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.     

Band gap uniformity and layer stability of HgTe-CdTe superlattices grown by photon-assisted molecular beam epitaxy

Two material properties important to the application of HgTe/CdTe superlattices for device fabrication are band gap uniformity and thermal stability. In this paper, we present the results of an infrared photoluminescence study of (211)B HgTe/CdTe superlattices grown by photon-assisted molecular beam epitaxy which show that cut-off wavelength uniformity can be controlled to a level commensurate with the demands of advanced infrared detector fabrication. Infrared photoluminescence and transmission electron microscopy were also employed to demonstrate that (211)B HgTe/CdTe superlattices are less prone to interdiffusion than previously believed.     

Growth kinetics and properties of heteroepitaxial (Cd,Zn)Te films prepared by metalorganic molecular beam epitaxy

Thermally precracked diethylzinc, dimethylcadmium, and diethyltelluride were used for the metalorganic molecular beam epitaxial growth of (001) ZnTe, CdTe, and CdZnTe films on GaAs substrates. Measurements of the growth rate as a function of the substrate temperature and the II/VI ratio were used to determine the growth kinetics of (001) ZnTe and CdTe. (001) CdTe,ZnTe, and CdZnTe films were deposited under near-stoichiometric growth conditions, as determined from the growth kinetics. The best heteroepitaxial films exhibited x-ray rocking curve full widths at half maximum of 200–210 arc-s. The photoluminescence spectra of the binary and ternary films at 5K were dominated by features associated with bound and free excitons. Secondary ion mass spectrometry measurements showed that the films were free of carbon and oxygen. A new mercury precursor, divinylmercury, was used for HgTe growth. Preliminary results indicated that divinylmercury is a viable mercury source for metalorganic molecular beam epitaxial growth when it is precracked.     

TEM Characterization of HgCdTe/CdTe Grown on GaAs(211)B Substrates

A microstructural study of HgCdTe/CdTe/GaAs(211)B and CdTe/GaAs(211)B heterostructures grown using molecular beam epitaxy (MBE) was carried out using transmission electron microscopy and small-probe microanalysis. High-quality MBE-grown CdTe on GaAs(211)B substrates was demonstrated to be a viable composite substrate platform for HgCdTe growth. In addition, analysis of interfacial misfit dislocations and residual strain showed that the CdTe/GaAs interface was fully relaxed except in localized regions where GaAs surface polishing had caused small pits. In the case of HgCdTe/CdTe/GaAs(211)B, the use of thin HgTe buffer layers between HgCdTe and CdTe for improving the HgCdTe crystal quality was also investigated.     

Direct growth of CdTe on (100), (211), and (111) Si by metalorganic chemical vapor deposition

CdTe epilayers were grown directly on (100), (211), and (111) silicon substrates by metalorganic chemical vapor deposition (MOCVD). The crystallinity and the growth orientation of the CdTe film were dependent on the surface treatment of the Si substrate. The surface treatment consisted of exposure of the Si surface to diethyltelluride (DETe) at temperatures over 600°C prior to CdTe growth. Direct growth of CdTe on (100) Si produced polycrystalline films whereas (lll)B single crystals grew when Si was exposed to DETe prior to CdTe growth. On (211) Si, single crystal films with (133)A orientation was obtained when grown directly; but produced films with (211)A orientation when the Si surface was exposed to DETe. On the other hand, only (lll)A CdTe films were possible on (111) Si, both with and without Te source exposure, although twinning was increased after exposure. The results indicate that the exposure to a Te-source changes the initial growth stage significantly, except for the growth on (111) Si. We propose a model in which a Te atom replaces a Si atom that is bound to two Si atoms.     

Understanding the Evolution of CdTe Buffer Layer Surfaces on ZnTe/Si(211) and GaAs(211)B During Cyclic Annealing

We present the results of a detailed study of the changes that occur on CdTe buffer layer surfaces grown on ZnTe/Si(211) and GaAs(211)B during the routine thermal cyclic annealing (TCA) process. Observations indicate that CdTe buffer layer surfaces are Te saturated when the TCA is performed under Te overpressure. In the absence of Te flux during the TCA step, the CdTe surface loses CdTe congruently and the typical CdTe nanowires show the presence of nodules on their surfaces. The observed changes in reflection high-energy electron diffraction patterns during TCA are explained in terms of surface chemistry and topography observations. Overall, the Te overpressure is necessary to maintain a smoother and pristine surface to continue the molecular beam epitaxy (MBE) growth.     

