Properties of Bi-doped PbTe epitaxial layers and pn junction diodes grown by TDM-CVP

Effects of Bi-doping in PbTe liquid-phase epitaxial layers grown by the TDM-CVP have been investigated. For Bi concentration in the solution, x_Bi, lower than 0.2 at.%, Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility is high, while carrier concentration is in the range 10¹⁷ cm⁻³. However, ICP emission analysis shows that, for x_Bi=1.0 at.%, Bi concentration in epitaxial layer is N_Bi=2.3–2.7×10¹⁹ cm⁻³.These results indicate that Bi behaves not only as a donor but also as an acceptor; the nearest neighbor or very near DA pairs are formed. Carrier concentration for Bi-doped layers takes a minimum value at a Te vapor pressure of 2.2×10⁻⁵ Torr for growth temperature 470°C, which is coincident with that of the undoped PbTe. And broad contact pn junctions with highly Bi-doped layers easily cause laser emission compared to undoped junctions. The result suggests that the nearest lattice site Bi–Bi DA pairs behave as strong radiative centers in PbTe.     

Measured In-plane hole drift and hall mobility in heavily-doped strained p-type Si1−xGex

Measured in-plane hole drift and Hall mobilities in heavily boron-doped strained Si_1−xGe_x layers are reported. In the range of boron dopings examined (1.5–2.1 × 10¹⁹ cm⁻³), the drift mobility is seen to increase with increasing germanium fraction. The Hall mobility decreases with increasing germanium fraction. Presented at the 1992 EMC, Boston.     

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.     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.     

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 properties of indium-doped LPE layers of Pb1−xSnx Te

Liquid-phase epitaxial (LPE) layers of Pb_1−xSn_xTe with an alloy composition 0≤×≤0.25 were doped n-type by adding from 0.002 to 10 at.% indium to the growth solution. Doping characteristics of indium and electrical properties of the epilayers at 77 and 4.2K were studied by Hall and resistivity measurements made directly on the grown layers. Electron concentration and mobility at 77 and 4.2K are presented as a function of indium doping for various x values. Doping coefficients of ~0.05 and ~0.03 are found for PbTe and Pb_0.8Sn_0.2Te, respectively, grown at ~450°C. For medium to high indium doping, the electron concentration saturates to a constant value independent of doping and LPE growth temperature. The saturation values decrease substantially with increasing x and increase with a decrease in sample temperature. Bulklike mobilities practically independent of doping are recorded up to an indium concentration N_ln~0.3 at.%, above which the mobility decreases with increasing indium concentration. The data shows that indium is a suitable donor in liquid-phase epitaxial layers of Pb_l-XSn_xTe.     

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.     

The growth of AlInSb by metalorganic chemical vapor deposition

We have grown Al_xIn_1−xSb epitaxial layers by metalorganic chemical vapor deposition using tritertiarybutylaluminum (TTBAl), trimethylindium (TMIn), and triethylantimony (TESb) as sources in a high speed rotating disk reactor. Growth temperatures of 435 to 505°C at 200 Torr were investigated. The V/III ratio was varied from 1.6 to 7.2 and TTBAl/(TTBAl+TMIn) ratios of 0.26 to 0.82 were investigated. Al_xIn_1−xSb compositions from x=0.002 to 0.52 were grown with TTBAl/(TTBAl+TMIn) ratios of 0.62 to 0.82. Under these conditions, no Al was incorporated for TTBAl/(TTBAl+TMIn) ratios less than 0.62. Hall measurements of Al_xIn_1−xSb showed hole concentrations between 5×10¹⁶ cm⁻³ to 2 × 10¹⁷ cm⁻³ and mobilities of 24 to 91 cm²/Vs for not intentionally doped Al_xIn_1−xSb.     

OMVPE growth of p-AlGaAs/GaAs heterojunctions using diethylberyllium

We report on the OMVPE growth of modulation doped p-type Al_0.43Ga_0.57As(Be)/GaAs heterojunctions which exhibit a two-dimensional hole gas (2DHG). Hole mobilities de-termined by Hall or cyclotron resonance measurements at 300, 77, and 4 K were 394, 3750, and 21200 cm²/V bs s respectively for a sheet carrier density of about 4.5 × 10¹¹ cm^s−2. Beryllium doping of Al_xGa_1−xAs using diethylberyllium is characterized by Hall measurements, secondary ion mass spectrometry, and photoluminescence. The depen-dence of free carrier concentrationvs AlAs% forp ⁺ layers of Al_xGa_1−xAs_x,x = 0–0.5, is determined. A free carrier concentration greater than 1 × 10¹⁸ cm^s−3 is achieved forx = 0–0.43 with no carrier freeze-out down to 77 K.     

