Radiative recombination in GaN nanocrystals at high intensities of optical excitation

The spectra of spontaneous UV luminescence of GaN nanocrystals excited by optical pumping at power densities ranging from 50 W/cm² to 50 MW/cm² are studied. At room temperature, radiation peaks related to the emission of free excitons and recombination of electron-hole plasma are revealed. The spectral characteristics of the emission of the electron-hole plasma in GaN is studied in the wide temperature range of 77–550 K.     

Properties of hot photoexcited holes in Cu-doped germanium at low temperature

A theoretical study is made of the properties of hot photoexcited holes in Cu-doped Germanium as a function of the lattice temperature and compensation density (N_d). Numerical solutions of the rate equation, obtained by an iterative technique, give the steady-state energy distribution of hot photoexcited holes due to aproximately blackbody room temperature radiation. Considering only a spherical heavy-hole band, simple acoustic deformation scattering and instantaneous optical phonon emission it is found that for the lowest values of N_d the energy distribution function is approximately Maxwellian and gives a good fit with experimental photohall mobilities. For higher values of N_d, a spherical light hole band is included in the calculations and this involves the use of a two-parameter deformation potential for inter- and intra-valley transverse and longitudinal scattering, and also instantaneous inter- and intra-valley optical scattering. The experimental photohall mobilities can then be used to give information on the deformation potential parameters for the valence band of Ge which cannot be derived from equilibrium transport measurements.     

Deep level spectroscopy,low temperature defect motion and nonradiative recombination in GaAs and GaP

Recent developments in junction capacitance measurements allow deep levels in semiconductors to be conveniently studied for the first time. Experiments carried out on deep levels in GaP and GaAs show that the levels are often strongly coupled to the lattice. This coupling can cause rapid nonradiative recombination in which the released electronic energy causes violent vibrations of the lattice near the defect. The vibrations can promote low temperature defect motion. These two phenomena, nonradiative recombination and defect motion, are fundamental to the understanding of how defects limit the efficiency of light emitting devices and cause junction devices to degrade when forward biased. The recent work of Lang on deep level spectroscopy, Lang and Kimerling on defect motion and Henry and Lang on nonradiative recombination by multiphonon emission will be reviewed.     

Characteristics of high power LEDs at high and low temperature

The high power light emitting diodes(LEDs) based on InGaN and AlGaInP individually are tested on line at temperatures from -30 to 100℃.The data are fitted to measure the relationship between temperature and the properties of forward voltage,relative light intensity,wavelength,and spectral bandwidth of two different kinds of LEDs.Why these properties changed and how these changes reflected on applications are also analyzed and compared with each other.The results show that temperature has a great influence on the performance and application of power LEDs.For applications at low temperature,the forward voltage rising and the peak wavelength blue-shifting must be considered;and at high temperature,the relative light intensity decreasing and the peak wavelength red-shifting must be considered.     

大功率LED高低温特性

在线测量了从－30至100度InGaN和AlGaInP大功率LED的各参数,拟合了两种LED正向压降、相对光强、波长、谱宽和温度的曲线,分析了两种LED参数变化的原因和这些变化对应用的影响。温度对大功率LED的性能和应用都有很大影响,低温时,应考虑正向压降的增大和波长的蓝移,高温时,应考虑光强的下降和波长的红移     

Low temperature recombination into shallow donors in germanium: Transient case

Recombination of electrons into shallow donors is traditionally described in terms of a single rate equation in the conduction band electron concentration n; this ignores the concentrations n_i of electrons in excited states (i) of the donors. Formally it should be described in terms of a set of rate equations in n and the n_i. It is proved here that if the sample purity is sufficiently high the two approaches give the same result. For this, suitable interpretation must be given to the ionization and recombination rates appearing in the simple rate equation; the latter represents the well-known cascade process; the former turns out to be dominated by an “inverse cascade”. The required sample purity is calculated as a function of temperature, applied electric field, and n. Calculations based on the linearized form of the rate equations, exact for small excursions, are used to test the theory.     