Interdiffusion behavior of Hg in HgTe/CdTe superlattices grown on Cd0.96Zn0.04 Te (211)B substrates by molecular beam epitaxy

Double-crystal x-ray rocking curve (DCRC) and secondary-ion mass-spectroscopy (SIMS) measurements have been performed to investigate the effect of rapid thermal annealing on the interdiffusion behavior of Hg in HgTe/CdTe superlattices grown on Cd_0.96Zn_0.04Te (211)B substrates by molecular beam epitaxy. The sharp satellite peaks of the DCRC measurements on a 100-period HgTe/CdTe (100Å/100Å) superlattice show a periodic arrangement of the superlattice with high-quality interfaces. The negative direction of the entropy change obtained from the diffusion coefficients as a function of the reciprocal of the temperature after RTA indicates that the Hg diffusion for the annealed HgTe/CdTe superlattice is caused by an interstitial mechanism. The Cd and the Hg concentration profiles near the annealed HgTe/CdTe superlattice interfaces, as measured by SIMS, show a nonlinear behavior for Hg, originating from the interstitial diffusion mechanism of the Hg composition. These results indicate that a nonlinear interdiffusion behavior is dominant for HgTe/CdTe superlattices annealed at 190°C and that the rectangular shape of HgTe/CdTe superlattices may change to a parabolic shape because of the intermixing of Hg and Cd due to the thermal treatment.     

The Surface Kinetics of MBE-Grown CdTe (211)B During In Situ Cyclic Annealing

The surface kinetics of CdTe (211)B grown by molecular beam epitaxy (MBE) is investigated using spectroscopic ellipsometry (SE) during in situ cyclic annealing. A method of measuring sublimation rates from high-index surfaces without use of reflection high-energy electron diffraction is presented. The effect of Te₂ overpressure on the activation energy of sublimation for the CdTe (211)B surface is reported. The sensitivity of SE to surface temperature and film thickness was leveraged to monitor sublimation rates of CdTe stabilized by a Te₂ overpressure. The sublimation activation energy was found to increase from 0.45 eV to 2.94 eV under the Te₂ beam pressure regime investigated.     

Structure of CdTe(111)B grown by MBE on misoriented Si(001)

Single domain CdTe(111)B has been grown on Si(001) substrates tilted 1^o 2^o, and 4^o toward [110]. All the layers started with a double-domain structure, then a transition from a double- to a single-domain was observed by reflection high energy electron diffraction. A microscopic picture of this transition is presented. We also measured the tilt between CdTe(111)B and Si(001). The result does not follow the tilt predicted by the currently existing model. A new model of the microscopic mechanism of CdTe(111)B growth is presented. New evidence indicates that optimizing the tilt of the substrate surface is very crucial in improving the CdTe(111)B crystal quality.     

Microstructural and optical characterization of CdTe(211)B/ZnTe/Si(211) grown by molecular beam epitaxy

CdTe layers have been grown by molecular beam epitaxy on 3 inch nominal Si(211) under various conditions to study the effect of growth parameters on the structural quality. The microstructure of several samples was investigated by high resolution transmission electron microscopy (HRTEM). The orientation of the CdTe layers was affected strongly by the ZnTe buffer deposition temperature. Both single domain CdTe(133)B and CdTe(211)B were obtained by selective growth of ZnTe buffer layers at different temperatures. We demonstrated that thin ZnTe buffer layers (<2 nm) are sufficient to maintain the (211) orientation. CdTe deposited at ∼300°C grows with its normal lattice parameter from the onset of growth, demonstrating the effective strain accommodation of the buffer layer. The low tilt angle (<1°) between CdTe[211] and Si[211] indicates that high miscut Si(211) substrates are unnecessary. From low temperature photoluminescence, it is shown that Cd-substituted Li is the main residual impurity in the CdTe layer. In addition, deep emission bands are attributed to the presence of As_Te and Ag_Cd acceptors. There is no evidence that copper plays a role in the impurity contamination of the samples.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

Metalorganic Vapor-Phase Epitaxial Growth of (211)B CdTe on (211) Si Using Ge Interfacial Layer

We have achieved metalorganic vapor-phase epitaxial growth of (211)B CdTe on Si without the requirement of a pregrowth high-temperature oxide desorption step. This was achieved by growing a thin Ge film on the starting (211) Si substrates. To get (211)B CdTe orientation, the Ge surface was exposed to As prior to the start of CdTe growth. A thin ZnTe interlayer between Ge and CdTe has been shown to improve the CdTe surface morphology.     

分子束外延CdTe（211）B／Si复合衬底材料

报道了用MBE的方法，在3英寸Si衬底上制备ZnTe／CdTe（211）B复合衬底材料的初步研究结果，该研究结果将能够直接应用于大面积Si基HgCdTe IRFPA材料的生长．经过Si（211）衬底低温表面处理、ZnTe低温成核、高温退火、高温ZnTe、CdTe层的生长研究，用MBE方法成功地获得了3英寸Si基ZnTe／CdTe（211）B复合衬底材料．CdTe厚度大于10μm，XRD FWHM平均值为120arc sec，最好达到100arc sec，无（133）孪晶和其他多晶晶向．     

HgTe/HgCdTe superlattices grown on CdTe/Si by molecular beam epitaxy for infrared detection

High-quality HgTe/CdTe superlattices (SLs) and device structures incorporating them were grown by molecular beam epitaxy (MBE) on CdTe/Si substrates. In-situ techniques, such as reflection, high-energy electron diffraction and spectroscopic ellipsometry, were extensively used to rigorously control the growth parameters. The full width at half maximum (FWHM) of x-ray double-crystal rocking curves (DCRCs) were 100–150 arcsec, comparable to those of HgCdTe alloys grown on the same type of substrates. The room-temperature Fourier transform infrared (FTIR) spectrum exhibits two-dimensional features characteristic of SLs. Trial devices in a p⁺-n⁻-n⁺ format were fabricated by diffusing gold in order to further evaluate the HgTe/CdTe SL performance. Gold diffusion was chosen to fabricate photovoltaic junctions in order to preserve the structural integrity of the SLs during the device processing. Though no attempt was made in the current study to optimize the junction properties by Au diffusion, this method has proven to be very useful for rapid preliminary evaluation. The measured spectral-response and detectivity data indicate the possibility to fabricate photovoltaic devices on an HgTe/CdTe SL, although further work is needed to optimize the p-n junction fabrication.     