Liquid phase epitaxial growth and characterization of InAsxSb1−;x AND In1−yGaySb ON (111)B InSb substrates

Liquid phase epitaxial growth of InAs_xSb_1−x, for 0<_x<0.27 and In_1−yGa_ySb, for 0<y<0.37, has been successfully accomplished on (111)B InSb substrates between the temperatures of 450 and 520°C. The phase diagrams and the growth conditions for high-quality planar epitaxial layers have been determined. For growth of InAs_xSb_1−x for high values of x, the strong tendency of the ternary melt to dissolve the substrate, even when the liquid is a few degrees below its melting point, was negated by using large supercooling. Small supercooling of zero to 5.6°C were required over the whole range of composition examined for (In.Ga)Sb, whereas, for example, supercooling greater than 30°C was required to grow InAs_o.26Sb_o.74 to avoid substrate dissolution. Lattice mismatch to the substrate was relieved by compositional grading. Etch pit studies in both materials yielded dislocation densities ranging from 5.8 × 10² to 2×10⁶ cm⁻² with most materials in the low 10⁴ range. Hall and resistivity measurements performed at 300K and 77K on most samples showed an impurity contamination of the epitaxial layers. Some samples were n-type (carrier concentration approximately 10¹⁷/cm³), with varying degrees of acceptor compensation and others were n-type (carrier concentration approximately l0¹⁷/cm³) at room temperature due to intrinsic conduction, but exhibited p-type conduction (carrier concentration approximately 5×l0^l6/cm³) at 77K. Hall measurements performed on one of the latter samples ofvery low As content from 77K to 4.2K to examine hole freeze-out yielded an acceptor level ionization energy of 0.0126eV which is close to the effective mass acceptor level ionization energy in InSb. The electron-to-hole mobility ratio was also found to be 65.9. Electron microprobe analysis showed silicon to be the dominant impurity.     

Control of Hole Concentration in Liquid-Phase Epitaxial Layers of PbTe by As Doping

Liquid-phase epitaxial layers of PbTe grown at ⋍55O‡C have been doped p-type over the concentration range 5 × 10¹⁶ − 5 × 10¹⁸cm^-3by adding from 0.004 − 10 at.% arsenic to the melts. The hole concentrations of the layers, which were grown on p+ substrates, were obtained from plasma reflectivity and thermoelectric probe measurements. The latter assessment technique is shown to be a reliable and simple method for determining the hole concentration in epitaxial layers with thicknesses greater than ≃ 5ym, provided the damage introduced by the probe is controlled. Damage can cause negative thermoelectric power signals to be obtained from p-type PbTe, showing that dislocations introduce donor centres in this material.     

Open tube vapor transport growth of Pb1−xSnxTe epitaxial films for infrared detectors

Epitaxial films of Pb_1−xSn_xTe have been grown by open tube vapor transport on (100) Pb_1−xSn_xTe substrates. The as-grown films are suitable for detector array fabrication with respect to both surface smoothness and electrical properties. Charge compositions from 1% excess metal to 1% excess Te were used. Growth rates up to 3–4 ym per hour were achieved. The asgrown carrier concentrations varied from 3 × 10¹⁶cm⁻³ to 3 × 10¹⁷cm⁻³ depending on growth temperature and charge composition. Schottky barrier detectors with semi-transparent electrodes were fabricated on as-grown layers with no surface preparation. Good uniformity of detector parameters was obtained with arrays of 20 to 40 elements. The array size is not limited by either substrate size or epitaxial quality. Resistance-area products on the order of 1 ohm-cm were obtained at 77 K for detectors with a 12 ym long wavelength cutoff. Quantum efficiencies for 8–12 urn radiation were 40–50%. Peak response and 50% cutoff occurred at 11 and 12 ym, respectively. Uniformity of cutoff wavelength across the arrays of ± 0.1 ym was obtained.     

Electroluminescence and lasing properties of highly Bi-doped PbTe epitaxial layers grown by temperature difference method under controlled vapor pressure

Electroluminescence (EL) from PbTe pn homojunctions with a highly Bi-doped n-type emission layer with a concentration of N_Bi > 10¹⁹ cm⁻³, grown by the temperature difference method (TDM) under controlled-Te vapor pressure has shown a positive shift of the peak-photon energy, which coincides with the model that Bi atoms act as both donors and acceptors, and they make the nearest lattice-site or very close donor-acceptor (DA) pairs. Broad-contact pn junctions with highly Bi-doped layers easily cause laser emission compared to the difficulty in the lasing operation of undoped pn junctions, which suggests that the nearest lattice-site Bi-Bi DA pairs act as strong radiative centers in PbTe.     

