Composition control of long wavelength MBE HgCdTe using In-situ spectroscopic ellipsometry

Improved composition control of Hg_1-xCd_xTe layers grown by molecular beam epitaxy using in-situ spectroscopic ellipsometry is described. This has increased our composition yields from <40% to approximately 70% for a specification of x to within 0.0015 of target composition. Knowledge of composition during growth also enables corrections to effusion cell temperatures so that the in-depth composition profile can be controlled. Further improvements were obtained after active composition control was implemented whereby the ellipsometer controls the Te cell temperature to maintain the desired composition.     

HgCdTe composition determination using spectroscopic ellipsometry during molecular beam epitaxy growth of near-infrared avalanche photodiode device structures

The application of spectroscopic ellipsometry (SE) for real-time composition determination during molecular beam epitaxy (MBE) growth of Hg_1−xCd_xTe alloys with x>0.5 is reported. Techniques previously developed for SE determination of composition in long-wavelength infrared (LWIR) HgCdTe have been successfully extended to near-infrared HgCdTe avalanche photodiode (APD) device structures with x values in the range of 0.6–0.8. Ellipsometric data collected over a spectral range of 1.7–5 eV were used to measure depth profiles of HgCdTe alloy composition through the use of an optical model of the growth surface. The optical model used a dielectric-function database collected through the growth of a set of HgCdTe calibration samples with x ranging from 0.6 to 0.8. The sensitivity of this SE method of composition determination is estimated to be Δx ∼0.0002 at x=0.6, which is sufficiently low to sense composition changes arising from flux variations of less than 0.1%. Errors in composition determination because of Hg-flux variations appear to be inconsequential, while substrate-temperature fluctuations have been observed to alter the derived composition at a rate of −0.0004/°C. By comparing the composition inferred from SE and postgrowth 300 K IR transmission measurements on a set of APD device structures, the run-to-run precision of the Se-derived composition (at x=0.6) is estimated to be ±0.0012, which is equivalent to the precision achieved with the same instrumentation during the growth of mid-wavelength infrared (MWIR) HgCdTe alloys in the same MBE system.     

Effect of incident angle in spectral ellipsometry on composition control during molecular beam epitaxial growth of HgCdTe

The effect of incident angle in spectral ellipsometry (SE) on composition control of Hg_1−xCd_xTe grown by molecular beam epitaxy (MBE) was investigated. Although a small uncertainty in the incident angle tends to have a significant impact on the ellipsometric data, and therefore the composition data, it was found that the incident angle uncertainty could be corrected in the SE model calculation, resulting in an “optimized” incident angle that would give the best fit between measured and calculated ellipsometric data. Experimental data supporting this simple corrective or optimization procedure for the incident angle are presented.     

Investigation of HgCdTe surface quality following Br-based etching for device fabrication using spectroscopic ellipsometry

We have examined the etching of HgCdTe (x=0.2) with bromine/ethylene glycol (Br/EG) solutions. Using a spectroscopic ellipsometer, we tracked the ellipsometric parameters (ψ and Δ) of the freshly etched HgCdTe surfaces. Parameters ψ and Δ were measured periodically as these values changed with the surface exposed to air. A second set of Br/EG-etched samples was stored in deionized (DI) water. We found that DI water effectively preserved the freshly etched HgCdTe surface for a period of several hours. Comparison with the literature on HgCdTe surface chemistry implies that oxide growth is inhibited on the etched HgCdTe samples immersed in DI water. Modeling results based on the measured ψ and Δ values agree with this assessment.     

Control of very-long-wavelength infrared HgCdTe detector-cutoff wavelength

Hg_1-xCd_xTe is an important material for infrared (IR) detection applications where the bandgap of the alloy varies from semimetal to 1.4 eV. The large variation in bandgap energy with HgCdTe composition causes difficulty in controlling detector-cutoff wavelength, particularly for the long-wavelength IR and very-long-wavelength infrared (VLWIR, greater than 12 μm) spectral bands. Our ability to control the HgCdTe composition and compositional profile during growth by molecular beam epitaxy (MBE) is improved significantly by using automated feedback control from spectroscopic ellipsometry (SE) measurements, where the standard deviation in the error in composition has improved by a factor of 5, from σ=0.0081 to σ=0.0016. To improve our ability to predict cutoff wavelength from IR transmission measurements, we have used a model of the absorption in HgCdTe to revise our past empirical cutoff relationship to include the effect of compositional grading. We have achieved a mean detector-cutoff wavelength of 14.1 μm and standard deviation of σ=0.25 μm for a series of 19 processed layers with a target cutoff of 14 μm. The excellent control in VLWIR detector cutoff we have observed is attributed to automated compositional control and an improved cutoff-prediction model.     

