Physico-chemical properties of Ga and In dopants during liquid phase epitaxy of CdxHg1−xTe

This work deals with the study by means of radioactive tracers and autoradiography, as well as measuring of galvanomagnetic properties, of Ga and In doping of epitaxial Cd_xHg_1−xTe layers during their crystallization from a Te-rich melt. Ga and In were introduced in the form of Ga⁷² and In¹¹⁴ master alloys with Te. The effective distribution coefficients of Ga and In during the crystallization of the Cd_xHg_1−xTe solid solutions with x=0.20 to 0.23 were determined by cooling the Te-base melt to 515–470°C. Depending on the concentration of the dopants and the time-temperature conditions of Cd_xHg_1−xTe growth, these ratios for Ga and In were 1.5–2.0 and 1.0–1.5, respectively. The electrical activity of Ga and In was determined after annealing of the Cd_xHg_1−xTe layers in saturated Hg vapor at 270–300°C. In doping of the epitaxial layers to (3–8)×10¹⁴ cm⁻³ with subsequent annealing in saturated Hg vapor at ∼270°C increases the carrier lifetime approximately by a factor of two as compared with the undoped material annealed under the same conditions.     

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.     

Measurement of minority carrier lifetime in n-type MBE HgCdTe and its dependence on annealing

Results are presented for minority carrier lifetime in n-type molecular beam epitaxy Hg_1−xCd_xTe with x ranging from 0.2 to 0.6. It was found that the lifetime was unintentionally degraded by post-growth annealing under Hg saturated conditions in a H₂ atmosphere that was both time and temperature dependent. This effect was minimal or non-existent for x∼0.2 material, but very strong for x ≥ 0.3. Hydrogen was identified as responsible for this degradation. Identical annealing in a He atmosphere avoids this degradation and results in neartheoretical lifetime values for carrier concentrations as low 1 × 10¹⁵ cm⁻³ in ≥0.3 material. Modeling was carried out for x∼0.2 and x∼0.4 material that shows the extent to which lifetime is reduced by Shockley-Real-Hall recombination for carrier concentrations below 1 × 10¹⁵ cm⁻³, as well as for layers annealed in H₂. It appears that annealing in H₂ results in a deep recombination center in wider bandgap HgCdTe that lowers the lifetime without affecting the majority carrier concentration and mobility.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

Electrical activity,mode of incorporation and distribution coefficient of group V elements in Hg1−xCdxTe grown from tellurium rich liquid phase epitxial growth solutions

Hg_1−xCd_xTe films were grown liquid phase epitaxially from tellurium rich solutions containing up to 10 at. % of the group V elements P, As, Sb, and Bi. Chemical analysis of the Te growth solutions and the films was carried out in conjunction with extensive Hall effect measurements on the films subsequent to various annealing treatments under Hg rich and Te rich conditions. Despite the presence of a large concentration of the group V elements in the Te source solution, the maximum concentration of these elements incorporated into the liquid phase epitaxially grown Hg_1-xCd_xTe appears to vary from <10¹⁵cm⁻³ for Bi up to 10¹⁷cm⁻³ for phosphorus and As implying a distribution coefficient varying from <10⁻⁵ for Bi up to 10⁻³ for P at growth temperature of ∼500° C. This low value of the distribution coefficient for group V elements for growths from Te rich solutions contrasts with the moderately high values reported in the literature to date for growth from Hg rich solutions as well as pseudobinary solutions (Bridgman growth). The widely differing distribution coefficients and hence the solubility of the group V elements for Hg rich and Te rich liquid phase epitaxial solutions is explained on the basis that the activity coefficient of the group V elements in Te rich solutions is probably orders of magnitude lower than it is in Hg rich solutions. Finally, the results of the anneals at 200° C under Hg saturated conditions with and without a 500° C Hg saturated preanneal have indicatedn top conversion in many of the films attesting to the amphoteric behavior of the group V elements in LPE grown Hg_1−xCd_xTe(s) similar to the previously reported behavior of P in bulk grown Hg_0.8Cd_0.2Te.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.     

