Excess low frequency noise/I-V studies in p-on-n MBE LWIR Hg1−xCdxTe detectors

Excess low frequency noise is investigated for the first time in infrared MBE grown LWIR Hg_1−xCd_xTe double layer planar heterostructure (DLPH) detectors grown on lattice matched substrates. LWIR detectors having R₀A_opt values at 40K in the 10¹–10⁷ Θ-cm² range have been characterized as a function of temperature between 120 and 20K. Detectors with R₀A_opt≥10³Θ-cm² at 40K have theoretical diffusion limited performance down to 78K and detectors with R₀A_opt ≥10⁵ Θ-cm² at 40K are within a factor of two of theoretical diffusion limited performance for T>65K. Activation energies extracted from noise (V_d=−100 mV) and dark current (V_d=−100 mV) vs temperature measurements were detector dependent. The activation energy for detectors with R₀A_opt≈10⁶ Θ-cm² at 40K is ∼0.90*E_g to 0.99*E_g. The noise measured between 78 and 105K in the intermediate performance (R₀A_opt∼10³–10⁴ Θ-cm² at 40K) detectors are higher than the noise measured in the higher performance (R₀A_opt∼10⁵–10⁷ Θ-cm²) detectors. In addition, the excess low frequency noise and the dark current at −100 mV in the intermediate and poor (R₀A_opt∼10¹ Θ-cm²) performance detectors are temperature independent. For each detector measured, the activation energy extracted from noise (V_d=−100 mV) vs temperature measurements is equal to the activation energy extracted from the total dark current (V_d=−100 mV) vs temperature measurements. For different dark current mechanisms, the excess low frequency noise varies with temperature and also with area within statistical accuracy in the same manner as the total dark current through the detector. At 78K, the Tobin¹⁴ expression holds in the general sense for equal area detectors dominated by different current mechanisms and also for detectors with a wide range of implant dimensions (A_imp=3.85×10⁻⁷ cm² to A_imp=6.25×10⁻⁴ cm²). Following measurements, the detectors were stripped of the passivation and overlaying metal layers and dressed by a defect etch to reveal defects in each detector. A correlation among noise, leakage current and defect type has been determined for each detector.     

Debye temperature and standard entropies and enthalpies of compound semiconductors of the type I-III-VI2

The heat capacities at constant pressure have been measured for CuInSe₂, CuInTe₂ and AgGaTe₂ in the temperature range 1K ≤T≤40K and for CuInS₂ and AgInTe₂ between 1 K and room temperature. From the low temperature data we derive the following Debye temperatures θ_D in the limit T → O K: θ_D(CuInS₂) = 273 K, θ_D(CuInSe₂) = 222 K, θ_D(CuInTe₂) = 191 K, θ_D(AgGaTe₂) = 182 K and θ_D(AgInTe₂) = 156 K. A plot of the average atomic heat capacity at constant volume C_v shows that the data scale to one general curve for all 5 compounds considered in this paper. This is, also, true for a plot C_v, i.e., all I-III-VI₂ compounds measured thus far deviate similarly from the Debye approximation. By integration of the general curves C_v(T/θ_D) and θ_D x C_v(T/θ_D) we derive tne standard entropies S ₀ ²⁹⁸ and energies E ₀ ²⁹⁸ -E ₀ ⁰ of 11 compounds of the type I-III-VI₂, for which the Debye temperatures are known. The difference between the energies E ₀ ²⁹⁸ -E ₀ ⁰ and enthalpies H ₀ ²⁹⁸ -H ₀ ⁰ is within the error limits of the experimental data (< 1%). The molar S ₀ ²⁹⁸ and H ₀ ²⁹⁸ -H ₀ ⁰ values for the I-III-VI₂ compounds are approximately twice the corresponding molar values for their II-VI isoelectronic analogs. The thermodynamic functions at standard state obtained by integration of the experimental data are all < 10% smaller than the corresponding values estimated on the basis of the Debye approximation.     