Spectroscopic ellipsometry for monitoring and control of molecular beam epitaxially grown HgCdTe heterostructures

A systematic study of the effect of measurement perturbation on in situ monitoring of the composition of molecular beam epitaxially (MBE) grown Hg_1−xCd_xTe using spectroscopic ellipsometry was carried out. Of the five variables investigated, which included angle of incidence, wavelength of the light beam, modulator rotation, analyzer rotation, and modulator amplitude, the angle of incidence and the modulator rotation had the strongest effect on the in situ Hg_1−xCd_xTe composition monitoring process. A wobble-free sample manipulator was installed to reduce the impact of these two variables. With these improvements, the spectroscopic ellipsometer is now routinely used to monitor Hg _1−xCd_xTe compositions during MBE growth of heterostructures and is a useful tool in diagnosing growth-related problems. Examples are included for both application areas, that include the control of the interface between Hg_1−xCd_xTe layers of different compositions, i.e. device engineering.     

Real-time control of HgCdTe growth by organometallic vapor phase epitaxy using spectroscopic ellipsometry

The use of spectroscopic ellipsometry for monitoring the vapor phase epitaxial growth of mercury cadmium telluride (Hg_1−xCd_xTe) in real-time is demonstrated. The ellipsometer is used to perform system identification of the chemical vapor deposition reactor used for the growth of CdTe and to measure the response of the reactor to different growth conditions. The dynamic behavior of the reactor is also studied by evaluating the gas transport delay. The optical constants of Hg_1−xCd_xTe are determined at the growth temperature for different compositions.In-situ real-time composition control is performed during the growth of Hg_1−xCd_xTe. The required target compositions are attained by the ellipsometer and appropriate corrections are also made by the controller when a noise input in the form of a temperature variation is introduced.     

In-situ monitoring of surface stoichiometry and growth kinetics study of GaN (0001) in MOVPE by spectroscopic ellipsometry

GaN surface stoichiometry and growth kinetics in MOVPE were studied by in-situ spectroscopic ellipsometry. The effect of MOVPE conditions on both the surface stoichiometry and growth kinetics was investigated. The surface stoichiometry, such as N-rich, Ga-rich and Ga-excess surfaces, was monitored, and was drastically changed by the variation of the NH₃ partial pressure. When the TMG supply was interrupted during the growth, the layer-by-layer decomposition/revaporation was observed in H₂/NH₃ ambient. The decomposition rate was measured as a function of the NH₃ flow rate at the conventional epilayer growth temperatures (1050–1140 C). The decomposition rate was decreased with the increase in the N coverage on the GaN surface. it was found that the surface stoichiometry is a very important parameter for the control of the MOVPE growth kinetics.     

In-situ ellipsometry studies of adsorption of Hg on CdTe(211)B/Si(211) and molecular beam epitaxy growth of HgCdTe(211)B

We study the adsorption of Hg on CdTe(211)B using an 88-wavelength spectroscopic ellipsometer mounted on a commercial, molecular beam epitaxy (MBE) chamber. A detailed analysis of the pseudo-dielectric function shows that Hg is present at the surface both in chemisorbed and physisorbed form. Effective medium models for a mixture of chemisorbed and physisorbed Hg on the microscopically rough CdTe surface could not fit our data. However, a proposed model in which a partial layer of physisorbed Hg sits on top of a partial layer of chemisorbed Hg fits the measured pseudo-dielectric function well and yields precise values for the thicknesses of the chemisorbed and the physisorbed Hg layers. These values change in the expected manner as a function of Hg flux, temperature, and Te coverage. An analysis of the uncertainty in the measured thicknesses is carried out in detail, and a study of the limitations of the ellipsometer used for this study is presented. The effects of these limitations on the precision and accuracy of in-situ data are enumerated.     

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.     

Investigation and optimization of InGaAs/InP heterointerfaces grown by chemical beam epitaxy using spectroscopic ellipsometry and photoluminescence

Spectroscopic ellipsometry (SE) has been used to investigate transition layers for InGaAs/InP heterointerfaces. For the case of InGaAs on InP, we have found that the samples can be best modeled by a strained In_xGa_1-xAs film with the possible presence of a thin interface region (15Å). We are unable to conclusively determine the existence of such a thin transition region. For InP on InGaAs, we find clear indications of As contamination in the bulk film, and that the addition of a thin interface region of In_0.75Gao_0.25As_0.5P_0.5 improves both the numerical fit and shape of the dielectric response curves, especially around E₁ and E₁ + Δ₁ where the effects of a transition region are most pronounced. However, difficulties in modeling the dielectric response of the contaminated InP film make identification of an interface transition region only speculative at this point. Multiple single quantum well structures have also been grown and analyzed with 7K photoluminescence. The quality of the quantum wells shows strong dependence on the gas switching sequence used at the heterointerfaces. The best switching sequence produced a 0.5 nm well with a 7K FWHM of only 12.3 meV. Multiple quantum wells have also been grown to investigate the uniformity and repeatability of our system. Twenty period MQWs with a well width of 1.6 nm display a 14K FWHM of 7.9 meV.     

