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.     

Amplification of electromagnetic waves at odd harmonics of the cyclotron resonance of heavy holes with negative effective masses in semiconducting diamond

It is shown theoretically that the absorption coefficient for circularly polarized electromagnetic waves at the cyclotron resonance of heavy holes with negative effective masses in diamond in parallel electric and magnetic fields oriented along the [001] crystal axis takes negative values at the frequency of any of the n+1 harmonics (n=0,4,8, etc.) for the right (electron) polarization and at the frequency of any of the n−1 harmonics (n=4,8,12, etc.) for the left (hole) polarization. In an electric field E≈10⁴ V/cm and magnetic fields H=30–80 kOe, at lattice temperatures of 77–100 K, and for a hole concentration of (3–5)×10¹⁵ cm⁻³, the absorption coefficient for an electromagnetic wave at the third harmonic ω ₃=3ω=2.5×10¹² s⁻¹ (wavelength λ₃=0.92 mm) is as high as η ₃=(−7)–(−30) cm⁻¹. Fiz. Tekh. Poluprovodn. 32, 504–508 (April 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.     

Effect of a high-energy proton-irradiation dose on the electron mobility in <Emphasis Type="Italic">n</Emphasis>-Si crystals

n-Si single crystals produced by the floating zone method are studied. The concentration of electrons in the crystals is 6 × 10¹³ cm⁻³. The samples are irradiated with 25-MeV protons at 300 K. The irradiation dose is varied in the range (1.8–8.1) × 10¹² cm⁻². The measurements are carried out by means of the Hall technique in the range of temperatures T = 77−300 K. In samples irradiated with different proton doses, a sharp increase in the experimental effective Hall mobility μ_eff or a deep minimum in the dependence μ_eff(T) in the region of phonon scattering of electrons is observed immediately after irradiation or after aging of the samples, respectively. The observed effect is attributed to the formation of high-conductivity (metal-like) inclusions in the irradiated samples and to changes in the degree of screening of the inclusions by impurity-defect shells in relation to the irradiation dose, the time of natural aging, and the temperature of measurements. The impurity-defect shells are formed around metal-like inclusions during isochronal annealing or natural aging of the irradiated samples. It is suggested that metal-like inclusions formed in the n-Si crystals on irradiation with protons with the energy 25 MeV are atomic nanoclusters with an 80-nm radius.     

Low-frequency noise in n-GaN

Low-frequency noise has been investigated in the hexagonal polytype of n-type gallium nitride (GaN) with equilibrium electron concentration at 300 K n ₀⋍7×10¹⁷ cm⁻³. The frequency and temperature dependence of the noise spectral density S _I/I² was studied in the range of analysis frequencies f from 20 Hz to 20 kHz in the temperature range from 80 to 400 K. Over the entire temperature range the frequency dependence of the dark noise is close to S _I/I²∼1/f (flicker noise). The rather weak temperature dependence of the noise level is characterized by very high values of the Hooge constant α⋍5–7. These large α values indicate a rather low level of structural quality of the material. The effects of infrared and band-to-band illumination on low-frequency noise in GaN are studied here for the first time. The noise level is unaffected by illumination with photon energy E _ph<E _g (E _g is the band gap) even for a relatively high value of the photoconductivity Δσ⋍50%. Band-to-band illumination (E _ph⩾E _g) influences the low-frequency noise level over the entire investigated temperature range. At relatively high temperatures the influence of illumination is qualitatively similar to that of band-to-band illumination on low-frequency noise in Si and GaAs. At relatively low temperatures the influence of illumination on the noise in GaN is qualitatively different from the results obtained earlier for Si and GaAs. Fiz. Tekh. Poluprovodn. 32, 285–289 (March 1998)     

Photovoltaic detector based on type II heterostructure with deep AlSb/InAsSb/AlSb quantum well in the active region for the midinfrared spectral range