Exciton characteristics of intercalated TlGaSe2 single crystal

It is shown that lithium-ion intercalation of TlGaSe₂ single crystal leads to a shift of the exciton peak associated with the direct edge toward longer wavelengths (ΔE=15 meV at 5 K). As a result, the temperature shift of the exciton peak in TlGaSe₂(∂E ^ex/∂T) decreases more than twofold in absolute value to −1.1×10⁻⁴ eV/K at 20≲T≲105 K and −0.25×10⁻⁴ eV/K at 5≲T≲20 K. Lithium-ion (Li⁺) intercalation of TlGaSe₂ has almost no effect on the energy position of the exciton associated with indirect transitions. Fiz. Tekh. Poluprovodn. 32, 145–147 (February 1998)     

High electron mobility in (InAs)n(GaAs)n short period superlattices grown by MOVPE for high-electron mobility transistor structure

(InAs)_n(GaAs)_n short period superlattices (SPSs) have been successfully grown by a continuous MOVPE process on InP substrates. Their structural, optical, and electrical properties have been studied. The periodic structures have been confirmed by x-ray measurements and (InAs)₁(GaAs)₁ SPSs have been clearly observed by transmission electron microscopic characterization. The optical quality of the material has been tested by 2K photoluminescence and excitonic recombinations have been observed. Mobilities as high as 10700 cm².V⁻¹.s⁻¹ and 64000 cm². V⁻¹.s⁻¹ for a sheet concentration of 3 × 10¹² cm⁻² have been obtained at 300K and 77K, respectively.     

Optical absorption in PbGa2Se4 single crystals

Optical measurements are performed in a PbGa2Se4 single crystal. The nature of the optical transitions is determined in the interval of photon energies 2.24–2.46 eV in the temperature range 77–300 K. It is shown that indirect and direct optical transitions take place in the energy intervals 2.28–2.35 eV and 2.35–2.46 eV, corresponding to E _gi =2.228 eV and E _gd =2.35 eV, respectively, at 300 K. The temperature coefficients of E _gi and E _gd are equal to −0.6×10⁻⁴ eV/K and −4.75×10⁻⁴ eV/K, respectively. Fiz. Tekh. Poluprovodn. 33, 39–41 (January 1999)     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.     

Phonon drag of electrons in Ag<Subscript>2</Subscript>S

The temperature dependences of the heat-conductivity coefficient χ and the thermopower 6h of Ag₂S are investigated in the range of 4.2–300 K. It is found that the value of 6h sharply increases (6h ∞ T^-3) with decreasing T at T < 100 K and passes through a maximum at 16–18 K. The heat-conductivity coefficient passes through a maximum at ≈30 K. The sharp increase in 6h is found to be caused by the effect of long-wavelength-phonon drag of electrons. It is shown that the shift of the 6h and χ peaks, as well as the temperature dependence of the phonon thermopower 6h_ph ∞ T^-3, agrees with the Herring theory.     

Circular and linear enhancement-mode 6H-SiC MOSFETs for high temperature applications

Direct current measurements are performed up to 673K at circular and linear (shown in parenthesis) enhancement-mode metal oxide semiconductor field effect transistors (MOSFETs). These devices are fabricated on a p-type 6H-SiC epitaxial layer with a doping concentration N_A ≈ 1 × 10¹⁶ cm⁻¹. The n⁺ source/drain regions and the p⁺ regions for the channel stops are achieved by ion implantation of nitrogen and aluminum, respectively. Both MOSFET geometries show excellent output characteristics with a good saturation behavior even at elevated temperatures. The inversion layer mobility μ_n extracted in the linear region is 38 cm²·V⁻¹·s⁻¹ (35 cm²·V⁻¹·s⁻¹) and reveals a weak dependence on temperature with a maximum of 46 cm²·V⁻¹·s⁻¹ (42 cm²·V⁻¹·s⁻¹) at about 473K. Regarding the transfer characteristics, the drain current I_D can be well modulated by the gate-source voltage V_GS resulting in an I_on/L_off-ratio of 10⁸ (10⁸) at 303K and 10⁵ (10⁶) at 673K. In the subthreshold regime, I_D can be pinched off well below 10 pA with a subthreshold swing of 150 mV/decade (155 mV/decade) at room temperature. The threshold voltage V_T as a function of temperature shows two linear sections with negative temperature coefficients of −6.8 mV·K⁻¹ (−6.8 mV·K⁻¹) from 303 to 423K and −2.5 mV·K⁻¹ (−2.0 mV·K⁻¹) from 423 to 673K. By measuring V_T as a function of bulk-source voltage V_BS at different temperatures, N_A can be directly estimated at a transistor and gives 9.6 × 10¹⁵ cm⁻³ (9.8 × 10¹⁵ cm⁻³). The measured bulk Fermi potential Φ_f of the p-type epitaxial layer deviates less than 10% from the calculated value at a given temperature.     

