Single crystals of CdTe or dilute alloys of Cd1−yZnyTe (y ≤ 0.04) and CdTe1−zSez (z ≤ 0.04) with low defect density and large single-crystal area (>30 cm2) are required as substrates for high-quality epitaxial Hg1−xCdxTe thin films in the infrared (IR) detector industry. Bridgman or gradient freeze has been the most common current technique used for growing these materials. This paper reviews the current status and the evolution at SBRC of one variation of the Bridgman technique, viz., vertical-modified Bridgman (VMB), for producing large-area substrates with excellent uniformity and reproducibility. CdTe, Cd1−yZnyTe (y ≤ 0.04) and CdTe1−zSez (z ≤ 0.04) boules of 5-to 7.5-cm diameter have been grown unseeded in the present version of the VMB growth system. In general, under optimum growth conditions, the boules have the smallest grain structure (several grains) at the tip end with enhancement of grain selection as the cylindrical body of the boule is approached, resulting in one predominant and large grain occupying 70 to 80 percent of the entire boule volume; {111}-oriented Cd1−yZnyTe and CdTe1−zSez substrates with single-crystal areas as large as 50 to 60 cm2 have been obtained from these boules. Crystal quality characterized by x-ray rocking curve, IR transmission (2.5 to 20 µm), low-temperature photoluminescence, and Hall-effect measurements as a function of temperature, showed a strong correlation with the starting material quality (especially that of elemental Te and Se). Analyses of the thermal history during growth reveals that the presence of the ampoule (with charge) increases the temperature inside the furnace by 10 to 15 degrees. The temperature gradient at the tip was measured to be 8 to 10°C/cm and it dropped to 4 to 5°C/cm beyond 2.5 cm from the tip - where rapid enhancement of grain selection takes place in most boules. The effect of this temperature rise on the initial crystallization near the tip of a boule can be explained from the numerical thermal model that was developed for the growth process with radiative and conductive heat transfer included and using a temperature profile similar to that existing in the actual growth furnace. Conditions for maximizing the fraction solidifying with a slightly convex interface, hence maximizing the single-crystal yield are discussed.

Methods used to grow bulk, CdTe crystals, effects of alloying on their perfection and typical single crystal properties are reviewed in this paper. Crystals grown by a modified horizontal Bridgman technique have lower dislocation densities than those grown by a modified vertical Bridgman method. Dislocation densities of the order of 1×103/cm2 have been observed in CdTeSe crystals grown by the former technique. Due to the difference in the distribution coefficients of Zn and Se in CdTe, CdTeSe ingots are chemically more uniform than CdZnTe ingots. Purity studies of starting materials indicate that Se substitutions may introduce more impurities than Zn additions.

The native defects introduced by Br2/CH3 OH etching and aging under atmospheric conditions have been investigated in In doped, bulk CdTe using photoluminescence (PL) spectroscopy. The results indicate a large enhancement of the Cd vacancy related 1.5896 eV excitonic feature with chemical treatment and aging. Hence, the primary perturbation is interpreted to be a small loss of Cd within the sampling region. This result is compared and contrasted with previous studies of etching induced modifications. The implications of Cd depletion on interpreting PL spectra, device processing and long term stability are considered.

The main lines in the photoluminescence spectra of Zn1Cd1−xTe single crystals grown by a modified Bridgman method in the compositional range of 0 ≤ X ≤ 0.25 have been identified. All crystals show only near-band-edge emission. To assist in the identification, various samples with different compositions were annealed under a Cd atmosphere. In the pure crystals, the prominent (A°,X) bound exciton line, as well as the doublet at longer wavelengths, disappear after the annealing. In contrast, the treatments do not change significantly the PL spectra of the mixed crystals.

Results from high resolution helium temperature photoluminescence (PL) spectroscopy have been correlated to precision lattice constant and transport measurements and to theoretical band gap versus composition behavior. It is found that low temperature PL spectra provide precise determination (+/- 0.02%) of ZnTe mole fraction as well as carrier type, relative impurity concentration and point defect properties of these substrates. In addition helium and room temperature PL results are correlated to determine the accuracy of room temperature measurements for composition determination.