Photodetectors for the spectral range 2–4 μm, based on an asymmetric type-II heterostructure p-InAs/AlSb/InAsSb/AlSb/(p, n)GaSb with a single deep quantum well (QW) or three deep QWs at the heterointerface, have been grown by metal-organic vapor phase epitaxy and analyzed. The transport, luminescent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A high-intensity positive and negative luminescence was observed in the spectral range 3–4 μm at high temperatures (300–400 K). The photosensitivity spectra were in the range 1.2–3.6 μm (T = 77 K). Large values of the quantum yield (η = 0.6−0.7), responsivity (S _λ = 0.9−1.4 A W^–1), and detectivity (D* _λ = 3.5 × 10¹¹ to 10¹⁰ cm Hz^1/2 W⁻¹) were obtained at T = 77–200 K. The small capacitance of the structures (C = 7.5 pF at V = −1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detection of the emission of quantum-cascade lasers and IR spectroscopy.     

Characterization of Single Barrier Microrefrigerators at Cryogenic Temperatures

The experimental characterization of single barrier heterostructure thermionic cooling devices at cryogenic temperatures is reported. The device studied was a cylindrical InGaAs microrefrigerator, in which the active layer was a 1 μm thick In_0.527Al_0.218Ga_0.255As heterostructure barrier with n-type doping concentration of 6.68 × 10¹⁶ cm⁻³ and an In_0.53Ga_0.47As emitter/collector of 5 × 10¹⁸ cm⁻³ n-doping. A full field thermoreflectance imaging technique was used to measure the distribution of temperature change on the device’s top surface when different current excitation values were applied. By reversing the current direction, we studied the device’s behavior in both cooling and heating regimes. At an ambient temperature of 100 K, a maximum cooling of 0.6 K was measured. This value was approximately one-third of the measured maximum cooling value at room temperature (1.8 K). The paper describes the device’s structure and the first reported thermal imaging at cryogenic temperatures using the thermoreflectance technique.     

Specific features of the temperature dependence of the conduction electron concentration in the narrow-gap and zero-gap states of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Results of studies of the conductivity σ and the Hall coefficient R in the Cd _x Hg_{1 − x} Te crystals with x = 0.1, 0.12, 0.14, and 0.15 are analyzed in the temperature range T = 4.2–300 K and the magnetic field range B = 0.005–2.22 T. Using data on the R(B) in low and high magnetic fields and the data on σ(T), electron and hole concentrations and mobilities are determined. It is shown that the electron concentration n in the studied samples is almost independent of T in the range 4.2–15 K, while as T increases, it increases according to the law n ∝ T ^r (r > 3/2), where r = f(n, T, x). It is found that r varies from 1.7 at x = 0.1 to 3.1 at compositions with x = 0.14 and 0.15. The results for n(T) are compared with theory, taking into account nonparabolicity of the variance law for ⃛(T), and with the theory of impurity states in narrow-gap and zero-gap semiconductors. It is shown that the constancy of n(T) up to ∼15 K and the strong dependence n(T) (r > 3/2) at higher temperatures are caused by the intense ionization of electrons localized at acceptor states.     

Specific features of intervalley scattering of charge carriers in <Emphasis Type="Italic">n</Emphasis>-Si at high temperatures

In n-Si, intervalley scattering of electrons can be of two types, f scattering and g scattering. With the purpose of establishing the contributions of f- and g-type transitions to intervalley scattering, the piezoresistance of n-Si crystals is studied in the temperature range T = 295–363 K. The initial concentration of charge carriers in the n-Si samples is 1.1 × 10¹⁴ cm⁻³, and the resistivity at 300 K is ρ = 30 Ω cm. As the temperature is increased, the region of leveling-off of the piezoresistance shifts to lower voltages. The characteristic feature of the dependence ρ = ρ(T) plotted in the double logarithmic coordinates (logρ = f(logT)) is the transition from the slope 1.68 to the slope 1.83 at T > 330 K. This is attributed to the substantial contribution of g transitions to intervalley scattering in the high-temperature region. For verification of the interpretation of the dependence ρ = ρ(T), the dependence is calculated on the basis of the theory of anisotropic scattering with consideration for intervalley transitions.     

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.     