Growth and doping of SiC-thin films on low-stress,amorphous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/Si substrates for robust microelectromechanical systems applications

The N₂-doped 3C-SiC thin films have been grown by low-pressure, chemical vapor deposition (LPCVD) on amorphous Si₃N₄/p-Si (111) substrates using the single, organosilane-precursor trimethylsilane [(CH₃)₃SiH]. The effects of N₂ flow rate and growth temperature on the electrical properties of SiC films were investigated by Hall-effect measurements. The electron-carrier concentration is between 10¹⁷–10¹⁸/cm³. The lowest resistivities at 400 K and 300 K are 1.12×10⁻² and 1.18×10⁻¹ cm, respectively. The corresponding sheet resistances are 75.02 Ω/□ and 790.36 Ω/□. The SiC film structure was studied by x-ray diffraction. The 3C-SiC films oriented in the 〈111〉 direction with a 2ϑ peak at 35.5° and line widths between 0.18–0.25° were obtained. The SiC/Si₃N₄ interface is very smooth and free of voids. The fabrication of microelectromechanical (MEMS) structures incorporating the SiC films is discussed.     

Synthesis and Electrical Properties of γ-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> via a Citrate Sol–Gel Method with Polyethylene Glycol 400

In this work, a citrate sol–gel method (Sol–Gel) with polyethylene glycol 400 (Sol-Gel-PEG400) was developed to prepare γ-Na _x Co₂O₄ by using sodium and cobalt nitrates as the raw materials, citric acid as a complexing agent, and PEG400 as a dispersant. At 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained using Sol-Gel-PEG400. With the addition of 1 vol.% PEG400, smaller, flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. Moreover, polycrystalline sintered bulk γ-Na _x Co₂O₄ with more highly oriented crystals and greater compact density was fabricated using the Sol-Gel-PEG400 synthesized powders compared with the powders synthesized by citrate Sol–Gel. The electrical conductivity (σ) values of Sol-Gel-PEG400 samples were higher than those of Sol–Gel samples between 400 K and 900 K. The σ value of Sol-Gel-PEG400 increased to 3.13 × 10⁴ Sm⁻¹ at 400 K and to 1.84 × 10⁴ Sm⁻¹ at 900 K. Between 400 K and 850 K, the Seebeck coefficient (α) values of Sol-Gel-PEG400 samples were slightly lower than those of Sol–Gel samples. Near 900 K, the α values of these two methods were nearly equal, at 164 μV K⁻¹. Between 400 K and 900 K, the power factor (P) of Sol-Gel-PEG400 was evidently larger than that of Sol–Gel.     

