The annealing of Cd1−xZnxTe in CdZn vapors

As-grown CdZnTe usually contains defects, such as twins, subgrain boundaries, dislocations, and Te precipitates. It is always important to anneal CdZnTe slices in Cd vapor to eliminate these defects, especially Te precipitates. The exchange of Zn atoms between the slices and the vapor plays an important role during the annealing process. In this paper, the effects of Zn partial pressure on the properties of the annealed slices are studied carefully by measuring the concentration profiles, the infrared (IR) transmission spectra, and the x-ray rocking curves. It was found that a surface layer with different compositions and possibly different structure from the bulk crystal formed during the annealing of CdZnTe samples in the saturated Zn vapor. The accumulation of excess Te in the surface layer helps to increase the IR permeability of the bulk crystal greatly. To improve the crystallization quality, a lower Zn-pressure annealing should be used following the high Zn-pressure annealing. The diffusion of Zn in the bulk crystal has also been analyzed at the temperatures of 700°C and 500°C. Calculations determined that D_Zn (700°C)=4.02 × 10⁻¹² cm²s⁻¹ and D_Zn (500°C)=1.22 × 10⁻¹³ cm²s⁻¹.     

Thermomigration of Te precipitates and improvement of (Cd,Zn)Te substrate characteristics for the fabrication of LWIR (Hg,Cd)Te photodiodes

(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates—to eliminate Te precipitates—prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.     

Reduction of Inclusions in (CdZn)Te and CdTe:In Single Crystals by Post-Growth Annealing

(CdZn)Te with the composition of 3% Zn and In-doped CdTe single crystals were annealed at various annealing temperatures and under various Cd or Te pressures with the aim of eliminating Te or Cd inclusions. Te inclusions were reduced by Cd-saturated annealing at temperatures above 660°C. Only small (<1 μm) residual dark spots, located at the original position of as-grown inclusions, were observed after annealing. The size of Cd inclusions was reduced by Te-rich annealing at temperatures higher than 700°C. A specific cooling regime was used to eliminate new small Te precipitates (∼1 μm) concurrently formed on dislocations during Te-rich annealing. Poor infrared transmittance of samples with Cd inclusions was detected after Te-rich annealing; therefore, Cd-saturated re-annealing of annealed samples was used for increasing infrared transmittance to a value above 60%. Alternative models explaining the formation of star-shaped corona-surrounding inclusions are discussed.     

The role of microstructure in the electrical properties of GaAs grown at low temperature

We have examined the role of As precipitates on transport through undoped GaAs grown at low temperature by molecular beam epitaxy and annealed at high temperature. Temperature dependent I–V measurements exhibit two regimes. At temperatures less than ∼200K, transport attributed to point defect-associated hopping conduction is observed even for samples annealed at 750°C. For temperatures greater than ∼200K, the transport is quantitatively consistent with calculations of thermally assisted tunneling emission of electrons from metallic As precipitates acting as buried Schottky barriers.     

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg_1−xCd_xTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm⁻². Void defect densities can routinely be achieved at levels below 1000 cm⁻². Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm⁻³. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.     

Influence of Cd-rich Annealing on Defects in Te-rich CdZnTe Materials

The influence of Cd-rich annealing at temperatures of 440–900 °C on the defect properties of Te-rich CdZnTe materials was studied. Cd-rich annealing at temperatures above the melting point of Te was confirmed to effectively reduce the size of Te-rich inclusions in the materials. However, dislocation multiplication occurred in the regions near Te-rich inclusions. Etch pit clusters were observed on the surfaces of annealed materials etched with Everson etchant. The etch pit clusters were much larger than the as-grown Te-rich inclusions. The dependence of the cluster size on that of the Te-rich inclusions and the annealing conditions was investigated. The density of etch pits in the normal region increased when the annealing temperature exceeded 750 °C. The mechanisms of the evolution of the Te-rich inclusions and the formation of new defects during the Cd-rich annealing are discussed.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.     