Scattering and electron mobility in combination-doped HFET-structures AlGaAs/InGaAs/AlGaAs with high electron density

Molecular-beam epitaxy is used for growing structures differing in doping technique and doping level and having a high two-dimensional-electron concentration n _s in the quantum well. The effect of doping combining uniform and δ doping on the electron-transport properties of heterostructures is investigated. A new type of structure with a two-sided silicon δ doping of GaAs transition layers located on the quantum-well boundaries is proposed. The largest value of electron mobility μ_H = 1520 cm²/(V s) is obtained simultaneously with a high electron density n _s = 1.37 × 10¹³ cm⁻² at 300 K with such a doping. It is associated with decreasing electron scattering by an ionized impurity, which is confirmed by the carried out calculations.     

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.     

Magnetron Deposition of In Situ Thermoelectric Mg2Ge Thin Films

Thin films of the semiconducting compound Mg₂Ge were deposited by magnetron cosputtering from source targets of high-purity Mg and Ge onto glass substrates at temperatures T _s = 300°C to 700°C. X-ray diffraction shows that the Mg₂Ge compound begins to form at a substrate temperature T _s ≈ 300°C. Films deposited at T _s = 400°C to 600°C are single-phase Mg₂Ge and have strong x-ray peaks. At higher T _s the films tend to be dominated by a Ge-rich phase primarily due to the loss of magnesium vapor from the condensing film.␣At optimum deposition temperatures, 550°C to 600°C, films have an electrical conductivity σ _600 K = 20 Ω⁻¹ cm⁻¹ to 40 Ω⁻¹ cm⁻¹ and a Seebeck coefficient α = 300 μV K⁻¹ to 450 μV K⁻¹ over a broad temperature range of 200 K to 600 K.     

Electronic properties and deep traps in electron-irradiated <Emphasis Type="Italic">n</Emphasis>-GaN

The study is concerned with the effect of electron irradiation (with the energies E = 7 and 10 MeV and doses D = 10¹⁶−10¹⁸ cm⁻²) and subsequent heat treatments in the temperature range 100–1000°C on the electrical properties and the spectrum of deep traps of undoped (concentration of electrons n = 1 × 10¹⁴−1 × 10¹⁶ cm⁻³), moderately Si-doped (n = (1.2−2) × 10¹⁷ cm⁻³), and heavily Si-doped (n = (2−3.5) × 10¹⁸ cm⁻³) epitaxial n-GaN layers grown on Al₂O₃ substrates by metal-organic chemical vapor deposition. It is found that, on electron irradiation, the resistivity of n-GaN increases, this is due to a shift of the Fermi level to the limiting position close to E _c −0.91 eV. The spectrum of deep traps is studied for the initial and electron-irradiated n-GaN. It is shown that the initial properties of the irradiated material are restored in the temperature range 100–1000°C, with the main stage of the annealing of radiation defects at about 400°C.     

High-voltage (3.3 kV) 4H-SiC JBS diodes

High-voltage 4H-SiC junction-barrier Schottky (JBS) diodes have been fabricated and studied. The working area of the diodes (anode contact area) is 1.44 mm². At currents in the range from 10⁻¹¹ to 1.5 A, the forward current-voltage characteristic of the diodes is described in terms of the thermionic emission model, with the series resistance taken into account: Schottky barrier height Φ_B = 1.16 eV, ideality factor n = 1.01, and series resistance R _s = 2.2 Ω (32 mΩ cm²). The value of R _s is governed by the resistance of the blocking epitaxial n-base (impurity concentration N = 9 × 10¹⁴ cm⁻³, n-layer thickness d = 34 μm). The diodes can block a reverse voltage of at least 3.3 kV (with a leakage current at room temperature on the order of 1 μA). It is suggested that the leakage mechanism is associated with crystal lattice defects (dislocations) in SiC. It is shown that the reverse-recovery characteristics of the diodes are determined by the flow of a purely capacitive reverse current.     