This is the first reported use of ion-implantation damage gettering of impurities in CdTe to provide high-quality substrates for the epitaxial growth of appropriate binary or ternary compounds, or for related applications. We describe the results of photoluminescence (PL) measurements performed on samples of Bridgeman-grown CdTe to study both the annealing behavior and gettering effects in this material. From the PL results, it was found that impurity gettering occurs at temperatures at which liquid phase epitaxy take place (∼500°C) so that these two fabrication procedures are compatible. It was also found that the optimum anneal time at this temperature is four hours.

The titlematerial is one of the least studied among the II-VI alloy systems. So far it has not been possible to prepare ZnTe in n-type and CdSe in p-type with appreciable conductivities. Moreover ZnTe crystallises in zincblende whereas CdSe crystallises in wurtzite structure. Because of the varied nature of the end compounds, an attempt has been made to prepare single crystals of (ZnTe)x(CdSe)1−x in the entire range of ‘x’. The alloy material in the entire range Xof composition in the single crystalline form was grown by a modified Piper-Plich method. The grown crystals of this alloy system have been subjected to chemical analysis. DTA and DTG studies carried out on these alloys did not show any phasec transitions. However two exothermic peaks associated with increase in mass were noticed. This has been attributed to oxidation effects of Se/Te or Cd/Zn. XRD data though showed some regularity near end compositions there is still some ambiguity for the middle compositions. Energy gap obtained from reflection spectra and also photocurrent spectral response showed bowing. However, there is a marked different feature at one of the end regions. The growth and the results of all the above mentioned investigations are presented and discussed in this paper.

We used photoluminescence spectroscopy to monitor CdTe in-situ during surface processing in order to optimize the surface electronic quality. Spectra of optimally-prepared (100) films grown by molecular beam epitaxy reveal low intensity emission from deep states relative to typical bulk-grown material.

Thin-film epitaxial layers on silicon substrates are being actively developed as an alternative to bulk substrates for infrared detectors. Success in this development is expected to yield improvements in size, strength, monolithic signal processing, and cost of the detector arrays. To provide feedback to the thin-film growth process, this study has investigated the structural and chemical properties of these films. CdTe or CdZnTe was grown on GaAs on Si wafers by metalorganic chemical vapor deposition (MOCVD) followed by the growth of HgCdTe by liquid-phase epitaxy (LPE). Development of TEM specimen preparation techniques permitted investigation of materials with up to four layers. Cross sectional TEM and AEM investigations resulted in the following observations: Selected area diffraction and CTEM show relations between the layers and structural defects. High resolution imaging of the multilayers reveals that most of the heteroepitaxial misfit is accomodated by misfit dislocations. Many, but not all, stacking faults and dislocations which propagate across the films appear to be related to structural interface defects. The evidence indicates that there is little, if any, contamination at MOCVD and LPE interfaces. Interface defects mainly appear to be extremely minute pits or hillocks. AEM investigations of interface regions have demonstrated a redistribution of material close to the CdZnTe / HgCdTe interface.

A description of incorporation and solubility limit of substitutional impurities is made using the alloy CdxHgloxTe as a model of analysis and a source of experimental data.

Then non-equilibrium incorporation of impurities under light excitation is considered. A model of photo assisted doping is presented which accounts for the high doping efficiency reported for the donor indium in photoassisted grown CdTe.

An x-ray diffraction method is described for determining thicknesses of thin films grown on single crystal substrates. The equations, based on the kinematical theory of x-ray diffraction and the mosaic crystal model, were developed. The thickness of the thin film was computed from the absorption of the integrated diffracted x-ray intensity from the single crystal substrate by the film. Since the diffracted intensity from the film is not required, the film does not have to be single crystal in nature. Thus, thicknesses of less ordered, polycrystalline or even amorphous films can be measured with high precision by this technique.

Recent progress in metalorganic chemical vapor deposition (MOCVD) of wide bandgap II-VI materials, especially of ZnSe, ZnS and their alloys, is discussed with emphasis on the general principles for obtaining uniform and high quality epitaxial layers and the current major issue of impurity doping for achieving conductivity control. The surface morphology and crystalline quality can be improved by a suitable choice of source materials and by lattice-matching the epitaxial layer to the substrate. By using appropriate sources, high conductivity n-type epitaxial layers of ZnSe and ZnS doped with impurities from group HI and VII of the periodic table have been successfully grown by low temperature MOCVD. We have also grown p-type ZnSe layers with carrier concentration ranging from low 1016 to high 1017 cm−3 using Li3N as the dopant. Extensive studies are now focussed on the better p-type control. High purity source materials, appropriate p-type dopants and low temperature growth are important keys.