Silver-indium joints produced at low temperature for high temperature devices

A two-step fluxless bonding process adopted to produce high temperature silver-indium joints (80 wt% silver and 20 wt% indium) at relatively low process temperature of 206/spl deg/C has been developed. After annealing the joint continuously for 26 h at 145/spl deg/C, its melting temperature increases to 765-780/spl deg/C, as confirmed by a de-bonding test. The technique thus developed provides a viable alternative to packaging many high temperature devices running at 350/spl deg/C and above. The bonding quality of the Ag-In joints produced was examined using scanning acoustic microscopy. The joint cross-section was also studied using a scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope to find the local microstructure and composition. The results have shown that the joint is nearly void-free and uniform in thickness ranging from 7.2 to 7.8 /spl mu/m. The annealed sample joint, as determined by EDX, is mainly composed of AgIn/sub 2/, Ag/sub 2/In, and AuIn/sub 2/ grains embedded in an Ag-rich Ag-In alloy matrix. During joint formation, the intermetallic compound AgIn/sub 2/, in particular, prevents the indium layer from oxidation, and therefore, no flux is needed. In addition, low process temperatures help to reduce the thermal stresses developed in the bonded structure due to thermal expansion mismatch. Finally, reliability tests were conducted on three sets of annealed joints using a high temperature oven running continuously at 500/spl deg/C for 10, 100, and 1000 h each. Scanning acoustic microscopy (SAM) images on these samples confirmed that the joints have an excellent survivability in a high temperature environment.     

Deep level studies in MBE GaAs grown at low temperature

The photo-induced current transient spectroscopy (PICTS), thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) spectroscopy have been used to characterize the deep levels in the GaAs materials grown at low temperature by molecular beam epitaxy. At least five hole traps and five electron traps have been identified by the TEES measurement employing a simplified sample arrangement. We have studied the behavior of various traps as a function of the growth temperature and the post-growth annealing temperature. Some of the shallower hole traps were annealed out above 650‡ C. Electron traps atE _c- 0.29 eV andE _c- 0.49 eV were present in the material, and have been identified as M3 and M4, respectively. The dominant electron trap, atE _c- 0.57 eV, is believed to be associated with the stoichiometric defect caused by the excess As in the material, and our data show evidence of forming a defect band by this trap. A possible model involving As precipitates is proposed for this trap atE _c-0.57 eV.     

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

Semiconductors - The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence...     

No trace of divacancies at room temperature in germanium

Alpha-particle irradiated n⁺p-mesa diodes of Ge were investigated by conventional deep level transient spectroscopy and high-resolution Laplace deep level transient spectroscopy. The electronic and annealing properties of the observed hole traps were studied. It is concluded that none of the observed traps which are stable at room temperature are related to the divacancy. These results are consistent with previous optical studies. They are, however, in disagreement with recent numerical density function calculations which predict a stable divacancy at room temperature with band-gap levels in the lower half of the band gap.     

Photoconductivity and luminescence of CuInSe2 single crystals at a high level of optical excitation

The luminance-current and spectral characteristics of photoluminescence of the CuInSe₂ single crystals are studied. The superlinear portion of the excitation-intensity dependence of photoconductivity at low excitation intensities in compensated p-CuInSe₂ crystals is explained on the basis of a recombination model. The emission band that peaked at 0.98 eV in the n-CuInSe₂ photoluminescence spectrum corresponds to radiative recombination of electrons at the donor level with a depth of 0.04 eV. The maximum in the band intensity corresponds to the energy gap between the trap level and the valence band.     

Avalanche ionization rates measured in silicon and germanium at low electric fields

Ionization rates in semiconductors can be measured at low values of electric field using a new method involving a field-effect transistor (FET) structure which offers greater sensitivity than reverse biased p-n junction diode methods. Carriers of only one polarity cause ionization in the FET and no correction is required for ionization caused by carriers of the opposite polarity. Since secondary carriers resulting from ionization are attracted to a different terminal (gate or substrate) than that used to collect the primary carriers (drain), very small ionization currents can be detected. Values of electron ionization rate αnand hole ionization rate αpas low as 10^-3cm^-1have been obtained for silicon. The approximate relationshipalpha = alpha_{infin}e^{-b/E}is observed and the values of α at high fields correspond to those obtained conventionally. Values of αpfrom 0.04^-1to 0.4 cm^-1have been obtained for germanium. Analytical determination of electric field was provided by a solution of Poisson's equation for the field-effect structure. Difficulty in accurately determining the FET channel doping introduces a ± 30 percent uncertainty in electric field values. The method is applicable to any semiconductor material where junction, MOS, or Schottky barrier techniques can be used to construct field-effect transistors.     

Bulk negative differential conductance in n-type germanium at low electric fields

Kalstalskii has recently reported a new current instability in n-type germanium at low fields and low temperatures. This letter shows that a theoretical calculation based on the equivalent intervalley transfer between [111] minima yields results that agree very well with the experimental observations. The treatment is based on Boltzmann's equation. The drifted Maxwellian distribution function is assumed.     