Differential resistance of Au/GaAs1−x Sbx tunneling contacts in the zero-bias anomaly region. I. Contacts to n-GaAs1−x Sbx

The current-voltage characteristics and the differential resistance R(V)=dV/dI of Au/n-GaAs_1−x Sb_x tunneling contacts were investigated. Schottky barriers were prepared on n-GaAs_1−x Sb_x epitaxial layers, which were specially not doped, in the composition range 0.01<x<0.125. It was shown that the curves R(V) in the electron density range 2×10¹⁸⩽n⩽7×10¹⁸ cm⁻³ and temperature range 4.2⩽T⩽295 K are described well by the tunneling theory employing a self-consistent calculation of the potential in the Schottky barrier region. A square-root dependence of the conductance G(V)=(dV/dI)⁻¹ on the bias voltage V was observed in the zero-bias anomaly region in accordance with the Al’tshuler-Aronov theory of quantum corrections introduced in the density of states at the Fermi level by the characteristic features of the electron-electron interaction in disordered metals. Fiz. Tekh. Poluprovodn. 32, 574–578 (May 1998)     

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)     

Very high-speed ultraviolet photodetectors fabricated on GaN

We report on the temporal and the frequency response of both metal-semiconductor-metal (MSM) and p-i-n ultraviolet photodetectors fabricated on single-crystal GaN. The best MSM devices show a fast 10–90% rise-time of ∼28 psec under comparatively low ultraviolet excitation of ∼0.1 W/cm² averagerirradiance. The fast-Fourier transform (FFT) of the pulse response data indicates a bandwidth, f_3dB, of ∼3.8 GHz at a reverse bias of 25 V. This agrees well with the direct frequency response measurement value of ∼3.5 GHz. For the p-i-n devices, we measured a rise-time of ∼43 psec at 15 V reverse bias for a 60 μm diameter mesa with 1 μm thick intrinsic region. The FFT of the p-i-n pulse response obtains f_3dB ≈1.4 GHz. Analysis in terms of reverse bias and geometric scaling indicates that the MSM photodetectors are transit-time limited. The p-i-n devices also show evidence of transit-time limited effects based on trends with respect to reverse bias and intrinsic region thickness. However, our larger area p-i-n devices show clear evidence of RC-limited behavior. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity. We have also found preliminary evidence of microplasmic effects in the p-i-n devices.     

On the synchronizing band of a rational harmonic injection locking

 Compared with the harmonic injection locking(HIL),a rationai harmonic injection locking (RHIL)can make the circuit even simpler and more flexible.A common formula for the synchro- nizing band ⊿ω(s_m)max of the RHIL is derived and analysed in this paper.The result shows that the formula given by Schmideg(1971)is only its special case.A method for extending the RHIL\'s synchronizing band with full-pass network is presented.Using the method,a 2/3 injection locking VHF divider consisting of the FZ1C,a type of IC,is developed,with a stability factor n=⊿f_((3/2)max)/ ⊿f_((-40-+85℃))≥4;while for the original circuit,n≈0.6.Thus,the RHIL may be of more practical value in applications.     

Avalanche Mechanism in p +-n −-n + and p +-n Mid-Wavelength Infrared Hg1−x Cd x Te Diodes on Si Substrates

Hg_1−x Cd _x Te mid-wavelength infrared (MWIR) p ⁺-n ⁻-n ⁺ and p ⁺-n ⁻ avalanche photodiodes (APDs) with a cut-off of 4.9 μm at 80 K were fabricated on Si substrates. Diode characteristics, avalanche characteristics, and excess noise characteristics were measured on two devices. Temperature-dependent diode and avalanche characterization was performed. Maximum 3 × 10⁶ Ω cm² and 9 × 10⁵ Ω cm² zero-bias resistance times active area (R ₀ A) products were measured for the p ⁺-n ⁻-n and p ⁺-n devices at 77 K, respectively. Multiplication gains of 1250 and 410 were measured at −10 and −4 V for the p ⁺-n ⁻-n ⁺ and p ⁺-n APDs at 77 K, respectively, in the front-illumination mode with the help of a laser with an incident wavelength of 632 nm. The gains reduce to 200 and 50 at 120 K, respectively. The excess noise factor in all APDs was measured to be in the range of 1 to 1.2.