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     

Magnetic Properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>2</Subscript>S<Subscript>4</Subscript> Alloy Single Crystals

This study concerns the magnetic properties of single crystals of Fe _x Mn_{1 − x} In₂S₄ alloys. The basically antiferromagnetic character of indirect exchange interactions between Fe²⁺ and Mn²⁺ cations is established. As the concentration of Fe²⁺ cations is increased, the magnetic ordering temperature increases from ∼12 K (x = 0) to ∼22 K (x = 1). Short-range-order ferromagnetic correlations are observed. The basic magnetic phase state of the alloys is the spin glass state, with the freezing temperature increasing from ∼5 K (x = 0) to ∼12 K (x = 1). As the external magnetic field is increased, the magnetic ordering temperature slightly decreases. The most probable causes and mechanisms of formation of the magnetic state of the alloys are discussed.     

Electrical properties of Pb1−xCdxS epitaxial films

We have measured the resistivity ρ and Hall coefficient R_H at 300, 77, and 4.2 K of p-type Pb_1−XCd_XS epitaxial films as a function of substrate temperature T_s, film thickness d, and composition x. The films were vapor deposited on cleaved (111) BaF₂ (111) SrF₂ , and (001) NaCl and polished (001) BaF₂ substrates. The Hall mobility μ_H at 77 K of p-type PbS films increased approximately linearly from 1 × 10⁴ to 2 × 10⁴ cm² V⁻¹ sec⁻¹ as T_s was varied from 400 to 500°C, respectively. Both μ_H and R_H increased with d due to the presence of a strong p-type surface layer on the exposed surface. The x of the films was controlled by the x of the source material and T_s. The mole fraction of CdS could be varied between 0.002 < x < 0.06 by varying T between 513 and 410°C, respectively, and using source material with x = 0.06. The electrical properties of samples grown on freshly cleaved (111) BaF₂ and (111) SrF₂ were essentially identical even though the lattice constant of SrF₂ is a better match to Pb_1−XCd_XS than BaF₂. The R_H and μ_H at 77 K were independent of thickness for low substrate temperatures and were observed to increase with increasing thickness for high substrate temperatures. The μ_H increased with decreasing temperature and became temperature independent below about 30 K, which is similar to the behavior observed in other lead salt compounds. However, the magnitude of μ_H was considerable lower throughout the 300 to 4.2 K temperature range than for PbS films. The R_H showed little temperature variation, which is typical lead salt behavior. Supported by Naval Surface Weapons Center Independent Research Funds.     

Electrophysical properties of solid solutions of silver in PbTe

The thermopower coefficient α₀ and the electrical conductivity σ of Pb_{1 − x} Ag _x Te solid solutions, where x = (0–0.007), are measured at T = 300 K. The hole concentration p is calculated. All samples are of the p type. With increasing silver content, α₀ decreases, while p and σ increase. For undoped crystals, α₀ = 251.0 μV/K, p = 1.1 × 10¹⁸ cm⁻³, and σ = 165 Ω⁻¹ cm⁻¹. At the silver-solubility limit for x = 0.007, α₀ = 193.8 μV/K, p = 2.3 × 10¹⁸ cm⁻³, and σ = 216 Ω⁻¹ cm⁻¹. The hole concentration in all samples is much lower than the concentration of introduced silver atoms. The hole gas in Pb_{1 − x} Ag _x Te solid solutions is weakly degenerate in the entire silver-concentration range.     

Low pressure metalorganic chemical vapor deposition of InP and related compounds

The low pressure metalorganic chemical vapor deposition epitaxial growth and characterization of InP, Ga_0.47In_0.53 As and Ga_xIn_1-xAs_yP_1-y, lattice-matched to InP substrate are described. The layers were found to have the same etch pit density (EPD) as the substrate. The best mobility obtained for InP was 5300 cm² V⁻¹S⁻¹ at 300 K and 58 900 cm² V⁻¹ S⁻¹ at 77₂K, and for GaInAs was 11900 cm² V⁻¹ S⁻¹ at 300 K, 54 600 cm² V⁻¹ S⁻¹ at 77 K and 90 000 cm V⁻¹S⁻¹ at 2°K. We report the first successful growth of a GaInAs-InP superlattice and the enhanced mobility of a two dimensional electron gas at a GaInAs -InP heterojunction grown by LP-MO CVD. LP MO CVD material has been used for GaInAsPInP, DH lasers emitting at 1.3 um and 1.5 um. These devices exhibit a low threshold current, a slightly higher than liquid phase epitaxy devices and a high differential quantum efficiency of 60%. Fundamental transverse mode oscillation has been achieved up to a power outpout of 10 mW. Threshold currents as low as 200 mA dc have been measured for devices with a stripe width of 9 um and a cavity length of 300 um for emission at 1.5 um. Values of T in the range 64–80 C have been obtained. Preliminary life testing has been carried out at room temperature on a few laser diodes (λ = 1.5μm). Operation at constant current for severalthousand hours has been achieved with no change in the threshold current.     

