temperature and effusion cell flux provide serious challenges to the uniform and reproducible growth of such structures. In this paper, we describe an integrated, multi-sensor approach for monitoring and controlling the variables that are most important for MBE growth of HgCdTe device structures used in advanced multi-color infrared detectors and high speed, low-noise avalanche photodiodes. Substrate temperature, effusion cell flux, and layer composition are monitored using absorption-edge spectroscopy (ABES), optical flux monitoring (OFM), an spectroscopic ellipsometry (SE), respectively. Flexible, custom software has been developed and implemented for analysis of sensor inputs and feedback control of the MBE system in response to those inputs. The sensors and their application to growth of HgCdTe will be described, and the use of a custom software framework for data analysis and system control will be discussed.     

Observation of low-T GaAs growth regimes by real-time ellipsometry

The molecular beam epitaxial growth of low temperature (LT) GaAs films has been studied by real-time ellipsometry. A modification in a GaAs (001) surface by cooling under a specific As₂ flux caused a change in the ellipsometry data. The thermocouple reading of this change was used as a signature to indicate the reproducible substrate temperature for the growth of LT-GaAs layers. The origin of this surface modification was studied by reflection high energy electron diffraction. The growth regimes of LT-GaAs layers were studied by real-time ellipsometry. The dielectric properties of the epitaxial layer and the critical thickness for epitaxial growth were extracted for various growth conditions. The microstructure beyond the critical point was found to be composed of amorphous as well as crystalline forms of GaAs.     

Monitoring of CdTe atomic layer epitaxy using in-situ spectroscopic ellipsometry

Atomic layer epitaxy or ALE has proven to be useful for the growth of epitaxial layers of high uniformity, good quality, and well-controlled thickness. In this study, we have carried out in-situ monitoring during the atmospheric pressure ALE of CdTe on GaAs (100) substrates using spectroscopic ellipsometry (SE). The susceptor temperature, reactant partial pressures, as well as the flow and flush duration for each precursor are crucial process variables for ALE growth. Growth was carried out for 20–25 cycles under different sets of these process conditions during the experiment and in-situ SE was used to verify the presence of layer-by-layer growth, which enabled the quick determination of the process window. We observed ALE growth of CdTe at 300°C, supporting the explanation that the growth of CdTe occurs via a surface catalyzed decomposition of the Te precursor di-isopropyltelluride (DIPTe). Investigation of ALE mode growth behavior for different susceptor temperatures and DIPTe flush times indicated that the growth was limited by competition between desorption and reaction of the adsorbed DIPTe species on the Cd terminated surface.     

A comparison of the critical thickness for MBE grown Lt-GaAs determined by In-Situ ellipsometry and transmission electron microscopy

The growth of low temperature (LT) GaAs by molecular beam epitaxy has been studied using ellipsometry. Different regimes of growth were observed in the data, depending on film thickness. Epitaxial growth of pseudomorphic LT-GaAs occurred immediately above the substrate, followed by a layer with changing dielectric properties. This upper layer can be modeled as a two-phase region consisting of epitaxial LT-GaAs and small grained, polycrystalline GaAs, which increases in volume fraction with increasing layer thickness. For sufficiently thick LT layers, cross-sectional transmission electron microscopy analysis showed pyramidal defects that were composed primarily of highly twinned regions. The ellipsometry data showed a deviation from the homogeneous growth model at a thickness less than the thickness at which the pyramidal defects nucleated in all samples.     

In situ optical diagnostics of growing surfaces in the process of nanoheterostructure fabrication

It is demonstrated that in situ reflectance and reflectance-anisotropy measurements can be used as efficient real-time monitoring tools for all stages of the growth of heterostructures with ultrathin (few-monolayer) GaAs and AlAs layers. Changes in the layer composition at normal GaAs/AlAs interfaces in the active region of resonant-tunneling diode structures are detected with a thickness resolution on the order of one monolayer. Resonant-tunneling diodes with a peak-to-valley ratio of 3.3 and peak current density of 6.6 × 10⁴ A/cm² are fabricated.     

Temperature dependence of the optical properties of Hg1−xCdxTe

The alloy composition of Hg_1−xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1−xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1−xCd_xTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂(X), and E₂(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.     

Continuous in situ growth rate extraction using pyrometric interferometry and laser reflectance measurement during molecular beam epitaxy

We describe a new, universal, and easy-to-use method for real-time thickness and growth rate extraction based on in situ optical monitoring during molecular beam epitaxy of GaAs/AlAs layered structures. The method is based on a novel least-square phase extraction algorithm and a model-reference comparison scheme. The new method demonstrates a significantly reduced growth rate estimation error (<2%) on both pyrometric interferometry (PI) and laser reflectance (LR) monitored data over conventional methods. Based on preliminary studies, the new method also shows excellent dynamic tracking of growth rate with a thickness resolution of only 30 nm when the growth rate is intentionally changed.     

