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.     

Photon assisted growth of HgTe by metalorganic chemical vapor deposition

Photon assisted metalorganic chemical vapor deposition (MOCVD) of HgTe by the reaction of mercury and diethyltelluride (DETe) on CdTe substrates is reported. The growths were done at 380–450°C and subatmospheric pressure, with helium as the carrier gas. A simple kinetic model for the MOCVD growth is presented. The model yields the growth rate as a function of the gas-phase partial pressures of the constituents in various limiting cases, and is corroborated by experimental results. The apparent activation energy for growth without photons (33–49 kcal/mol) exceeds the activation energy of pyrolysis of DETe (∼25 kcal/mol) and is significantly reduced in photon assisted growth (18–28 kcal/mol). The effect of photon assisted growth on morphology, thickness uniformity, and location along the susceptor is also presented and discussed.     

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.     

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.     

p-type HgTe/CdTe superlattices for very-long wavelength infrared detectors

HgTe/CdTe superlattices (SL) have been studied for applications involving the detection of very-long wavelength infrared (VLWIR) detectors. In this study, p-type HgTe/CdTe SLs were grown by molecular beam epitaxy (MBE). As-grown arsenic δ-doped HgTe/CdTe SLs and undoped HgTe/CdTe SLs were characterized before and after annealing under different conditions using XRD, temperature-dependent Hall-effect measurements, SIMS, and TEM. Incorporated arsenic atoms in a HgTe/CdTe SL were electrically activated without the typical thermal annealing of alloy HgCdTe materials.     

Two-dimensional molecular beam epitaxy of {001} CdTe on Cd and Zn terminated {001} GaAs

Amorphous layers of CdTe deposited on Cd or Zn terminated GaAs {001} surfaces can be recrystallized above ∼200°C. Subsequent molecular beam epitaxy of CdTe proceeds in a two-dimensional mode and leads to layers which are specular and single domain {0011}. Threading dislocation density in these layers was 1–2 x 10⁵ cm⁻². Values of full width at half maximum for x-ray rocking curves were as low as 80 arc-s.     

Plasma characteristics and the growth of group III-nitrides by metalorganic molecular beam epitaxy

Improved quality and controllability of growth processes are key issues for the maturation of III-N technologies. One of the most important concerns for the growth of III-N materials in ultra high vacuum is the ability to provide an effective nitrogen flux to the growth surface. This work has sought to correlate radio frequency (rf) plasma parameters and their impact on the growth of GaN by plasma-assisted metalorganic molecular beam epitaxy. Utilizing optical emission spectrometry, the atomic nitrogen production has been optimized as a function of rf power and N₂ flow rate. Growth experiments indicate that the abundance of atomic nitrogen alone does not control growth. Increasing energy per molecule in the rf source, with a constant level of atomic nitrogen, dramatically decreases the GaN growth rate. High levels of atomic nitrogen with a low energy per molecule resulted in restoration of the growth rate to ∼0.5 μm/h.     

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.     

Reaction kinetics control in preparation of CdTe and HgCdTe by hot wall metalorganic vapor phase epitaxy

The growth rates of CdTe and HgCdTe by hot wall metalorganic vapor phase epitaxy were studied as functions of growth temperatures and partial pressures of precursors. It is suggested that the growth of CdTe and HgCdTe was controlled by reaction kinetics. The relationship between growth processes and epilayer properties was discussed and high quality epilayers were obtained.     

Growth of high quality CdTe on Si substrates by molecular beam epitaxy

We have systematically studied the growth of CdTe (lll)B on Si(001)with different atomic step structures, defined uniquely by miscut tilt angle and direction. X-ray double crystal rocking curve (DCRC) analysis has been used to evaluate the crystalline quality and twin content of the films. High-resolution electron microscopy has been used to examine the CdTe(lll)B/Si(001) interface and to follow the microstructural evolution as a function of distance from the interface. Our results show that the formation of double domains and twins is very sensitive to the tilt parameters. When growth conditions are optimized, twins are not observed at distances greater than about 2.5 microns from the substrate surface. The best quality films exhibit a DCRC FWHM of 60 arc sec, for a film thickness of 17 μm, the lowest value ever reported for heteroepitaxial growth of CdTe on Si or GaAs. In efforts to improve the nucleation process, precursors such as Te and As have been used, and we have shown that they improve the stability of the heterointerface.     

Low temperature growth of (100) HgCdTe layers with DtBTe in metalorganic vapor phase epitaxy

Growth characteristics of (100) HgCdTe (MCT) layers by MOVPE at low temperature of 275°C were studied using ditertiarybutyltelluride as a tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell at atmospheric pressure. Cd composition of MCT layers were controlled from 0 to 0.98 using dimethylcadmium (DMCd) flow. The growth rate was constant for increase of DMCd flow. During the growth, Cd was incorporated preferentially into the MCT layers. Enhancement of Cd incorporation in the presence of Hg was also observed. Crystal quality and electrical properties were also evaluated, which showed that high quality MCT layers can be grown at 275°C. Strain in CdTe layers grown at 425 and 275°C was also evaluated. Lattice parameter of layers grown at 425°C approached bulk value at thickness of 5 μm, while layers grown at 275°C relaxed at 1 μm. The rapid strain relaxation of layers grown at 275°C was considered due to the layer growth on the strain relaxed buffer layer. The effect of the thermal stress on the relaxation of CdTe lattice strain was also discussed.     

