Study of electrically active defects in n-GaN layer

Deep level defects in both p⁺/n junctions and n-type Schottky GaN diodes are studied using the Fourier transform deep level transient spectroscopy. An electron trap level was detected in the range of energies at E_c−E_t=0.23–0.27 eV with a capture cross-section of the order of 10⁻¹⁹–10⁻¹⁶ cm² for both the p⁺/n and n-type Schottky GaN diodes. For one set of p⁺/n diodes with a structure of Au/Pt/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au and the n-type Schottky diodes, two other common electron traps are found at energy positions, E_c−E_t=0.53–0.56 eV and 0.79–0.82 eV. In addition, an electron trap level with energy position at E_c−E_t=1.07 eV and a capture cross-section of σ_n=1.6×10⁻¹³ cm² are detected for the n-type Schottky diodes. This trap level has not been previously reported in the literature. For the other set of p⁺/n diodes with a structure of Au/Ni/p⁺–GaN/n–GaN/n⁺–GaN/Ti/Al/Pd/Au, a prominent minority carrier (hole) trap level was also identified with an energy position at E_t−E_v=0.85 eV and a capture cross-section of σ_n=8.1×10⁻¹⁴ cm². The 0.56 eV electron trap level observed in n-type Schottky diode and the 0.23 eV electron trap level detected in the p⁺/n diode with Ni/Au contact are attributed to the extended defects based on the observation of logarithmic capture kinetics.     

Steady-State Electron Transport and Low-Field Mobility of Wurtzite Bulk ZnO and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O

Steady-state electron transport and low-field electron mobility characteristics of wurtzite ZnO and Zn_1−x Mg _x O are examined using the ensemble Monte Carlo model. The Monte Carlo calculations are carried out using a three-valley model for the systems under consideration. Acoustic and optical phonon scattering, intervalley (equivalent and nonequivalent) scattering, ionized impurity scattering, and alloy disorder scattering are used in the Monte Carlo simulations. Steady-state electron transport is analyzed, and the population of valleys is also obtained as a function of applied electric field and ionized impurity concentrations. The negative differential mobility phenomena is clearly observed and seems compatible with the occupancy and effective nonparabolicity factors of the valleys in bulk ZnO and in Zn_1−x Mg _x O with low Mg content. The low-field mobilities are obtained as a function of temperature and ionized impurity concentrations from the slope of the linear part of each velocity–field curve. It is seen that mobilities begin to be significantly affected for ionized impurity concentrations above 5 × 10¹⁵/cm³. The calculated Monte Carlo simulation results for low-field electron mobilities are found to be consistent with published data.     

Fine structure and Zeeman effect of the excited state of the green emitting copper centre in zinc oxide

In the excitation spectrum of the green emission band of ZnO:Cu crystals, zero phonon lines have been detected for the first time. In the liquid-helium temperature range, a line with Δ = 2 cm⁻¹ at λ = 433.5 nm coincides with the well-known zero-phonon line of the emission. Additional lines are located at 432.2 and 431.4 nm. In a distance of 525 cm⁻¹ from these zero-phonon lines, phonon satellites superpose on the low-energy part of the broad excitation band. On its high-energy slope pronounced structures appear in the excitonic region. With selected highly-doped crystals, the new zero-phonon lines could also be detected in absorption.Zeeman experiments produce anisotropic splittings of the common ground state of all three lines with g = 0.7 ± 0.1 and g = 1.5 ± 0.1 which are approximately the values known from emission. Different anisotropic g-factors around g = 2 are derived for the terminal states of the absorption lines.The experimental data are compatible with a model of the investigated centre in which the ground state is Cu²⁺ and the excited state is a Cu⁺ with a loosely bound hole: (Cu⁺)⁺ [3]. As to the position of the impurity levels within the forbidden gap, still two possibilities exist: the excited hole could either be localized near the oxygen neighbours so that a depth of some tenth of an eV results, or effective-mass-like states could be involved in a distance of only a few meV above the top of the valence band.     