LT-GaAs with high breakdown strength at low temperature for power MISFET applications

Low temperature grown GaAs has been fabricated containing a limited amount of excess arsenic. The material has a low conductivity in the order of 100KΩ cm, due to hopping in a deep donor band. This σ-LT-GaAs was grown reproducibly by using the lattice mismatch as the primary parameter for substrate temperature calibration. Breakdown fields, in the order of 100kV/cm, are observed for planar structures and increased at low measurement emperatures. Low hopping conductivity and high breakdown field are also observed in the lossy dielectric metal-insulator-semiconductor field-effect transistor device using σ-LT-GaAs as a surface layer. The record radio frequency power density of 4.0W/mm at 77K is extracted from the dc output characteristics.     

Charge transport in poly(p-phenylene vinylene) at low temperature and high electric field

Charge transport in poly(2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)-based hole-only diodes is investigated at high electric fields and low temperatures using a novel diode architecture. Charge carrier densities that are in the range of those in a field-effect transistor are achieved, bridging the gap in the mobility versus charge carrier density plot between polymer-based light-emitting diodes and field-effect transistors. The extended field range that is accessed allows us to discuss the applicability of current theoretical models of charge transport, using numerical simulations. Finally, within a simple approximation, we extract the hopping length for holes in MEH-PPV directly from the experimental data at high fields, and we derive a value of 1.0 ± 0.1 nm.     

The growth of laser oscillations at high excitation

The buildup of single-mode laser oscillations is investigated for the 0.633-μ transition of a helium-neon laser at levels of relative excitation significantly above threshold (1.5 leq n leq 4) using a fast intra-cavity chopper. The time response agrees with theory. The evolution of the signal provides information on the saturation mechanism and allows a very simple determination of resonator quality, relative excitation, unsaturated gain, and degree of saturation of a laser. The saturation parameter is found to depend on the discharge current. This is attributed to a variation of the gas density in the capillary discharge tube caused by gas heating, which affects the collision-dominated atomic lifetimes.     

Minority-charge-carrier mobility at low injection level in semiconductors

From the kinetic equations, the distribution functions for majority and minority charge carriers are obtained at a low injection level. For describing the electron-hole collisions, the Landau collision integral is used. The carrier scattering at ionized or neutral impurity and at acoustic phonons is taken into account. The majority-carrier distribution function is presented in the analytical form. The minority-carrier mobility is calculated and analyzed, and the features of its behavior at low temperatures are revealed. It follows from the developed theory that the hole mobility in an n-type material increases with doping and neutral-impurity concentration. This effect is attributed to mutual charge-carrier collisions and different effective masses of different-sign carriers.     

An indirect measurement of energy gap for silicon and germanium

Using temperature characteristics obtained from the reverse saturation current of p-n junctions and the forward-bias voltage, the energy gaps of Si and Ge can be indirectly measured at 0 K by means of linear extrapolation. Considering the different components of the saturation currents of p-n junctions for Si and Ge, we have forsaken the methods prevalent in the usual CAD of electronic circuits and, instead, have derived two distinct equations to measure the E_g0 of Si and Ge. The paper shows, after including band-gap narrowing effects, that the extrapolated intrinsic E_g0 of the sample Si approaches the generally accepted values, with an error within 0.8–1.0% for Si. The E_g0 for doped Ge is 0.67–0.75 eV approximately, when using, a small power transistor as sample. The model of physics used in the test has been shown to be clear-cut and simple, thus providing a simple way for testing the parameter E_g0, both practically and pedagogically.     

Observation of electromigration at low temperature

Electromigration and mean time-to-failure were investigated for unglassed thin Al stripes over the temperature range of 223 K to 347 K. Thermal effects are minimized by sinking heat from the linestrip through the substrate to a miniature cryogenic refrigerator. This test technique allows the investigation of structure and current interactions while suppressing the effects of an added temperature factor. The circuit was stressed by direct current densities greater than 4×10⁶ A/cm². Electromigration damage was induced in a test stripe at temperatures near 0°C. For the temperature range of 347 K to 267 K, an activation energy of 0.30 eV was calculated, indicating that surface migration is the dominant failure mechanism. For temperatures between 267 K and 223 K, a calculated activation energy of 0.12 eV suggests a different failure mechanism, which was subsequently identified as stripe separation from the substrate     

Anomalous relaxation of photoconductivity in silicon at high excitation levels

Anomalous relaxation of excess carries at high injection levels is observed in silicon samples upon their excitation with a pulse of light from a laser diode (1060 nm/500 mW). Microwave conductivity measurements and conventional photoconductivity-relaxation measurements were employed in these experiments. Unusual behavior takes place at the initial stage of the photoconductivity’s relaxation after the end of a pulse of light. To explain the anomalous-relaxation effect, we suggest a simplified model that involves excitons at high excitation levels.