Supercooling studies and LPE growth of Hg1−xCdxTe from Te-Rich solutions

Hg_1-xCd_xTe liquid phase epitaxial (LPE) layers were grown from well-stirred large (100 g) Te-rich Hg-Cd-Te solutions by the dipping method. Supercooling below the liquidus temperature in Te-rich solutions was studied by differential thermal analysis (DTA) and film growth results. Although supercooling of 20 to more than 100° C was routinely measured in small (2 g) sample melts, supercooling in larger melts (>100 g) was erratic and smaller. Factors affecting the degree of supercooling were identified and a Hg-reflux was found to be a major cause of erratic melt behavior. The LPE reactor was modified to correct the Hg-reflux action and a visual technique was developed for in situ determination of the liquidus temperature. A limited amount of supercooling was found in the melt after reactor modification but it was difficult to maintain for extended durations before spontaneous nucleation occurred. Consequently, programmed cooling rather than isothermal LPE was employed to grow many of the films reported here. Hg_1−xCd_xTe epitaxial layers ofx = 0.2 to 0.25 were grown on (111)B oriented CdTe substrates by cooling the melts only 1–2° C below the previously measured crystallization temperature. The small amount of cooling minimized composition variation with film thickness. Excellent surface morphology was obtained when slow cooling rates of 0.02–0.05° C/ min were used. Cooling rates greater than 0.2° C/min created rough, pitted surface. Precise substrate orientation was important in reducing surface terracing. Composition and thickness uniformities of the epitaxial films were excellent as a result of substrate rotation. Run-to-run reproducibility of film composition was ±0.01 inx. Hall measurements showed carrier concentrations in the range 2–20 × 10¹⁴ cm⁻³ with photoconductive lifetimes of 0.5–3.0 dms forx = 0.20 to 0.25.     

Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

Growth method,composition, and defect structure dependence of mercury diffusion in CdxHg1–xTe

Mercury radiotracer diffusion results are presented, in the range 254 to 452°C, for bulk and epitaxial Cd_xHg_1–xTe, and we believe this to be the first report for metalorganic vapor phase epitaxy (MOVPE) grown Cd_xHg_1–xTe. For all growth types studied, with compositions of x_Cd=0.2±0.04, the variation of the lattice diffusion coefficient, D_Hg, with temperature, under saturated mercury partial pressure, obeyed the equation: D_Hg=3×10⁻³ exp(−1.2 eV/kT) cm² s⁻¹. It was found to have a strong composition dependence but was insensitive to changes of substrate material or crystal orientation. Autoradiography was used to show that mercury also exploited defect structure to diffuse rapidly from the surface. Dislocation diffusion analysis is used to model defect tails in MOVPE Cd_xHg_1–xTe profiles.     

Liquid phase epitaxial growth and assessment of Pb1−xSn x Te alloys

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3.     

Te and Ge — doping studies in Ga1−xAlxAs

The temperature dependence in the range 77–400 K of the carrier concentration, resistivity and mobility of a series of n and p-type single crystal, liquid-phase epitaxial layers of Ga_1−xAl_xAs are presented. These layers were doped, n-type with tellurium, and p-type with germanium to yield carrier concentrations in the range 10¹⁷ – 10¹⁸cm⁻³ at 295 K. Donor and acceptor ionization energies, ε_D and ε_A, are derived from the data. The dependence of ε_D on alloy composition is interpreted in terms of the known band structure variation in the alloy system.     

Errata

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3. The online version of the original article can be found at     

Nickel Ohmic Contacts to N-Implanted (0001) 4H-SiC

Samples for transmission line model (TLM) and Hall measurements were fabricated on (0001) 4H-SiC implanted with nitrogen at 1 × 10¹⁸ cm⁻³, 4 × 10¹⁸ cm⁻³, 1 × 10¹⁹ cm⁻³, 4 × 10¹⁹ cm⁻³, and 1 × 10²⁰ cm⁻³. Following high-temperature activation, the activation percentage dropped from ~90% to ~20%, and the Hall mobility decreased from ~100 cm²/V · s to ~20 cm²/V · s as the implant concentration increased from 1 × 10¹⁸ cm⁻³ to 1 × 10²⁰ cm⁻³. The specific contact resistance as a function of Hall concentration is compared with published data for Ni contacts to epitaxial layers. The specific contact resistance as a function of activation temperature was also studied for two fixed implant concentrations of 5 × 10¹⁸ cm⁻³ and 1 × 10²⁰ cm⁻³.