Application of spectroscopic ellipsometry for real-time control of CdTe and HgCdTe growth in an OMCVD system

A multi-wavelengthinsitu spectroscopic ellipsometer system is described. The hardware can acquire accurate ellipsometric data at 44 wavelengths in less than one second, is simple and compact, and is well suited forin-situ monitoring of chemical vapor deposition. The software used for data analysis is capable of determining the growth rate and composition of the growing layer in real time. These tools were used to study the organometallic chemical vapor deposition of CdTe, HgTe, and HgCdTe on GaAs. We could obtain the dielectric constants of these materials at the growth temperature and also the growth rate and composition of the layers in real time. Feedback control of CdTe growth was performed by connecting an analog control voltage line from the data acquisition/ analysis computer to the dimethylcadmium mass flow controller. Using dielectric constants of HgCdTe for two different compositions at the growth temperature, composition control of HgCdTe was attempted in a similar manner.     

In-Situ monitoring of temperature and alloy composition of Hg1−xCdxTe using FTIR spectroscopic techniques

Optical real-time in-situ sensors play a very important role in the processing of semiconductor devices because of their noncontact remote nature and excellent compatibility with UHV systems. In this work, we report on progress in developing an in-situ temperature sensor for HgCdTe structures grown by molecular beam epitaxy (MBE). Based on the Fourier transform infrared (FTIR) spectrometer, this sensor is capable of continuous real-time monitoring of the surface temperature, thickness and alloy composition of HgCdTe epilayers. The accuracy and sensitivity of this FTIR technique were studied in all temperature ranges of interest. Also compared are two different methods of temperature determination obtained from the normalized spectral radiance. The influence of stray radiation and of sample holder rotation on the measurement accuracy have been studied. Reflectivity spectra for HgCdTe/CdZnTe(211) and HgCdTe/CdTe(211)/Si(211) structures have been analyzed in real time in order to determine the layer thickness and alloy composition for growing layers. Also discussed is a multilayer-structure optical model developed to solve the problem of composition determination at early stages of growth. The application of this model for fitting the transmission spectra is demonstrated.     

Modeling of in situ monitored laser reflectance during MOCVD growth of HgCdTe

An effective way to in situ monitor the metalorganic chemical vapor deposition (MOCVD) of HgCdTe/CdTe/ZnTe on GaAs or GaAs/Si substrates is presented. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphology for each layer in the grown multilayer structure. In situ monitoring has enabled precise measurements of ZnTe nucleation and CdTe buffer layer thicknesses. Monitoring the constancy of reflectance during the thicker CdTe buffer growth where absorption in the CdTe reduces reflectance to just the surface component has led to optimum buffer growth ensuring good quality of subsequently grown HgCdTe. During the interdiffused multilayer process (IMP) HgCdTe growth, because multiple interfaces are present within the absorption length, a periodic reflectance signal is maintained throughout this growth cycle. A theoretical model was developed to extract IMP layer thicknesses from in situ recorded experimental data. For structures that required the growth of a larger band gap HgCdTe cap layer on top of a smaller band gap active layer, in situ monitored reflectance data allowed determination of alloy composition in the cap layer as well. Continuous monitoring of IMP parameters established the stability of growth conditions, translating into depth uniformity of the grown material, and allowed diagnosis of growth rate instabilities in terms of changes in the HgTe and CdTe parts of the IMP cycle. A unique advantage of in situ laser monitoring is the opportunity to perform “interactive” crystal growth, a development that is a key to real time MOCVD HgCdTe feedback growth control.     

分子束外延HgCdTe薄膜的CdTe缓冲层特性研究

CdTe是GaAs衬底上分子束外延(MBE)HgCdTe薄膜时的缓冲层,引入缓冲层的目的是减小失配位错,CdTe缓冲层的生长直接影响到后续HgCdTe薄膜的制备质量,然而目前现有文献鲜有报道CdTe缓冲层的最佳厚度.采用X射线双晶衍射、位错腐蚀坑密度(EPD)、FT-IR和椭圆偏振光谱的方法,从CdTe缓冲层厚度对位错密度的影响入手,分析并确定了理想的CdTe缓冲层厚度.     

In-situ ellipsometric measurements of the MBE growth of CdTe/HgTe and CdTe/ZnTe superlattices

Phase modulated ellipsometric data recorded during molecular beam epitaxial growth of CdTe/HgTe and CdTe/ZnTe superlattices on (100) and (211)B oriented Cd_0.96Zn_0.04Te and GaAs substrates are presented. The measurements provide a continuous monitor of the growth process, thickness, growth rate, compositional data, and evidence of interdiffusion in CdTe/HgTe superlattices at elevated temperatures. The thickness measurements are independent of growth kinetics and surface orientation and agree well with those obtained from x-ray diffraction and reflection high energy electron diffraction. Ellipsometry shows that the incorporation of Hg in CdTe is significantly higher on (100) oriented surfaces than on (211)B oriented surfaces. Fine structure in the data from CdTe/ZnTe superlattices may be associated with a surface reconstruction during deposition of each CdTe layer. The experimental results for CdTe/HgTe superlattices compare well with results of thin film multi-layer calculations. The general applicability of ellipsometry as an in-situ analytical technique for epitaxial growth of a range of semiconductor superlattices is discussed.     

