The magnetic field dependence of R0A products in n-on-p homojunctions and p-on-n heterojunctions from Hg0.78Cd0.22Te liquid phase epitaxy films

The analysis of R₀A products as a function of magnetic field in n-on-p diodes using a simple diffusion current model has previously been shown to yield both J_ep/J_total ratio (the relative contribution of the p-side diffusion current) and μ_ep (the minority carrier, electron mobility). In this paper, we report the good agreement between the experimental and theoretical dependence of μ_ep on the hole concentration over a wide range between 1 x 10¹⁶ and 4 x 10¹⁷ cm⁻³ in n-on-p homojunction diodes fabricated on undoped p-type Hg_0.78Cd_0.22Te liquid phase epitaxial (LPE) films. The averaged J_ep/J_total ratio varied between 68 and 90% with the hole concentration. These J_ep/J_total ratios indicate that other leakage current mechanisms than the p-side diffusion current were not negligible. Also, for the first time, comparative measurements were made on p⁺/n heterojunction diodes consisting of As-doped Hg_0.07Cd_0.30Te and In-doped Hg_0.78Cd_0.22Te LPE layers. Unlike a typical change in R₀A products by a factor of 2–3 in n-on-p homojunction diodes, the R₀A products in p⁺/n heterojunction diodes at 7 kG were typically only 2–3% higher than that at the zero field. The typical J_ep/J_total ratio in p⁺/n heterojunction diodes was about 3–4 %, which confirms the general belief that the p⁺ cap layer, due to the high doping and a larger bandgap, contributes very little to the total leakage current.     

The minority carrier lifetime in doped and undoped p-type Hg0.78Cd0.22Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg_1-xCd_xTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τ_A7 ⁱ/τ_A1 ⁱ ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 10¹⁵ to 1.5 x 10¹⁷ cm^-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 10¹⁵ and 8 x 10¹⁶ cm^-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τ_A7 ⁱ/τ_A1 ⁱ, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (E_v+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.     

Minority carrier lifetime in indium-doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x ≈ 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as ≈2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of τ_n0 and τ_p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.     

Electrical properties and compositional distributions of CVT and PVT grown Hg1−xCdxTe epilayers

Epitaxial layers of Hg_1−xCd_x Te were grown on CdTe substrates by the chemical vapor transport technique using Hgl₂ as a transport agent. The epilayers were of nearly uniform composition both laterally and to a depth of about one-half of the layer thickness. By comparison, the composition varied continuously throughout the depth of the layer for epilayers grown by the physical vapor transport technique. Layers were grown both p- and n-type with carrier concentrations on the order of 10¹⁷ cm⁻³. Low-temperature annealing was used to convert the p-type layers into n-type. The room-temperature carrier mobilities of as-grown and converted n-type layers ranged from 10³ to 10⁴ cm²/V-s depending on the composition and are comparable to previous literature values for undoped Hg_1−xCd_xTe crystals.     

N-Type Hg1−xCdxTe: Undoped x = 0.3 LPE material for sprite IR detectors

The requirement for two color Sprite detectors, with elements sensitive in the ranges 3-5 μn (MW) and 8-14 μn (LW) at 77K, is met using Hg_1−xCd_xTe elements of composition x = 0.3 and x = 0.2, respectively. The need for low defect levels for increased performance indicates the use of liquid phase epitaxy (LPE). While LW material is fairly well characterized, the growth and conversion to n-type of MW LPE has proved more difficult. Reported work shows limited data and limited success in converting MW LPE to n-type, and this primarily in donor-doped material. This paper describes the growth, annealing to n-type and characterization of Hg_0.7Cd_0.3Te. High n-type conversion yields were obtained, with low donor levels (mid-10¹³ to mid-10¹⁴ cm⁻³), high mobility (>10⁴ cm² (Vs)⁻¹) and long minority carrier lifetime (>10 us).     

Full band structure calculation of minority carrier lifetimes in HgCdTe and thallium-based alloys

We have calculated the full band structures-based minority carrier lifetimes in small-gap semiconductor alloys. The contribution from first-order Coulomb interactions and second-order electron-electron interactions coupled through optical phonons are included. Our results agree reasonably well with experiments in Hg_0.78Cd_0.22Te. Similar calculations were carried out for lifetimes in In_0.67Tl_0.33P, In_0.85Tl_0.15As, and In_0.92Tl_0.08Sb. The minority carrier lifetimes in In_0.67Tl_0.33P and In_0.92Tl_0.08Sb are shorter than that in Hg_0.78Cd_0.22Te at all temperatures. However, the low-temperature minority carrier lifetime in In_0.85Tl_0.15 As is an order of magnitude longer than that in Hg_0.78Cd_0.22Te. Our calculations further suggest the possibility of increasing the lifetimes of minority carriers either by decreasing the density of states inside a critical energy and momentum region or by increasing the total hole population outside that critical region. Experimental observations that substantiate this suggestion are discussed.     