Formation of nickel disilicide using nickel implantation and rapid thermal annealing

Transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS), and x-ray photoemission spectroscopy (XPS) have been used to investigate the nucleation, growth, and ripening behavior of nickel-disilicide precipitates formed by Ni implantation in an amorphous-Si layer on (100) Si and followed by a two-step annealing treatment. The TEM and XPS results show that amorphous-disilicide precipitates are formed in a depth of ∼21 nm in the amorphous-Si layer when pre-annealed at 380°C for 30 sec. It is also shown that the second-step annealing at temperatures in the range of 450–600°C causes the amorphous precipitates to transform to randomly oriented crystalline ones embedded in the amorphous-Si layer. Annealing above 550°C is shown to induce the crystallization of amorphous Si by solid-phase epitaxial growth (SPEG). It is further shown that, in a prolonged annealing at high temperatures, the disilicide has dissolved and reprecipitated on the Si surface. Based on the roles of the silicide-mediated crystallization (SMC), the dissolution and reprecipitation of silicides, and SPEG, possible mechanisms are given to explain how the surface-disilicide islands are formed during annealing at temperatures of 550–950°C.     

High resistivity LT-In0.47Ga0.53P grown by gas source molecular beam epitaxy

Low-temperature (LT) growth of In_0.47Ga_0.53P was carried out in the temperature range from 200 to 260°C by gas source molecular beam epitaxy using solid Ga and In and precracked PH₃. The Hall measurements of the as-grown film showed a resistivity of ∼10⁶ Ω-cm at room temperature whereas the annealed film (at 600°C for 1 h) had at least three orders of magnitude higher resistivity. The Hall measurements, also, indicated activation energies of ∼0.5 and 0.8 eV for the asgrown and annealed samples, respectively. Double-crystal x-ray diffraction showed that the LT-InGaP films had ∼47% In composition. The angular separation, Δθ, between the GaAs substrate and the as-grown LT-InGaP film on (004) reflection was increased by 20 arc-s after annealing. In order to better understand the annealing effect, a LT-InGaP film was grown on an InGaP film grown at 480°C. While annealing did not have any effect on the HT-InGaP peak position, the LT-InGaP peak was shifted toward the HT-InGaP peak, indicating a decrease in the LT-InGaP lattice parameter. Cross-sectional transmission electron microscopy indicates the presence of phase separation in LT-InGaP films, manifested in the form of a “precipitate-like” microstructure. The analytical scanning transmission electron microscopy analysis of the LT-InGaP film revealed a group-V nonstoichiometric deviation of ∼0.5 at.% P. To our knowledge, this is the first report about the growth and characterization of LT-InGaP films.     

Thermal stability of heavily tellurium-doped InP grown by metalorganic molecular beam epitaxy

The thermal stability of tellurium in InP has been examined in samples doped with Te up to an electron concentration of 1.4 × 10²⁰ cm⁻³. Annealing was conducted using rapid thermal annealing for a period of one minute at temperatures over the range 650–800°C. Secondary ion mass spectroscopy analysis showed virtually no change in the Te profile before and after annealing, even at the highest annealing temperatures. High resolution x-ray diffraction and Hall measurements revealed a general decrease in the lattice strain and carrier concentration for annealing temperatures above 650°C. No evidence of strain relief was found in the form of cross-hatching or through the formation of a dislocation network as examined by scanning electron microscopy or transmission electron microscopy (TEM). These results are most likely due to the formation of Te clusters, though such clusters could not be seen by crosssectional TEM.     

P-type doping of GaAs by carbon implantation

The electrical properties of C-implanted <100> GaAs have been studied following rapid thermal annealing at temperatures in the range from 750 to 950°C. This includes dopant profiling using differential Hall measurements. The maximum p-type activation efficiency was found to be a function of C-dose and annealing temperature, with the optimum annealing temperature varying from 900°C for C doses of 5 × 10¹³ cm⁻² to 800°C for doses ≥5 × 10¹⁴cm⁻². For low dose implants, the net p-type activation efficiency was as high as 75%; while for the highest dose implants, it dropped to as low as 0.5%. Moreover, for these high-dose samples, 5 × 10¹⁵ cm⁻², the activation efficiency was found to decrease with increasing annealing temperature, for temperatures above ∼800°C, and the net hole concentration fell below that of samples implanted to lower doses. This issue is discussed in terms of the amphoteric doping behavior of C in GaAs. Hole mobilities showed little dependence on annealing temperature but decreased with increasing implant dose, ranging from ∼100 cm²/V·s for low dose implants, to ∼65 cm²/V·s for high dose samples. These mobility values are the same or higher than those for Be-, Zn-, or Cd-implanted GaAs.     