Real-time optical techniques and QMS to characterize growth in a modified commercial OMVPE reactor

We describe a modified commercial OMVPE reactor that incorporates quadrupole mass spectrometry (QMS) with a broadband parallel-processing optical spectrometer that simultaneously performs spectroscopic ellipsometry (SE) and reflectance-difference spectroscopy (RDS) measurements. We demonstrate its use by determining the surface temperature of Si to a precision of ±1°C and investigating the initial stages of GaP heteroepitaxy on Si(100). Analysis of the real-time SE data indicates that under our conditions GaP and Si interpenetrate as optically identifiable materials on a thickness scale of 100Å.     

Improved model for the analysis of FTIR transmission spectra from multilayer HgCdTe structures

This paper reports the further development of the model for the analysis of FTIR transmission spectra from the dual-color Hg_1−xCd_xTe (MCT) structures for the constituent layer thickness and alloy composition. The previously reported model¹ was shown to suffer from excessively high uncertainty in the provided individual layer thickness and low convergence rate for some types of structures, attributed primarily to inaccuracies in the model representation of the MCT dielectric function. Since last report, we have substantially improved the FTIR analysis accuracy by developing a better MCT dielectric function approximation, which is based on the interpolation of the measured spectroscopic ellipsometry (SE) experimental spectral dielectric functions at few discrete alloy compositions. Based on this, the optical model for graded layers was also created and calibrated against the traditional FTIR data reduction technique. The new model was shown to produce the most accurate fits to the experimental FTIR transmission spectra from single- and two-color detector structures, and has demonstrated a better convergence rate. The new model was tested to predict both band cutoff wavelengths for the actual two-color MWIR/LWIR SUMIT detectors.¹⁵ We have demonstrated that the model prediction from as-grown structures was in good agreement with the actual two-color device data, as measured on performance evaluation chips (PECs), thus validating the modeling technique for routine postgrowth wafer screening.     

Ellipsometric study of ultrathin AlxGa1−x As layers

A method for rapid monitoring of the parameters of thin-layer semiconductor structures by ellipsometry is proposed. The results of ellipsometric analysis of the material thickness and composition distribution in Al_xGa_1?x As films grown by low-temperature liquid-phase epitaxy (LPE) are presented. The ellipsometric data are compared to those obtained by the Raman scattering spectroscopy.     

Phase modulated ellipsometry used for composition control during MBE growth of CdHgTe: An analysis of instrumental factors and an assessment of the material produced

We have found phase modulated ellipsometry (PME) to be a sensitive analytical technique capable of providing real time information on composition, epilayer thickness, growth rate, interdiffusion and surface roughness. To fully exploit the benefits of PME, the instrument must be carefully calibrated and the many factors affecting its performance need to be understood and allowed for. In this paper we examine how the accuracy of the determination of composition of molecular beam epitaxially grown CdHgTe alloy films is affected by misalignment of the optical components, the presence of vacuum windows, signal conditioning prior to analog to digital conversion, temperature changes, and modulator settings. We conclude by presenting results which demonstrate the quality of CdHgTe layers grown on (211)B CdZnTe and (211)B GaAs substrates using our techniques.     

Determination of individual layer composition and thickness in multilayer HgCdTe structures

The reproducible molecular-beam epitaxy (MBE) growth of dual-band Hg_1−xCd_xTe (MCT) heterostructures requires routine post-growth wafer analysis for constituent layer thickness and alloy composition, therefore, demanding nondestructive characterization techniques that offer quick data feedback. This paper reports a multilayer structure model, which can be least-square fit directly to either Fourier transform infrared (FTIR) transmission or reflection spectra to provide individual layer thickness, alloy composition, and grading information for various complex structures. The model, we developed, is based on an accurate representation of both the real and imaginary parts of the MCT dielectric function across and above E _g as a function of alloy composition. The parametric, MCT optical-dielectric function for compositions varying between x=0.17 to x=0.5 was developed in the range from 400 cm⁻¹ to 4,000 cm⁻¹, based on a semi-empirical model for the absorption coefficient and extrapolation of the refractive index across E _g . The model parameters were refined through simultaneous fits to multiple reflection and transmission data sets from as-grown, double-layer planar heterostructure (DLPH) structures of variable thickness. The multilayer model was tested on a variety of simple DLPH structures with thick absorber layers (>8 μm) and was compared against traditional FTIR analysis and cross-section optical microscopy and showed good agreement in both composition and thickness. Model fits to dual-color MCT data and subsequent analysis of the internal parameter correlation have demonstrated that error bars on absorber layer composition and thickness could be as low as ∼0.0005 and ∼0.02 μm, correspondingly.