Vacancies in Hg1−xCdxTe

Measurements have been performed of the carrier concentrations in vacancy-doped Hg_1−xCd_xTe with x=0.22, 0.29, 0.45, and 0.5. Anneals to establish the carrier concentrations were performed on both the mercury- and tellurium-rich sides of the phase field. When these results were added to earlier data for x=0.2 and 0.4, and assuming that all vacancies are doubly ionized, then vacancy concentrations for all values of x and anneal temperature can be represented by simple equations. On the mercury side of the phase field, the vacancy concentrations varied as 2.50×10²³(1−x) exp[−1.00/kT] for low concentrations, and as 3.97×10⁷(1−x)^1/3n _i ^2/3 exp[−0.33/kT] for high concentrations, where n_i is the intrinsic carrier concentration. On the tellurium rich side, the vacancy concentrations varied as 2.81 × 10²²(1−x) exp[−0.65/kT] for low concentrations and as 1.92×10⁷(1−x)^1/3n _i ^2/3 exp[−0.22/kT] for high concentrations.     

Effect of cooling procedure after annealing on electrical properties of Cd0.2Hg0.8Te epitaxial films grown by liquid phase epitaxy

Effect of cooling procedure after annealing on the electrical properties of Cd_0.2Hg_0.8Te (CMT) epitaxial films grown by liquid phase epitaxy has been investigated to obtain the CMT films with low carrier concentration of 10¹⁴ cm⁻³ reproducibly. Annealing has been performed at the temperature range from 260 to 350°C for 8 h in a fixed Hg vapor pressure. The quenching and the gradual cooling over a duration of 200 min after annealing have been employed for the cooling procedures. For quenched CMT samples, hole concentration decreases with decreasing anneal temperature and conduction type conversion from p to n is observed at 300°C. For the gradual cooling, all samples show n-type conduction for all annealing temperatures. Electrical properties of annealed layers strongly depend on the cooling procedure. The difference in electrical properties of the annealed CMT between two types of cooling procedure is mainly attributed to the difference in the annihilation of Hg vacancies during cooling procedure. The decrease of Hg vacancies during quenching is negligible, while Hg vacancies are annihilated during gradual cooling by rapid Hg diffusion. The diffusion coefficient of Hg is estimated more than 10⁻⁹ cm²/s and this value is two orders of magnitude larger than that obtained by radiotracer technique.     

Rapid capless annealing of28Si,64Zn,and9Be implants in GaAs

We report the use of tungsten-halogen lamps for rapid (−10 s) thermal annealing of ion-implanted (100) GaAs under AsH₃/Ar and N₂ atmospheres. Annealing under flowing AsH₃/Ar was carried out without wafer encapsulation. Rapid capless annealing activated implants in GaAs with good mobility and surface morphology. Typical mobilities were 3700–4500 cm²/V-s for n-layers with about 2×10¹⁷cm⁻³ carrier concentration and 50–150 cm²/v-s for 0.1–5xl0¹⁹ cm⁻³ doped p-layers. Rapid thermal annealing was performed in a vertical quartz tube where different gases (N₂, AsH₃/H₂, AsH₃/Ar) can be introduced. Samples were encapsulated with SiO when N₂ was used. Tungsten-halogen lamps of 600 or 1000 W were utilized for annealing GaAs wafers ranging from 1 to 10 cm² in area and 0.025 to 0.040 cm in thickness. The transient temperature at the wafer position was monitored using a fine thermocouple. We carried out experiments for energies of 30 to 200 keV, doses of 2×10¹² to 1×10¹⁵ cm⁻², and peak temperatures ranging from 600 to 1000‡C. Most results quoted are in the 700 to 870‡C temperature range. Data on implant conditions, optimum anneal conditions, electrical characteristics, carrier concentration profiles, and atomic profiles of the implanted layers are described. Presented at the 25th Electronic Materials Conference, Burlington, VT, June 22, 1983.     