Superionic conductivity in TlGaTe<Subscript>2</Subscript> crystals

Temperature dependences of electrical conductivity σ(T) and permittivity ɛ(T) of one-dimensional (1D) TlGaTe₂ single crystals are investigated. At temperatures higher than 305 K, superionic conductivity of the TlGaTe₂ is observed and is related to diffusion of Tl⁺ ions via vacancies in the thallium sublattice between (Ga³⁺Te₂^{2− −} nanochains. A relaxation character of dielectric anomalies is established, which suggests the existence of electric charges weakly bound to the crystal lattice. Upon the transition to the superionic state, relaxors in the TlGaTe₂ crystals are Tl⁺ dipoles ((Ga³⁺Te₂²⁻)⁻ chains) that arise due to melting of the thallium sublattice and hops of Tl⁺ ions from one localized state to another. The effect of a field-induced transition of the TlGaTe₂ crystal to the superionic state is detected.     

Nuclear magnetic resonance spectra of 119Sn and 125Te in SnTe and SnTe:Mn

A study is reported of the nuclear magnetic resonance spectra of ¹¹⁹Sn and ¹²⁵Te in SnTe with hole concentrations p ₇₇=1.42×10²⁰−2.3×10²¹ cm⁻³ and in SnTe:Mn (N _Mn=0.5 and 5 at. %, p ₇₇=8×10²⁰ cm⁻³) at T=4.2–300 K. Considerable broadening of NMR lines due to hyperfine magnetic interactions between nuclear and electron spins was observed in SnTe with p ₇₇>2ײ⁰ cm⁻³. Asymmetric broadening of the resonance lines was observed in the rhombohedral phase of SnTe and SnTe:Mn. The temperature dependence of the NMR line width of ¹²⁵Te in SnTe:Mn is in agreement with the magnetic phase diagram for N _Mn=5 at. %. The superparamagnetic phase of SnTe:Mn is formed at T=20±2 K and the ferromagnetic phase is formed at T=4.2 K. Fiz. Tekh. Poluprovodn. 31, 1187–1191 (October 1997)     

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.     

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.     

Thermoelectric Oxides: Effect of Doping in Delafossites and Zinc Oxide

The electrical resistivity (ρ) and Seebeck coefficient (S) of the three delafossites CuFe_0.9Cr_0.1O₂, CuCr_0.98Mg_0.02O₂, and CuRh_0.9Mg_0.1O₂ have been measured and their power factor (PF) calculated. These p-type oxides show PF values at 800 K from 1.4 × 10⁻⁴ W K⁻² m⁻¹ to 6.9 × 10⁻⁴ W K⁻² m⁻¹. In contrast to delafossites containing Fe or Cr, for which ρ exhibits a regime, the Rh-based delafossite shows a metallic regime from 5 K to 1000 K. This points toward the role of the transition-metal electronic configuration in the transport properties. Otherwise, similar PF values are obtained in the case of the n-type Al-doped ZnO. For these oxides, the ρ and PF values are minimum and maximum, respectively, for x = 0.01. However, the presence of spinel impurities even for x = 0.01 in Zn_1−x Al _x O or for x = 0.02 in CuCr_1−x Mg _x O₂ calls into question the role of the doping element in the physical properties. This should motivate a deeper insight into the physics of thermoelectric oxides.