Luminescence decay kinetics in GaP:Bi and GaP:N

The lifetime of excitons bound to isoelectronic traps in GaP is studied as a function of temperature, excitation energy and intensity. The photoluminescence was excited both selectively and above the gap by using a pulsed, tunable dye laser with photon flux varying in the range of 10¹⁴–10¹⁹ photons/cm² per pulse. GaP:Bi shows a strong intensity dependence of τ(T) in the temperature range of 40–60 K due to the thermal activation of the electron. Similarly, GaP:N shows this behavior in the range of 20–100 K. In this case, two activation processes can be identified: release of the bound exciton into the free exciton band and dissociation of the exciton. The observed dependence of τ(T) on both excitation energy and intensity indicate that saturable deep traps (shunt paths) deplete the excess free carriers. These traps can be completely saturated in GaP:N while only partial saturation is achieved in GaP:Bi. The kinetic equations are written for GaP:Bi and solved numerically assuming quasiequilibrium conditions. Fitting this model to the experimental results yields the capture cross sections for carriers by Bi and by the deep traps as well as the concentration of the latter.     

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.     

New electronic effects in double injection diodes with one deep level

Double injection diodes made of high resistivity semiconductors compensated with deep levels show a negative differential resistance region in the stationary I - V characteristic. At lower temperatures the injection level of free carriers can be altered within the prebreakdown region without changing the space charge situation. Therefore is valid for several orders of magnitude of the current. Furthermore, the switching of the diode from the “off state” (prebreakdown region) to the “on-state” (high injection or semiconductor regime) can be delayed by applying a corresponding voltage V >V_BD, the breakdown voltage. An exponential dependence of the delay time t_D on the applied voltage is found. The lower limit of t_D is determined by the free carrier lifetime, an upper limit does practically not exist if the temperature is low enough.     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

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.     

Classification of macroscopic defects contained in p-type EFG ribbon silicon

Recombination behavior of the grown-in defects contained in p-type ribbon silicon has been examined. Minority carrier lifetime at various defect sites was measured as a function of temperature by the electron beam induced current (EBIC) method. Based on the lifetime vs temperature characteristics, we classify the defects into three macroscopic (> 20 μm scale) categories: (i) plastically deformed single crystal region; (ii) dislocation arrays with associated impurity atmosphere; and (iii) crystallographic structural line boundaries including twins and grain boundaries. The recombination processes described by category (ii) include a set of shallow electron donor traps near the dislocation core with apparent activation energies E_c − E_t = 0.066−0.087eV. The recombination lifetime of category (iii) can be described by Shockley-Read-Hall statistics with a recombination level located at the lower half of the band gap. Under the measurement condition, the activation energies of the acceptor-like center in category (iii) were found to be E_t − E_v = 0.086−0.114 eV. The recombination properties of category (i) and (ii) defects are consistent with the presence of compensating donor states in the p-type ribbon. Thus, it is believed that these types of defects are primarily responsible for the observed minority carrier lifetime dependence on the photoexcitation level in the EFG ribbon silicon.     

多晶MAPbBr_(3)薄膜中束缚载流子热发射复合过程北大核心CSCD

有机-无机卤化铅钙钛矿(organic inorganic lead halide perovskite,OLHP)半导体材料内部的陷阱是影响OLHP的光电性能的重要因素。为了理解多晶的甲胺溴基钙钛矿((Methylammonium)PbBr_(3),MAPbBr_(3))薄膜中陷阱对光生载流子复合的影响,本文采用了时间分辨微波光电导(time resolved microwave conductivity,TRMC)技术探究了多晶MAPbBr_(3)薄膜的光生载流子复合动力学过程。实验测量结果表明多晶MAPbBr_(3)薄膜的载流子复合过程包括自由载流子复合与束缚载流子的热发射复合两部分。其中,与束缚载流子热发射复合相关的能级远离连续带,且对应的能级深度约为0.6 eV,分布宽度约为89.2 meV。本文同时利用变激发波长TRMC对比实验,分析浅束缚光生电子与导带光生电子复合过程的差异。相比于导带上的电子,实验结果表明浅束缚电子跃迁到深束缚能级的概率更大。     

Electron recombination between Landau-levels in n-InSb

Experimental and theoretical investigations of electron recombination between Landau levels and associated impurity levels have been performed for n-InSb. The electronic lifetime in the first Landau level is found to be determined by electron-electron scattering what is experimentally confirmed by an inversely linear dependence on the electron concentration n₁ in the 1st Landau level. Values of 10⁻¹⁰ sec are obtained for n₁ 10¹³ cm⁻³. For n₁ ≤ 10¹⁰ cm⁻¹ acoustic phonon scattering determines the lifetime in the first Land level. For the recombination of electrons between the lowest Landau level and the impurity ground state times between 200 nsec and 50 nsec dependent on the magnetic field but independent of electron concentration are found indicating a phonon capture process. These times are also responsible for the observation that relaxation times from the (110) impurity level to the (000) ground state are the bottle neck in a three step process.     