Automated compositional control of Hg1−xCdxTe during MBE, using in situ spectroscopic ellipsometry

The implementation of a feedback control system for maintaining a desired compositional value in Hg_1−xCd_xTe epilayers is reported. An 88-wavelength ellipsometer monitored the Cd content (x) of a Hg_1−xCd_xTe film during molecular beam epitaxy, and deviations from a pre-determined set-point were automatically corrected via adjustments in the CdTe effusion cell temperature. The accuracy of this system (Δx∼0.002) was confirmed by Fourier transform infrared transmission measurements made ex situ on the epilayers.     

GaN外延膜的红外椭偏光谱研究

利用红外椭偏光谱法（IRSE）对生长在蓝宝石衬底上的非故意掺杂的GaN外延膜进行了研究。通过对椭偏光谱的理论计算，拟合了本征GaN中的声子振动参量和等离子振荡的频率及阻尼常量，并由此得到了各向异性的折射率和消光系数的色散曲线以及载流子浓度和迁移率。将得到的电学参数同霍耳测量结果进行了比较。     

Low-k dielectrics: a non-destructive characterization by infrared spectroscopic ellipsometry

An infrared spectroscopic ellipsometer devoted to the characterization of silicon microelectronics has recently been developed at SOPRA. Its main feature is the ability to measure on a small spot (80×200 μm) with a high signal/noise ratio. An original patented optical design suppresses back face reflection and ensures good-quality spectral measurements in the 600–7000 cm⁻¹ range. The excellent signal/noise ratio allows the performance of measurements in less than 30 s. Automation and real-time analysis are included to offer an operator-orientated metrology tool. Details of the instrument are presented, and its use for the characterization of different kinds of low-k dielectrics.     

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.     

Optical properties of Cd0.9Zn0.1 Te studied by variable angle spectroscopic ellipsometry between 0.75 and 6.24 eV

Optical properties of Cd_0.9Zn_0.1Te (CZT) were studied by variable angle spectroscopic ellipsometry (VASE). Measurements made by VASE were performed on CZT and CdTe samples in air at room temperature at multiple angles of incidence. A parametric function model was employed in the VASE analysis to determine the dielectric functions ɛ=ɛ₁+iɛ₂ in the range of 0.75 to 6.24 eV. A two-oscillator analytical model was used to describe the dielectric response of native oxides on CZT. Surface oxide optical properties and thickness on CZT were also determined in conjunction with the VASE measurement and analysis of a CdTe sample. Two samples of CZT of different oxide thicknesses were measured and their optical constants were coupled together in a multiple-sample, multiple-model VASE analysis to resolve correlations between fitting parameters. Effective medium approximation was used to describe the optical properties of the CZT oxide with roughness. A Kramers-Kronig self-consistency check of the real and imaginary parts of the Cd_0.9Zn_0.1Te dielectric functions was performed over the energy range 0.75 to 6.24 eV. A five-Lorentz-oscillator model was employed to describe the dielectric response of CZT in the range of 1.6 to 6.24 eV. Intensity transmission measurements were made on the Cd_0.9Zn_0.1Te and CdTe, showing the absorption energy band edges of ∼1.58 and 1.46 eV, respectively.     

Use of ellipsometry to characterize the surface of HgCdTe

Ellipsometry is a sensitive, rapid, and nondestructive optical technique for characterizing materials, especially surfaces and films. By measuring the change in the state of polarization of a light beam reflecting from the sample, one may infer certain characteristics of the sample. We present a review of the applications of ellipsometry to HgCdTe and related materials. The fundamentals of the technique are discussed briefly and the optical parameters at the wavelength 6328A for several materials of interest to infrared technology are listed. The emphasis of this paper is on the interpretation of the ellipsometric data, expressed in terms of the usual parameters Ψ and Δ obtained at a single wavelength. Methods and limitations of the analysis of single films, both nonabsorbing and absorbing, are discussed. Examples of an acceptance window for process monitoring are presented. The ellipsometric signatures of amorphous Te films and microroughness are described, along with a graphical method for interpreting the readings from very thin films. Spectroscopic applications and in situ monitoring of molecular beam epitaxial growth processes are briefly reviewed.     

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.     

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.