Diffusion of Selenium in Liquid-Phase Epitaxy–Grown Hg<Subscript>0.78</Subscript>Cd<Subscript>0.22</Subscript>Te

We present preliminary results on Se diffusion in liquid-phase epitaxy (LPE)–grown HgCdTe epilayers. The LPE Hg_0.78Cd_0.22Te samples were implanted with Se of 2.0 × 10¹⁴/cm² at 100 keV and annealed at 350–450°C in mercury saturated vapor. Secondary ion mass spectrometry (SIMS) profiles were obtained for each sample. From a Gaussian fit, we find that the Se diffusion coefficient D _Se is about 1–2 orders of magnitude smaller than that of arsenic. The as-implanted Se distribution is taken into account in case of small diffusion length in Gaussian fitting. The D _Se was found to satisfy the Arrhenius relationship .     

Improvement in HgCdTe diode characteristics by low temperature post-implantation annealing

Hg_1−xCd_xTe diodes (x∼0.22) with different carrier concentrations in p type materials have been fabricated by employing an ion-implantation technique. The performances of the diodes, prior to and after low temperature postimplantation annealing, have been investigated in detail by model fitting, taking into account dark current mechanisms. Prior to the annealing process, dark currents for diodes with relatively low carrier concentrations are found to be limited by generation-recombination current and trap-assisted tunneling current, while dark currents for diodes with higher carrier concentrations are limited by band-to-band tunneling current. These dark currents in both diodes have been dramatically decreased by the low temperature annealing at 120∼150°C. From the model fitting analyses, it turned out that trap density and the density of the surface recombination center in the vicinity of the pn junction were reduced by one order of magnitude for a diode with lower carrier concentration and that the carrier concentration profile in a pn junction changed for a diode with higher carrier concentration. The improvements are explained by changes in both carrier concentration profile and pn junction position determined by interaction of interstitial Hg with Hg vacancy in the vicinity of the junction during the annealing process.     

Ten-inch molecular beam epitaxy production system for HgCdTe growth

Growth of Hg_1−xCd_xTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC), formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In this paper, we present data on Hg_1−xCd_xTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED) and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs) in the low 10⁴ cm⁻² range, Hall carrier concentration in low 10¹⁴ cm⁻³, and void density <1000 cm². The epilayers were grown on near lattice-matched (211)B Cd_0.96Zn_0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R₀A values of 7.2×10⁶ Ω-cm² at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K. For short wavelength infrared (SWIR) devices, an R₀A value of 9.4×10⁵ Ω-cm² at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at 37 K. These R₀A values are comparable to our trend line values in this temperature range.     

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.     

Effect of the concentration of uncompensated impurities on the properties of CdTe-based X- and γ-ray detectors

Measurements of the ⁵⁵Fe-isotope emission spectra and the photosensitivity of CdTe detectors with a Schottky diode, and also the temperature dependence of the resistivity of a CdTe crystal ((2–3) × 10⁹ Ωcm at 300 K) have been used to determine the concentration of uncompensated donors (1–3) × 10¹² cm⁻³. Similar measurements performed for Cd_0.9Zn_0.1Te crystals with the resistivity (3–5) × 10¹⁰ Ω cm at 300 K have shown that the concentration of uncompensated donors in this case is lower by approximately four orders of magnitude. The results of calculations show that, due to such a significant decrease in the concentration of uncompensated donors, the efficiency of X- and γ-ray radiation detection in the photon energy range 59 to 662 keV can decrease by one-three orders of magnitude (depending on the photon energy and the lifetime of charge carriers in the space-charge region). The results obtained account for the apparent poor detecting properties of the Cd_0.9Zn_0.1Te detectors.     

Slider LPE of Hg1-xCdxTe using mercury pressure controlled growth solutions

Homogeneous, nearly perfect single crystals of Hg_1-xCd_xTe are extremely difficult to prepare due primarily to the high vapor pressure of mercury. However, epitaxially grown Hg_1-xCd_xTe layers have a high potential for yielding material of a substantially higher quality. Using a new, open-tube, horizontal slider-type liquid phase epitaxial (LPE) growth technique, in which mercury pressure controlled growth solutions are used, a high degree of growth solution compositional control has been demonstrated. LPE layers of Hg_1-xCd_xTe have been grown on CdTe substrates and their high quality has been confirmed by optical, transport and electron microprobe measurements. Layer thicknesses are uniform and have been varied from 5 to 40 μ by changing the degree of supercooling or the growth time. An electron carrier concentration as low as 8.6 × 10¹⁵/cm³ and electron Hall mobilities up to 2.8 × 105 cm²/V-sec at 77K have been measured on in situ annealed samples. This work was sponsored by the Department of the Air Force and the U.S. Army Research Office.     