Modelling of semiconductor diodes made of high defect concentration,irradiated, high resistivity and semi-insulating material: The capacitance–voltage characteristics

Full modelling is reported of the capacitance of a long PIN semiconductor diode with a high concentration of generation–recombination (g–r) centres and different concentrations of deep traps. There are considerable differences from the textbook results given for normal lifetime diodes which have low concentrations of g–r centres. For a low density of g–r centres, the capacitance is the usual value. That is it decreases as V^−1/2 with increasing reverse bias while it increases rapidly with increasing forward bias. For high density of g–r centres and in reverse bias a departure from this voltage dependence is observed, while in forward bias a negative capacitance appears. This agrees with experiment. From these results we present a physical understanding of the processes involved. There are specific applications of these results to radiation damaged devices, lifetime killed diodes and devices made from high resistance and semi-insulating materials, especially in the interpretation of the C–V curves to evaluate the fixed space charge density.     

Monitoring of carrier lifetime distribution in high power semiconductor device technology

Non-uniform distribution of carrier lifetime over the area of power bipolar semiconductor devices results in a non-uniform distribution of on-state current density and switching loses. Consequently, it results in non-uniform temperature distribution which can negatively influence the device reliability. Several methods can be used for measuring carrier lifetime distribution both in starting single crystal material and in device structures after high-temperature processes. Advantages and disadvantages of individual methods and an optimum area of applications are discussed in this paper. This paper is mostly oriented on a possibility to use LBIC method for measuring carrier lifetime distribution in the bulk of high voltage large-area devices, especially N⁺NPP⁺ diode structures.     

Photoluminescence in CuGaSe2

The photoluminescence spectra of CuGaSe₂ have been investigated at 76 K in the range 0.65 < λ < 1.1μ. The crystals were grown by iodine transport, and had p-type conductivity. The photoexcitation was provided by the single-line outputs of a CW Ar⁺ laser, and the luminescence spectra were analyzed using a m grating monochromator.The luminescence observed consisted of a rather sharp structure near the energy gap, plus two broad peaks at considerably smaller energies. The broad peaks, centered around 1.52 and 1.30 eV respectively, are attributed to transitions involving deep localized states. The sharper structure near the energy gap is resolved into two peaks, one at 1.72 eV and another at 1.68 eV. This structure is explained by radiative recombination of free excitons (peak at 1.72 eV), and by recombination of free electrons with bound holes (peak at 1.68 eV).A thermal treatment done on the CuGaSe₂ crystals changed both their electrical properties and their luminescence spectra. These changes produced by sample treatment, plus measurements of the excitation power dependence and the polarization of the luminescent radiation, are in agreement with the interpretation presented.     

A method for the determination of electron capture cross-section at imperfection centers in gallium arsenide of electroluminescent diodes

A simple method is described for the determination of the electron capture cross-section at the imperfection centers with a dominant deep level in semiconductor electroluminescent diodes. In this method, the electron capture cross-section is determined through simultaneous measurements of the temperature dependence of the minority carrier lifetime and the external quantum efficiency of the EL diodes. When the method is applied to Zn-diffused GaAs EL diodes, the average electron capture cross-section is found to be 10^?16 cm² at a level either 0·1 or 0·2 eV away from the mid-gap.     

Non uniform recombination in thin silicon-on-sapphire films

Recombination parameters of SOS films are deduced from the study of the magnetoconcentration effect in double-injecting structures. The method of measurement is based on an original theory succintly developed; it takes into account general SRH bulk and surface recombination laws; moreover inhomogeneous distributions of recombination centers are considered.Experimental results (current-voltage characteristics of such “magnetodiodes”), when analysed according to the proposed method, lead to more realistic values of the global recombination parameters (τ_v, S₁, S₂) of the SOS film. It is proved, for example, that the previous simplified analysis overestimates the carrier recombination velocity on the Si-Al₂O₃ surface. On the contrary our method gives both a moderate value for this recombination velocity and a lower carrier lifetime near the Sapphire interface, which well agrees with the continuity of recombination rates at the surface and in the underlying bulk; the higher recombination region is found to be 100–1000 Å thick.     