Real-Time In?Situ Monitoring of GaAs (211) Oxide Desorption and CdTe Growth by Spectroscopic Ellipsometry

We describe the growth of CdTe (211)B by molecular beam epitaxy on large-area epiready GaAs (211)B substrates. Prior to CdTe growth, GaAs substrates were thermally cleaned under an As₄ flux. Oxide desorption was verified by in?situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction. The use of in?situ SE played a significant role in the study of CdTe-on-GaAs growth and annealing processes. An effective medium approximation (EMA) was used to model the overlayer thickness variation of CdTe epilayers throughout growth and in?situ annealing cycles. A correlation between SE-derived EMA thickness values and surface defect formation mitigation is discussed. All annealed samples (11.5???m to 13???m thick) exhibited excellent crystalline quality with average double crystal rocking curve full-width at half-maximum (FWHM) values of ~60?arcsec.     

Microstructural Characterization of CdTe(211)B/ZnTe/Si(211) Heterostructures Grown by Molecular Beam Epitaxy

Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures. Thin ZnTe buffer layers and subsequent thick CdTe layers were grown on Si(211) substrates using molecular beam epitaxy. Many {111}-type stacking faults were found to be present throughout the entire ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Local lattice parameter measurement and elemental profiles indicated that some intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd,Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Determining the [001] crystal orientation of CdTe layers grown on (001) GaAs

We discuss various possibilities for determining the orientation of CdTe layers grown on (001) GaAs and in particular, determining the (001) orientation. This growth orientation is characterized by a three dimensional growth mechanism which controls the growth in the (111) orientation. We show that a thin layer of ZnTe deposited directly on the oxide free GaAs surface can be used to determine the (001) orientation, eliminate (111) phases and enhance a two dimensional growth of the CdTe layer, resulting in an improved crystalline quality and a smooth surface morphology. CdTe layers grown in the (111) direction on oxide free (001) GaAs substrates contain (111) microtwins and an intermixed (001) phase. This work is a part of a Ph.D. thesis to be submitted to the Weizmann Institute of Science.     

The growth and characterization of (211) and (133) Oriented (Hg,Cd)Te epilayers (211)B GaAs by organometallic vapor phase epitaxy

The growth of CdTe buffer layers on (211)B GaAs substrates by organometallic vapor phase epitaxy (OMVPE) was studied, and it was found that, depending on the growth conditions, either the (211) or (133) epitaxial orientation could be formed. In some cases, an epilayer showing a mixed (211) and (133) orientation was also observed. The influence of several growth parameters on the orientation of the CdTe layer was investigated, and it was found that the Te/Cd ratio, together with the growth temperature, have the most significant effect in determining the epilayer orientation. From these results, it was then possible to select nominally optimized growth conditions for CdTe buffer layers of both orientations. (Hg,Cd)Te layers of the same orientations could then be grown and characterized. Although double crystal x-ray diffraction measurements indicated a somewhat better crystalline perfection in the (133) (Hg,Cd)Te layers, these layers showed a poor surface morphology compared to the (211) orientation. Measurement of etch pit densities also indicated defect densities to be typically half an order of magnitude higher in the (133) orientation. Diodes were formed by ion implantation in both orientations and significantly better results were obtained on the (211) (Hg,Cd)Te layers.     

Surface Structure of Molecular Beam Epitaxy (211)B HgCdTe

The as-grown molecular beam epitaxy (MBE) (211)B HgCdTe surface has variable surface topography, which is primarily dependent on substrate temperature and substrate/epilayer mismatch. Nano-ripple formation and cross-hatch patterning are the predominant structural features observed. Nano-ripples preferentially form parallel to the \( [\bar {1}11] \) and are from 0 Å to 100 Å in height with a wavelength between 0.1 μm and 0.8 μm. Cross-hatch patterns result from slip dislocations in the three {111} planes intersecting the (211) growth surface. The cross-hatch step height is 4 ± 1 Å (limited data set). This indicates that only a bi-layer slip (Hg/Cd + Te) in the {111} slip plane occurs. For the deposition of MBE (211)B HgCdTe/CdTe/Si, the reorientation of multiple nano-ripples coalesced into “packets” forms cross-hatch patterns. The as-grown MBE (211)B CdTe/Si surface is highly variable but displays nano-ripples and no cross-hatch pattern. Three types of defects were observed by atomic force microscopy (AFM): needle, void/hillock, and voids.