Redistribution of implanted dopants in GaN

Donor (S, Se, and Te) and acceptor (Mg, Be, and C) dopants have been implanted into GaN at doses of 3–5×10¹⁴ cm⁻² and annealed at tem peratures up to 1450°C. No redistribution of any of the elements is detectable by secondary ion mass spectrometry, except for Be, which displays behavior consistent with damageassisted diffusion at 900°C. At higher temperatures, there is no further movement of the Be, for peak annealing temperature durations of 10 s. Effective diffusivities are ≤2×10⁻¹³ cm²·s⁻¹ at 1450°C for each of the dopants in GaN.     

Solid-state phase formation between Pd thin films and GaSb

Knowledge of the interaction between a thin metal film and a compound semiconductor can be used to engineer electrical contacts to the semiconductor. In this study, we examine the reaction between a 50 nm layer of Pd and a GaSb substrate annealed at 100–350°C for 10–360 min using transmission electron microscopy (TEM) and x-ray diffraction (XRD). We report on the formation of Pd-rich nanocrystalline and polycrystalline ternary phases at temperatures below 200°C, followed by Pd-Ga and Pd-Sb binary phases above 200°C.     

Effect of annealing temperature on 1.5 μm photoluminescence from Er-lmplanted 6H-SiC

The effect of post-implantation anneal on erbium-doped 6H-SiC has been investigated. 6H-SiC has been implanted with 330 keV Er⁺ at a dose of 1 × 1013 /cm2. Er depth profiles were obtained by secondary ion mass spectrometry (SIMS). The as-implanted Er-profile had a peak concentration of∼1.3 × 10¹⁸/cm³ at a depth of 770Å. The samples were annealed in Ar at temperatures from 1200 to 1900°C. The photoluminescence intensity integrated over the 1.5 to 1.6 μm region is essentially independent of annealing temperature from 1400 to 1900°C. Reduced, but still significant PL intensity, was measured from the sample annealed at 1200°C. The approximate diffusivity of Er in 6H SiC was calculated from the SIMS profiles, yielding values from 4.5 × 10⁻¹⁶ cm²/s at 1200°C to 5.5 × 10⁻¹⁵ cm²/s at 1900°C.     

A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Correlation of arsenic incorporation and its electrical activation in MBE HgCdTe

The behavior of arsenic for p-type doping of MBE HgCdTe layers has been studied for various annealing temperatures and arsenic doping concentrations. We have demonstrated that arsenic is in-situ incorporated into HgCdTe layers during MBE growth. The carrier concentration has been measured by the Van der Pauw technique, and the total arsenic concentration has been determined by secondary ion mass spectroscopy. After annealing at 250°C under an Hg over pressure, As-doped HgCdTe layers show highly compensated n-type properties and the carrier concentration is approximately constant (∼mid 10¹⁵ cm⁻³) until the total arsenic concentration in the HgCdTe layers approach mid 10¹⁷ cm⁻³. The source of n-type behavior does not appear to be associated with arsenic dopants, such as arsenic atoms occupying Hg vacancy sites, but rather unidentified structural defects acting as donors. When the total arsenic concentration is above mid 10¹⁷ cm⁻³, the carrier concentration shows a dependence on the arsenic concentration while remaining n-type. We conjecture that the increase in n-type behavior may be due to donor arsenic tetramers or donor tetramer clusters. Above a total arsenic concentration of 1∼2×10¹⁸ cm⁻³, after annealing at 300°C, the arsenic acceptor activation ratio rapidly decreases below 100% with increasing arsenic concentration and is smaller than that after annealing at 450°C. The electrically inactive arsenic is inferred to be in the form of neutral arsenic tetramer clusters incorporated during the MBE growth. Annealing at 450°C appears to supply enough thermal energy to break some of the bonds of neutral arsenic tetramer clusters so that the separated arsenic atoms could occupy Te sites and behave as acceptors. However, the number of arsenic atoms on Te sites is saturated at ∼2×10¹⁸ cm⁻³, possibly due to a limitation of its solid solubility in HgCdTe.     