New challenges to the modelling and electrical characterisation of ohmic contacts for ULSI devices

Continuing developments in semiconductor process and materials technology have enabled significant reductions to be achieved in the contact resistance R_c of devices. This reduction is commonly assessed in terms of the specific contact resistance (SCR) parameter ρ_c (Ω cm²) of the metal–semiconductor interface. Such a reduction in SCR is essential, for as device dimensions decrease, then so also must ρ_c and the corresponding contact resistance in order not to compromise the down-scaled ULSI device performance. Thus the ability to accurately model contacts and measure ρ_c is essential to ohmic contact development. The cross kelvin resistor (CKR) test structure is commonly used to experimentally measure the Kelvin resistance of an ohmic contact and obtain the specific contact resistance ρ_c. The error correction curves generated from computer modelling of the CKR test structure are used to compensate for the semiconductor parasitic resistance, thus giving the SCR value. In this paper the increased difficulty in measuring lower ρ_c values, due to trends in technology, is discussed. The challenges presented by the presence of two interfaces in silicided contacts (metal-silicide–silicon) is also discussed. Experimental values of the SCR of an aluminium–titanium silicide interface is determined using multiple CKR test structures.     

Comparison of theoretical and empirical lifetimes for minority carriers in heavily doped silicon

The minority carriers determine essential electrical characteristics of bipolar devices and bipolar-like parasitic paths in field effect devices. The electrical behavior of such devices is frequently described by detailed device models. Compared to the other input parameters for detailed device models, the minority carrier lifetimes due to traps or defects as functions of doping density have great uncertainty. Detailed device models, which include the contributions of both Auger recombination and Shockley-Read-Hall processes to this lifetime, give correct values for the d.c. common emitter gain of npn transistors with shallow, heavily doped emitters. However, those models which include commonly used empirical expressions for the above lifetime do not predict correct values for the gain. The physical significance of these device modeling results involves competition between trapping of carriers and Auger recombination when donor densities lie within the decade of 10¹⁹ cm^?3. An example of the foregoing competition is given by applying detailed device models to an npn transistor with an emitter surface concentration of 2.3 × 10²⁰ cm^?3 and with an emitter-base junction depth of 1.1 μm. A major finding in this paper is that the commonly used empirical expressions for the lifetime due to defects may not give correct results when included in detailed models of shallow, heavily doped silicon emitters.     

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.     

Characterization of oxide traps in 0.15 μm MOSFETs using random telegraph signals

Random telegraph signals (RTS) have been used to characterize oxide traps of W×L=0.97×0.15 μm² medium-doped drain n-MOSFETs. RTS have been measured in the linear and saturation regions of operation, both in forward and reverse modes where the drain and source are reversed. The contribution of mobility fluctuations as well as number fluctuations to the amplitude of RTS has been investigated. The scattering coefficient due to screened Coulomb scattering effect is computed from the measured data as a function of channel carrier density. The depth of the position of the trap in the oxide from Si–SiO₂ interface is calculated utilizing the dependence of the emission and capture times on the gate voltage. In addition, the position of the trap along the channel with respect to the source is obtained using the difference in the drain voltage dependence of the capture and emission times between the forward and reverse modes. Knowing the location of the trap in the oxide and along the channel, the energy associated with the trap can be extracted accurately from the data. This technique allows one to evaluate the trap energy at the point where the trap is located without any assumptions about the location of the trap or the need for variable temperature measurements. The probed trap was found to be an acceptor type center (repulsive for an n-MOSFET) located at about 27 Å deep the oxide, half-way between drain and source with an energy of E_Cox−E_T=3.04 eV, slightly above the conduction band edge.     

Characterization of multiple deep level systems in semiconductor junctions by admittance measurements

The small signal admittance, Y, of a junction device, in the presence of deep lying majority carrier traps, is obtained as a solution to a simple differential equation (dC/dχ) = (C²/ε)?(ρ_ac/ψ_ac), where CY/jω is the complex capacitance, x is the distance within the depletion region from the neutral bulk semiconductor, ρ_ac is the a.c. incremental change in charge density at χ when the bias is incremented by ψ_ac. This equation can be numerically integrated with one boundary condition, the flat band capacitance of the bulk semiconductor. An analytic solution to the above differential equation is possible over a wide frequency range without the use of a truncated space charge approximation. The admittance of one half of a junction device can then be modelled by a 3p + 1 lumped element equivalent circuit involving 3p + 2 device parameters, where p is the number of species of deep lying majority carrier traps that are virtually unionized in the bulk. These circuit elements bear simple direct relationships to the deep level parameters. Impedance vs frequency measurements at a single bias and temperature yield only 2p + 1 equations and are not sufficient to define the elements uniquely. One therefore needs p + 1 additional equations for a unique synthesis. We also show how additional equations can be obtained from impedance vs voltage or temperature measurements.