Si-defect concentrations in heavily Si-DOPED GaAs: annealing-Induced changes-II

Isothermal annealing produces changes in the free carrier density, defect-induced localized vibrational mode (LVM) infrared absorption, microstructure as measured by transmission electron microscopy (TEM), and critical resolve shear stress of heavily Si-doped GaAs. The changes have been measured and correlated for three different Si concentrations for several annealing temperatures. The measurements reveal temperature dependent annealing-induced changes in several specific defect concentrations. The observations indicate the following behavior for two ingots with [Si] ≳ 2 × 10¹⁹ cm⁻¹³: (1) when the anneal temperature, T_A = 400°C, the concentration of Si_ga donors, as determined from LVM spectra decreases probably due to the generation of V_Ga defects followed by the formation of Si_Ga-V_Ga pairs. This change is responsible for observed decreases in carrier density and the large increase in yield stress. The yield stress shows a dependence of the form σ-σ_o ∝ [Si_Ga-V_Ga]^1/4. (2) When T_A = 500°C, the LVM spectra indicate that all of the observed Si defect concentrations change. The decrease in [Si_Ga] alone cannot explain the decrease in carrier density, and a previous suggestion that a new acceptor is required is confirmed. Both the LVM measurements and the shear stress indicate that only a small fraction of the [Si_Ga] reduction is by the formation of Si_Ga-V_Ga pairs. (3) When T_A = 700°C, a new acceptor is still required and the other experimental observations at T_A = 500°C are also still seen here. There is a large decrease in [Si_Ga] and [Si_As] observed for short anneal times which coincides with the formation of Si-rich extrinsic loops and the loop area/vol increases with [Si]. (4) When T_A > 700°C, all of the changes become smaller as T_A increases. For lower [Si] ~ 1.5 × 10¹⁸ cm⁻³, no significant annealing-induced changes are observed for any of the T_A given above.     

Structures and physical properties of sputtered amorphous SiC films

Optical and electrical properties have been measured for amorphous SiC films prepared by rf sputtering in a pure Ar atmosphere with a sintered 6H-SiC target. The absorption edge E₀ determined from the relation of αhΝ = B(hΝ-E₀)² ranged from 1.45 to 1.80 eV depending on the film thickness and the substrate temperature. The room temperature electrical conductivity is in the range of 5.4×10⁻¹¹ and 1.4×10⁻⁵ Ω⁻¹cm⁻¹. The absorption edge decreases and the conductivity increases with increasing film thickness. The absorption edge shifts to shorter wavelengths (blue shift) and the conductivity decreases during annealing below 400‡C for 60 min, whereas the absorption edge shifts to the longer wavelength side (red shift) and the conductivity increases during annealing at 800‡C It is proposed that the two annealing processes cause structural changes in amorphous SiC films, one of which involves removal of defects or voids while the other involves rearrangement or rebonding of the component atoms.     

Growth and characterization of GaSb bulk crystals with low acceptor concentration

GaSb bulk single crystals with low acceptor concentration were grown from a bismuth solution by the traveling heater method. The result is isoelectronic doping by Bi which gives a variation of the opto-electronic properties as a function of grown length as well as a pronounced microscopic segregation. Photoluminescence spectra at 4K show a decrease of the natural acceptor during growth, which is confirmed by Hall measurements. The electrical properties of this isoelectronic doped GaSb are hole concentrations and mobilities of N_A − N_D = 1.7 × 10¹⁶ cm⁻³ and μ = 870 cm²Vs at room temperature and N_A-N_D = 1 × 10¹⁶ cm⁻³ and μ = 4900 cm²/Vs at 77K, respectively. The lowest p-type carrier concentration measured at 300K is N_A − N_D = 3.3 × 10¹⁵ cm⁻³     

Electron scattering by acceptor centers in <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te at low temperatures

Resonant electron scattering in p-Ag₂Te at acceptor concentrations N _a < 4.2 × 10¹⁶ cm⁻³ has been observed in the temperature range of 50–80 K. The contribution of the resonant scattering to the temperature dependences of the conductivity σ(T) and thermopower α₀(T) has been calculated. It is shown that this contribution exceeds that of charge carrier scattering by acoustic phonons.     