Observation of phase separation in Hg1−xCdxTe solid solutions by low incident angle x-ray diffraction

The low incident angle (surface analysis) and the conventional wide angle (bulk analysis) x-ray diffraction techniques were employed to investigate the existence of a miscibility gap in the Hg_1−xCd_xTe system. Samples of initial composition Hg_0.46Cd_0.54Te were annealed at 140 and 400°C, respectively, for four weeks. The diffraction planes (531) and (642) have been selected for the x-ray diffraction analysis. The results of this work provide the first, direct experimental evidence for the existence of a miscibility gap at lower temperature in the Hg_1−xCd_xTe system. The phase separation occurs primarily in a thin surface layer at 140°C and is reversible after annealing at 530°C. The compositions of the two compounds at the tie-line at 140°C are Hg_0.22Cd_0.78Te and Hg_0.63Cd_0.37Te.     

Improved morphology and crystalline quality of MBE CdZnTe/Si

We report on continuing efforts to develop a reproducible process for molecular beam epitaxy of CdZnTe on three-inch, (211) Si wafers. Through a systematic study of growth parameters, we have significantly improved the crystalline quality and have reduced the density of typical surface defects. Lower substrate growth temperatures (∼250–280°C) and higher CdZnTe growth rates improved the surface morphology of the epilayers by reducing the density of triangular surface defects. Cyclic thermal annealing was found to reduce the dislocation density. Epilayers were characterized using Nomarski microscopy, scanning electron microscopy, x-ray diffraction, defect-decoration etching, and by their use as substrates for HgCdTe epitaxy.     

Amorphous chromium silicide formation in hydrogenated amorphous silicon

The interaction between thin films of hydrogenated amorphous silicon and sputter-deposited chromium has been studied. Following deposition of the chromium films at room temperature, the films were annealed over a range of times and temperatures below 350°C. It was found that an amorphous silicide was formed only a few nanometers thick with the square of thickness proportional to the annealing time. The activation energy for the process was 0.55±0.05 eV. The formation process of the silicide was very reproducible with the value of density derived from the thickness and Cr surface density being close to the value for crystalline CrSi₂ for all films formed at temperatures ≤300°C. The specific resistivity of the amorphous CrSi₂ was ≈600 μΩ·cm and independent of annealing temperature.     

Annealing effect on the P-type carrier concentration in low-temperature processed arsenic-doped HgCdTe

We report the results of annealing effects on the As-doped alloy HgCdTe grown by molecular beam epitaxy (MBE), arsenic (As) diffusion in HgCdTe from Hg-rich solutions at low temperatures, and As ion implantation at room temperature. Hall-effect measurements, secondary ion mass spectrometry and p-on-n test photodiodes were used to characterize the As activation. High As-doping levels (10¹⁷−10¹⁹ cm⁻³) could be obtained using either MBE growth, As diffusion or As ion-implantation. Annealed below 400°C, As doping in HgCdTe shows n-type characteristics, but above 410°C demonstrates that all methods of As doping exhibit p-type characteristics independent of As incorporation techniques. For example, for samples annealed at 436°C (P_Hg≈2 atm), in addition to p-type activation, we observe a significant improvement of p/n junction characteristics independent of the As source; i.e. As doping either in situ, by diffusion, or ion implantation. A study of this As activation of As-doped MBE HgCdTe as a function of anneal temperature reveals a striking similarity to results observed for As diffusion into HgCdTe and implanted As activation as a function of temperature. The observed dependence of As activation on partial pressure of Hg at various temperatures in the range of 250 to 450°C suggests that As acts as an acceptor at high Hg pressure (>1 atm) and as a donor at low Hg pressure (<1 atm) even under Hg-rich conditions.