Wurtzite CdS on CdTe grown by molecular beam epitaxy

Growth of single crystal wurtzite cadmium sulfide on CdTe(111)B substrates has been achieved using molecular beam epitaxy. Reflection high-energy electron diffraction (RHEED) indicates smooth surface morphology for several hundreds of nanometers after nucleation. X-ray diffraction measurements confirm the crystalline orientation to be [0001] in the growth direction. X-ray photoelectron spectroscopy (XPS) indicates mostly stoichiometric CdS layers and the existence of a reaction at the interface. Sulfur incorporation into CdTe for various S fluxes has been investigated by Auger electron spectroscopy (AES). High-resolution TEM images of the interface between such epilayers were recorded. During the growth In was used as an in-situ dopant. The concentration and uniformity of In was determined by secondary ion mass spectrometry. Indium profiles were obtained for concentrations ranging from 5 × 10¹⁷ to 1.4 × 10²¹ cm⁻³. The experimental concentration agrees well with the variation expected from the In flux.     

Magnetoluminescence properties of Hg1−xCdxTe epitaxial layers and superlattice structures grown by metalorganic molecular beam epitaxy

We present a study of the electro-optical properties ofHg _1- xCd_xTe epitaxial layers and Hg_1-x Cd_xTe/CdTe (0.28 < x < 0.30) superlattice structures by x-ray diffraction, lateral transport and photo- and magneto-luminescence measurements. Systematic studies of the excitation intensity and magnetic field dependence of the photoluminescence revealed direct evidence of an excitonic contribution to the observed luminescence in Hg_{1- x}Cd_xTe epitaxial layers. Similar investigations of the superlattice structures indicated that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm⁻¹ were obtained for an excitation intensity of 100 kW/cm² indicating suitable electro-optical properties for making efficient mid-infrared laser diodes.     

The stability of α-Sn grown on CdTe by molecular beam epitaxy

We have examined the effect of the substrate orientation, thickness, growth rate, substrate temperature and inclusion of small amounts of Ge on the transition temperature of α-Sn films grown on CdTe. The transition temperature from α-Sn to β-Sn was determined by optical microscopy to be as high as 132° C. CdTe(ll0) is a somewhat better orientation than CdTe(100), and CdTe(lll)B appears to be totally unacceptable. The transition temperature from α-Sn to β-Sn depends on the film thickness; thinner films have a somewhat higher transition temperature than thicker films. The film quality can be increased by lowering the growth rate and raising the growth temperature to about 75° C. Since the transition from α-Sn to β-Sn starts at defects in the film, improving the film quality by lowering the growth rate and raising the growth temperature raises the transition temperature. Also by adding small amounts of Ge (∼2%) the transition temperature of films grown on CdTe can be significantly increased.     

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.     

Selective growth of CdTe on Si and GaAs substrates using metalorganic vapor phase epitaxy

Epitaxial lateral overgrowth (ELO) is a new technique to grow low-defect-density thin films on lattice-mismatched substrates. For the ELO growth of CdTe and HgCdTe on Si substrate to be successful, the first requirement is that the growth should be selective. To that end, we have used several mask materials, and several growth conditions in order to obtain selective growth. A number of growth-experiments have been carried out, with temperatures in the range from 380°C to 550°C, and pressures in the range from 380 torr to less than 20 torr. Perfectly selective growth of CdTe has been achieved on Si and GaAs substrates using Si₃N₄ as the mask layer. Successful lateral epitaxial growth of CdTe was also achieved.     

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].     

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.     

Reflectance and photoreflectance for in-situ monitoring of the molecular beam epitaxial growth of CdTe and Hg-based materials

Epitaxial growth of Hg-based semiconductors by molecular beam epitaxy (MBE) and metalorganic MBE (MOMBE) has progressed sufficiently to shift emphasis to the control of factors limiting the yield of both materials and devices. This paper reports on anex-situ study to evaluate the suitability of reflectance and photoreflectance (PR) asin-situ characterization techniques for the growth of CdTe and HgCdTe. Photoreflectance yields information about CdTe layers, with largest utility for doped and multi-layer structures. However, caution must be taken in interpretation of the spectra since the near-bandedge PR spectra consists of multiple transitions and the E₁ transition energy is very sensitive to the sample history. Photoreflectance appears to be of limited utility for HgCdTe single layer growth with x<0.4. However, reflectance measurements of the E₁ peak can be used to determine composition in HgCdTe single layers with an accuracy Δx = ±0.01, which can be useful for growth control. A tight binding model was used to calculate the E₁ peak energy as a function of bandgap for HgCdTe and HgTe/CdTe superlattices. Comparisons are made with experimental observations. Surface interdiffusion in HgTe-CdTe superlattices was probed using reflectance measurements.