The Auger-effect in Hg1−xCdxTe

The temperature dependence of the Auger-lifetime of n-Hg_1−xCd_xTe is investigated both theoretically and experimentally for several values of bandgap and of extrinsic carrier-concentration n_ex in the whole range between room and He-temperatures. For semiconducting compounds a pronounced minimum of the lifetime between 10 and 50K and an exponential increase at still lower temperatures are found. The position of the minimum and the exponent depend mainly on the value of the bandgap and on the ratio of the conduction- and valenceband effective masses. Apart from the extended temperature range, which does not allow the use of classical statistics, our calculation differs from earlier work in so far, as we take the band energies and the overlap integrals of conduction- and valenceband Bloch-functions from a k·p-calculation. For semiconducting compounds, we compare the results with those obtained from an estimate of the overlap integrals given by Antončik and Landsberg. Whereas both results are compatible at high temperatures, characteristic differences occur at low temperatures, where we find the lifetime to be proportional to rather than to n_ex⁻². For semimetallic compounds we calculate a weakly temperature dependent Auger-time of the order of 0.1–1 nsec.     

Hot electron energy relaxation in quantizing magnetic fields

A systematic study of the radiative recombination of hot electrons between Landau levels in several n-InSb samples has been carried out as a function of the free electron concentration and the applied electric and magnetic fields (E ～ H). The magnetic field range investigated was 0.5 to 3.0 T. Electron temperatures are derived in dependence of the total electric and the magnetic field applied. The carrier heating is impeded with increasing electron concentration and also with increasing magnetic field. From the direct observation of the recombination radiation intensity on electric field response the electron lifetime in the first Landau level is found to be less than 10^?8. The results obtained are discussed in light of a new theoretical approach. We give evidence that electron-electron scattering plays an important role both for the carrier distribution established and for the electron lifetime in the first Landau level.     

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.     

Frequency bandwidth and conversion loss of a semiconductor heterodyne receiver with phonon cooling of two-dimensional electrons

The temperature and concentration dependences of the frequency bandwidth of terahertz heterodyne AlGaAs/GaAs detectors based on hot electron phenomena with phonon cooling of two-dimensional electrons have been measured by submillimeter spectroscopy with a high time resolution. At a temperature of 4.2 K, the frequency bandwidth at a level of 3 dB (f _{3 dB}) is varied from 150 to 250 MHz with a change in the concentration n _s according to the power law f _3dB ∝ n _s ^−0.5 due to the dominant contribution of piezoelectric phonon scattering. The minimum conversion loss of the semiconductor heterodyne detector is obtained in structures with a high carrier mobility (μ > 3 × 10⁵ cm² V⁻¹ s⁻¹ at 4.2 K).     

Remote phonon and surface roughness limited universal electron mobility of In0.53Ga0.47As surface channel MOSFETs

The inversion layer electron mobility in n-channel In_0.53Ga_0.47As MOSFET’s with HfO₂ gate dielectric with several substrate impurity concentrations (∼1 × 10¹⁶ cm⁻³ to ∼1 × 10¹⁸ cm⁻³) and various surface preparations (HF surface clean, (NH₄)₂S surface clean and PECVD a-Si interlayer with a HfO₂ gate dielectric) have been studied. The peak electron mobility is observed to be strongly dependent on the surface preparation, but the high field mobility is observed to be almost independent of the surface preparation. A detailed analysis of the effective mobility as a function of electric field, substrate doping, and temperature was used to determine the various mobility components (surface roughness, phonon, and coulombic scattering limited mobility components). For the substrates with high doping concentration, the electron mobility at low vertical electric field is dominated by Coulomb scattering from the substrate dopants, whereas, for lower substrate doping the Coulombic scattering is dominated by the disorder induced gap states. Low temperature measurements were used to determine the surface roughness scattering and phonon components. The results show that room temperature mobility of In_0.53Ga_0.47As surface channel MOSFETs with HfO₂ gate dielectric at high electric field is limited primarily by remote phonons whereas the Al₂O₃ gate dielectric is limited by surface roughness scattering.     

Carrier effective mobilities in germanium MOSFET inversion layer investigated by Monte Carlo simulation

This paper presents the Monte Carlo studies of inversion mobility in Ge MOSFETs covering a wide range of bulk-impurity concentrations (10¹⁴ cm⁻³–10¹⁷ cm⁻³), and substrate bias (0–10 V). Carrier mobilities in Ge MOSFETs have obviously increased compared with those in Si MOSFETs. At low effective field, both electron and hole mobilities have increased over 100%; while at high effective field the increase is reduced due to the effect of surface roughness. Similar to Si MOSFETs, the carrier effective mobilities in Ge MOSFETs also have a universal behavior. The universality of both electron and hole mobilities holds up to a bulk-impurity concentration of 10¹⁷ cm⁻³. On substrates with higher bulk-impurity concentrations, the carrier effective mobilities significantly deviate from the universal curves under low effective field because of Coulomb scattering by the bulk impurity.     