Recently, with the advent of thermal nonequilibrium growth techniques like molecular beam epitaxy and metalorganic chemical vapor deposition, great progress has been made in overcoming some of the problems traditionally encountered in the growth and doping of ZnSe. Breakthroughs have been made in several areas including the growth of high quality undoped films, in intentional n-type doping and, most importantly, in p-type doping. In this paper we will review the progress made in the growth and doping of ZnSe by molecular beam epitaxy.

Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques.

ALE is a chemical vapor deposition technique which deposits an atomic layer of material during each cycle of the deposition process. The thickness of a film deposited by ALE depends only on the number of cycles. Multilayers deposited by ALE should be smooth and uniform with precise d-spacings which makes ALE an excellent technique for producing multilayer x-ray optics.

We have designed and built an ALE system and we have used this system to deposit ZnSe using diethyl zinc and hydrogen selenide.

In the study reported here, the GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe to form epitaxial ZnSe/epitaxial GaAs interfaces. The structures were grown by molecular beam epitaxy and evaluated by several techniques including capacitance-voltage (C-V) measurements. A dramatic reduction of interface state density occurred when the GaAs epilayer was made As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.

A series of pseudomorphic ZnTe/AlSb/GaSb epilayer/epilayer heterostructures, aimed at the realization of novel wide bandgap light emitting devices, were grown by molecular beam epitaxy. The low temperature photoluminescence (PL) spectra of ZnTe epilayers showed dominant near-band-edge features related to free, and shallow impurity bound excitons. The PL could be seen at room temperature. Both GaSb and AlSb were doped n-type using a PbSe source.

Interfaces of pseudomorphic (100)ZnSe/GaAs and (100)CdTe/InSb heterostructures grown by molecular beam epitaxy have been studied by transmission electron microscopy. High resolution electron microscope images show dark bands with thicknesses of one or two monolayers at the interfaces. The interfaces appear as bright lines in dark field images of the 200 type reflections, while they become dark lines in dark field images of the 400 type reflections. These observations are explained by assuming the existence of interfaces layers of III2VI3 compounds which have structural vacancies in the sublattices of the group III atoms.

ZnSe:Cl epilayers have been grown on (100) GaAs by MBE using a Zn to Se beam flux ratio of 2:1 and substrate temperatures as low as 225 °C. The ZnSe:Cl epilayers are highly conducting and exhibit bright blue-violet edge luminescence at 300 K. A mobility of 2480 cm2 /V-s at 40 K was measured for an n-type ZnSe:CI film that was Cl-doped to ∼2.3×1017 cm−3 and for which a compensation ratio (NA/ND) of ∼3% was calculated. Carrier concentrations as large as 6.7×1018cm−3 were obtained by increasing the temperature of the MBE oven containing the C1 dopant.

We have investigated excimer laser annnealing of Na doped ZnSe, with emphasis on photoluminescence (PL) characterization as a probe of resultant changes in materials properties. We observed a relative increase, after annealing, of Na on substitutional (acceptor) sites vs. Na on interstitial (donor) sites. Another result was the occurrence of extensive twinning; unfortunately such twinning has complicated the analysis of other changes.

Plasma-assisted epitaxy (PAE) was applied to the growth of ZnSe films on (100) GaAs for low temperature epitaxial growth in hydrogen plasma. High purity ZnSe films were successfully grown by the control of hydrogen gas flow rate and VI/II supply ratio. Hydrogen-chloride gas and nitrogen gas mixed in pure hydrogen gas plasma around 2% respectively resulted in the growth of highly conductive n-type layers (630Scm−1 ) and nitrogen-acceptor doped layers (N-acceptor level∼100meV), however, the control of VI/II supply ratio is also very important for the efficient N-acceptor doping. The plasma optical emission spectroscopy indicated that Se-N is composed by the reaction of -ion with SeH (or Se) in hydrogen and nitrogen mixed gas plasma with Se supply.