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.     

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.     

Electrical properties of HgCdTe epilayers doped with silver using an AgNO3 solution

We employed AgNO₃ solutions for doping Ag in liquid phase epitaxy (LPE) grown Hg_0.78Cd_0.22Te epilayers and found that the minority carrier lifetimes became longer so that the diode properties improved. After annealing LPE grown Hg(1-x)Cd(x)Te layers (x=0.22) in Hg atmosphere, the epilayers were immersed in an AgNO₃ solution at room temperature. The typical carrier concentrations of holes was 3 × 10¹⁶ cm⁻³ at 77K. These values were almost the same as for the nondoped wafers. Also, its acceptor level was 3 to 4 meV. This shows that the Ag was activated. The doped crystals have lifetimes several times longer than those of the nondoped crystals. Numerical fitting showed the lifetime was limited mostly by the Auger 7 process. The Shockley-Read-Hall recombination process was not effective. To examine the Ag-doped wafer, we fabricated photodiodes using standard planar technology. The diodes have an average zero-bias resistance of several MΩ and a shunt resistance of about 1 GΩ for a 10 μm cutoff wavelength at 78K. These values are about four times higher than those of nondoped diodes. The photo current is also two times higher at the same pixel size. This shows that the quantum efficiency is increased. The extension of the lifetime contributes to the high resistance and the high quantum efficiency of the photodiode.     

Tellurium-rich growth and laser fabrication of lead-tin-telluride (Pb1−xSnxTe: 0.06<x<0.08)

Single crystals of Pb_1−x Sn_x Te (0.06<x<0.08) have been grown by using an ingot-nucleation technique from a Te-rich source. The as-grown crystals have a p-type carrier concentration around 10¹⁹ cm⁻³ and dislocation density as low as 10³ cm⁻². Diode lasers fabricated from these crystals have contact resistances of 2×10⁻⁵ Ω-cm² and a single-mode single-ended output power of 750 μW at heat sink temperatures around 15 K.     

Open-tube vapor transport epitaxy of Hg1−xCdxTe

A novel set-up for horizontal open-tube vapor transport epitaxy of Hg_1−xCd_xTe films is described. Mirror-like Hg_1−xCd_xTe epitaxial layers with thicknesses up to 40 Μm were grown and characterized. The growth temperature ranged from 380 to 550‡C, with growth rates of the order of 0.5–7 Μm per hour. The concentration depth profiles and the optical and electrical properties of relatively uniform films with x≈0.3–0.4 are reported. The process kinetics are studied. A simple model which takes into account the reactions occurring at the boundaries of the epitaxial layer and the interdiffusion in the epilayer is presented and discussed. The model fits the experimentally observed characteristics of the epitaxial growth process. A constant growth rate leading to a linear dependence of film thickness upon deposition time y–y_i=k_s t is derived. The reaction rate constant k is given by k_s=k_oe^−Ea^/kT with k_o=0.18 cm-sec⁻¹and the energy of activation E_a=1.12 eV.     

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.     

Time relaxation of point defects in p- and n-(HgCd)Te after ion milling

Time relaxation of the electrical conductivity σ(77 K) and Hall coefficient R_H(77 K) of the n-type layer created by ion milling is investigated in Hg vacancy-doped, As-doped, and In-predoped p-type, and In-doped n-type Hg_1−xCd_xTe (0.2 < x < 0.22) samples. We show that the n-type layer is formed, and the temperature-activated relaxation occurs in all cases. The annealing at 75°C results in a gradual degradation of the converted n-type layer and a back n-to-p conversion within 8 days. The existence of a high-conducting, surface-damaged region with a high-electron density (∼10¹⁸ cm⁻³) and a low mobility (∼10³ cm²/Vs) is confirmed, and its influence on the relaxation is studied.