A note on trap recombination in high voltage device structures

The carrier lifetime is a very important parameter influencing all important characteristics of bipolar devices both discrete and integrated structures and carrier lifetime tailoring is an important part of power semiconductor device technology. In presented paper, recombination through traps (centres with a deep energy level between edge of bandgap and Fermi level) is discussed in more details. It has been shown that some traps can considerably influence recombination rate in silicon and that at some traps a considerable temperature dependence of the centre cross-sections may be found. This is demonstrated in the case of iridium traps with a deep energy level 0.28 eV below the conduction band which capture cross-section temperature dependence has been found σ_p+σ_n∝T^−6.5. Further, the problem on low injection carrier lifetime in low-doped layers of high voltage semiconductor devices is also discussed.     

An investigation of surface state capture cross-sections for metal–oxide–semiconductor field-effect transistors using HfO2 gate dielectrics

MOSFETs and MOSCs incorporating HfO₂ gate dielectrics were fabricated. The I_DS–V_DS, I_DS–V_GS, gated-diode and C–V characteristics were investigated. The subthreshold swing and the interface trap density were obtained. The surface recombination velocity and the minority carrier lifetime in the field-induced depletion region measured from the gated diodes were about 2.73 × 10³ cm/s and 1.63 × 10⁻⁶ s, respectively. The effective capture cross section of surface state was determined to be 1.6 × 10⁻¹⁵ cm² using the gated-diode technique in comparison with the subthreshold swing measurement. A comparison with conventional MOSFETs using SiO₂ gate oxide was also made.     

Raman spectra of GaPAs–GaP heterostructure waveguides

A GaPAs waveguide with GaP cladding layers, has been fabricated in only two steps of LPE growth. One-way optical loss of the fabricated GaPAs–GaP waveguide is 12% mm⁻¹ (α=1.9 cm⁻¹). The free carrier concentration of the GaPAs layer is 1×10¹⁶ cm⁻³ (α=0.05 cm⁻¹). The backward spontaneous Raman spectrum from the GaPAs waveguide shows a GaP-like longitudinal optical (LO) phonon line. The LO band is intensified and shifted to a higher frequency compared to the LO phonon of GaP bulk crystal.     

Carrier recombination and deep levels in PbTe

The results of transient photoconductivity lifetime measurements and of impurity photoconductivity as well as Hall effect measurements obtained on PbTe films show that deep electron trapping levels exist in monocrystalline epitaxially grown PbTe. To our knowledge this is the first direct observation of deep levels in the gap of PbTe. The energetic position of the levels and their concentration were determined. The observed levels profoundly influenced the efficiency of PbTe infrared emitter diodes. This was shown experimentally by comparing the luminescence intensity of various diodes with different carrier concentration in the active zone. A strong decrease of luminescence intensity was observed for carrier concentrations greater than 10¹⁷ cm⁻³. This decrease could be explained by recombination via the observed levels in the gap.     

Hooge parameter of InGaAs bulk material and InGaAs 2DEG quantum well structures based on InP substrates

The 1/f noise of various InGaAs layers lattice matched to InP is investigated systematically at room temperature. To elucidate the origin of the 1/f noise in this type of III–V compound semiconductor, thick n-type doped InGaAs layers, typical InP based InAlAs/InGaAs or InP/InGaAs heterostrucuture field-effect transistor (HFET) structures, respectively, as well as InP based InAlAs/InGaAs quantum well structures with doped InGaAs two-dimensional electron gas (2DEG) channel are analysed. From the experiments it is found that mobility fluctuation is the only origin for the 1/f noise observed both in InGaAs bulk material and in 2DEG heterostructures of high quality. A Hooge parameter attributed to phonon scattering α_Hphon in the bulk material is found to be about 7×10⁻⁶ and agrees with those obtained from HFET structures with the highest mobilities (α_H≈1.5×10⁻⁵). Furthermore, the Hooge parameter of 2DEG structures strongly depends on the channel design and on the doping concentration in the n-type doped 2DEG channels.     