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.     

HgCdTe mid-wavelength IR photovoltaic detectors fabricated using plasma induced junction technology

Preliminary characterization results are presented for mid-wave infrared (MWIR) mercury cadmium telluride n-on-p photodiodes fabricated using a plasma induced type conversion junction formation technology. The diodes have been fabricated on three different vacancy doped p-type epitaxial starting materials, grown by liquid phase epitaxy (LPE) on CdZnTe, LPE on sapphire, and P/p isotype heterojunction material grown by molecular beam epitaxy (MBE) on CdZnTe. All materials had CdTe mole fraction in the active region of the device of ∼0.3. The process uses a H₂/CH₄ plasma generated in a parallel plate reactive ion etching (RIE) system to type convert the p-type material to n-type. The process is different from previously reported type conversion techniques in that it does not require a high temperature anneal, does not expose the junction at the surface to atmosphere after formation, and requires significantly fewer process steps than other planar processes. Homojunction devices fabricated using this process exhibit R₀A values >10⁷ Ω·cm² at 80 K. The R₀A is diffusion limited for temperatures >∼135 K. Results for responsivity, bias dependence of dynamic resistance — junction area product and 1/f noise show that the resulting diodes are comparable to the best planar diodes reported in the literature.     

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.     

Planar n-on-p ion milled mid-wavelength and long-wavelength infrared diodes on molecular beam epitaxy vacancy-doped CdHgTe on CdZnTe

Planar mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) photodiodes were fabricated by ion milling molecular beam epitaxy (MBE) Cd_xHg_1−xTe (CMT) layers with and without compositional grading in the layer. Linear arrays with 32 and 64 diodes, as well as test diodes of varying size, were fabricated. Good quantum efficiencies were measured, and MWIR diodes, with cutoff wavelength λ_CO=4.5 μm, had zero-bias resistance-area values (R₀A) in excess of 1×10⁷ Ωcm², whereas LWIR diodes with λ_CO=8.9−9.3 μm had R₀A=3×10² Ωcm² at 77 K. Comparison between a limited number of layers indicates that in layers with a gradient the RA values are a factor of ∼10 larger, and possibly more uniform, than in layers without a gradient.     

Perimeter governed minority carrier lifetimes in 4H-SiC p+n diodes measured by reverse recovery switching transient analysis

Minority carrier lifetimes in epitaxial 4H-SiC p⁺n junction diodes were measured via an analysis of reverse recovery switching characteristics. Behavior of reverse recovery storage time (t_s) as a function of initial ON-state forward current (I_F) and OFF-state reverse current (I_R) followed well-documented trends which have been observed for decades in silicon p⁺n rectifiers. Average minority carrier (hole) lifetimes (τ_p) calculated from plots of t_s vs I_R/I_F strongly decreased with decreasing device area. Bulk and perimeter components of average hole lifetimes were separated by plotting 1/τ_p as a function of device perimeter-to-area ratio (P/A). This plot reveals that perimeter recombination is dominant in these devices, whose areas are all less than 1 mm². The bulk minority carrier (hole) lifetime extracted from the 1/τ_p vs P/A plot is approximately 0.7 μs, well above the 60 ns to 300 ns average lifetimes obtained when perimeter recombination effects are ignored in the analysis. Given the fact that there has been little previous investigation of bipolar diode and transistor performance as a function of perimeter-to-area ratio, this work raises the possibility that perimeter recombination may be partly responsible for poor effective minority carrier lifetimes and limited performance obtained in many previous SiC bipolar junction devices.     

The effect of annealing temperature on the carrier concentration OF Hg0.6Cd0.4Te

The carrier concentration dependence of Hg_0.6Cd_0.4Te on annealing temperature for the Hg and Te saturation condi-tions is presented in this paper. At low annealing tempera-tures, T_A < 350‡ C, residual donor impurities apparently limit the carrier concentration. In contrast, at higher annealing temperatures, 350‡ C < T < 700‡ C, the stoichio-metric acceptor density is increased such that the residual donors are compensated and the material is converted to p-type with an acceptor density as large as 10¹⁷ cm⁻³ . An empirical expression describing this dependence of the acceptors on annealing temperature is given for both the Hg and Te saturation condition: P (Hg saturation) = 1.46 × 10²² exp (−0.84/kT_A), P (Te saturation) = 1.90 × 10¹⁸ exp (−0.15/kT_A). Supported in part by Air Force Materials Laboratory, WPAFB, Ohio under Contract AF61533-C-74-5041.     

Carrier Lifetimes in Lightly-Doped <Emphasis Type="Italic">p</Emphasis>-Type 4H-SiC Epitaxial Layers Enhanced by Post-growth Processes and Surface Passivation

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (N _A ～ 2 × 10¹⁴ cm^?3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of V_C-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO₂ followed by POCl₃ annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the V_C-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.     

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.