Negative electron diffusivity in high electric fields

The prediction that the electron diffusivity may be negative in high electric fields under some conditions has been examined starting from the Boltzmann equation and assuming a Maxwellian distribution function. It is found that the diffusion constant is positive for predominant acoustic phonon, polar optical phonon or impurity atom scattering. But the constant may be negative when effects of nonparabolicity are important and energy relaxation is limited by non-polar optical phonon scattering but the momentum relaxation is dominated by impurity atom scattering. Calculations with the parameter values of InSb, silicon and germanium show that only in materials like germanium at low temperatures of about 27 K the diffusion constant may be negative for impurity concentrations of 10¹⁷–10¹⁸ cm^?3.     

Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy

We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg_0.1Zn_0.9O layers are grown when [Ga] is less than 1×10¹⁸ cm⁻³. For [Ga] between 1×10¹⁸ cm⁻³ and 1×10²⁰ cm⁻³, ZnO and Mg_0.2Zn_0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1×10²⁰ cm⁻³, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries.     

Conductivity in transparent anatase TiO2 films epitaxially grown by reactive sputtering deposition

The synthesis of semiconducting TiO₂ thin films deposited by reactive sputtering is discussed. In particular, defect doping of the anatase polymorph that is epitaxial stabilized on (0 0 1) LaAlO₃ was explored using either oxygen or water vapor as the oxidizing species. For films grown in oxygen, a transition from insulating to metallic conductivity of the films is observed as the O₂ pressure is reduced. X-ray diffraction measurements show the presence of the Ti_nO_2n−1 phase when the oxygen pressure is reduced sufficiently to induce conductive behavior. Hall measurements indicate that these materials are p-type. In contrast, the use of water vapor as the oxidizing species enabled the formation of n-type semiconducting TiO₂ with carrier density on the order of 10¹⁸ cm⁻³ and mobility of 10–15 cm²/V s.     

Optimization of electric properties of high-k zirconium dioxide by varying deposition and annealing conditions

ZrO₂ thin films with a smooth surface were synthesized on silicon by atomic vapor deposition™ using Zr[OC(CH₃)₃]₄ as precursor. The maximum growth rate (7 nm min⁻¹) and strongest crystalline phase were obtained at 400 °C. The increase of the deposition temperature reduced the deposition rate to 0.5 nm min⁻¹ and changed the crystalline ZrO₂ phase from cubic/tetragonal to monoclinic. These films showed no enhancement of the dominating monoclinic phase by annealing. The values of the dielectric constant (up to 32) and leakage current density (down to 1.2×10⁻⁶ A cm⁻² at 1×10⁶ V cm⁻¹) varied depending on the deposition temperature and film thickness. The midgap density of interface states was N_it=5×10¹¹ eV⁻¹ cm⁻². The leakage current and the density of interface states were lowered by the annealing to 10⁻⁷ A cm⁻² at 1×10⁶ V cm⁻¹ and to 10¹⁰ eV⁻¹ cm⁻², respectively. However, this also led to a decrease of the dielectric constant.     

Effect of superstoichiometric components on the spectral and kinetic characteristics of the luminescence of ZnSe crystals with isovalent impurities

The spectral and kinetic parameters of the X-ray luminescence of ZnSe crystals doped with Zn, Se, and Te were investigated during the growth process at temperatures in the range 80–500 K, and also after annealing in Zn vapor. ZnSe crystals grown from a stoichiometric mixture, or mixture containing chalcogenide impurities, typically produce the minimum level of afterglow and a rapid rise of X-ray luminescence, as well as a shift of its peak from the infrared region toward shorter wavelengths after annealing in zinc. ZnSe crystals grown from material with excess of Zn have a relatively low X-ray luminescence yield and a substantial level of afterglow. It is assumed that the growth of Te-activated crystals is accompanied by the development of thermally stable complexes of the form V _ZnTe_Se that act as radiative recombination centers. The introduction of excess Zn into the initial mixture produces a reduction in the concentration of V _Zn and, hence, in the concentration of radiative recombination centers. It is shown that, for free-electron concentrations n<10¹⁸ cm⁻³, the afterglow time constant τ can be described as a function of n by a model of radiative recombination that involves a single impurity level, whereas for n>10¹⁸ cm⁻³, the time constant decreases with increasing n, which cannot be explained in terms of the simple model. It is suggested that radiative recombination centers of a new type are produced as a result of prolonged annealing in Zn vapor. Fiz. Tekh. Poluprovodn. 31, 1211–1215 (October 1997)