宽带隙半导体材料光电性能的测试

主要通过光致发光的实验手段,研究分析了在自支撑GaN衬底上生长的InGaN/GaN多量子阱(InGaN/GaN MQW)有源层中的载流子复合机制,实验中发现多量子阱的光致发光光谱中有一个与有源区中的深能级相关的额外的发光峰。在任何温度大功率激发条件下,自由激子的带边复合占主导地位,并且带边复合的强度随温度或激发功率的下降而减弱;在室温以下小功率激发条件下,局域化能级引入的束缚激子复合占主导地位,其复合强度随温度的下降而单调上升,随激发功率的下降而上升。带边复合在样品温度上升或者激发功率变大时发生蓝移,而局域的束缚激子复合辐射的峰值波长,随样品温度和激发功率的变化没有明显变化。     

Quantitative investigation of the recombination involving free particle scattering processes in highly excited blend type II–VI compounds

Inelastic scattering processes involving free excitons and either free carriers or excitons are important radiative recombination processes in highly excited direct gap semiconductors.Using ZnTe as a model substance, we have investigated the luminescence under low and high excitation intensities from liquid helium to room temperature. The comparison between our experimental results and two theories published recently shows that the observed recombination processes are dominantly due to inelastic exciton-exciton scattering at lower temperatures (T ≤ 40 K) and due to exciton-hole scattering at higher temperatures (T > 40 K).We find good agreement between experiment and theory in the lineshape and the spectral position of the emission maxima as function of temperature.     

Theory of the temperature dependence of the threshold current of an InGaAsP laser

The threshold current of an InGaAsP laser is calculated, where the radiative emission, reflection and absorption losses, and Auger recombination are considered. Moreover, the enhancement of the threshold carrier density at high temperatures is an important point. A mechanism for this enhancement is discussed. Then we obtain an excellent agreement with the measured temperature dependence of the threshold current, in particular the To-values for T≷TBand the break point TB. The reason for this break point is that the radiative recombination dominates for T < TB, whereas the strongly temperature dependent valence band Auger process becomes more and more important for T > TB. It is this process which causes the strong increase of the threshold current in the room temperature range.     

The first 70 semiconductor Auger processes

Ten basic Auger recombination processes are considered: two phononless and two phonon-assisted band-band processes, four processes involving one type of trap and two donor-acceptor processes. Expressions are obtained for the recombination coefficient by making a constant matrix element approximation, working out the impact ionization rate, and using detailed balance, noting that impact ionization is the inverse process of the Auger effect. Assuming all bands involved (excepting the band which contains the impact ionizing carrier) to be parabolic, described by a diagonalized effective mass tensor, new results for eight of the ten cases are found. Different band structure types lead to a multiplicity of each of the ten processes, yielding seventy different types. They are classified by utilising recent work on impact ionization thresholds as van Hove singularities. Processes involving excitons, pairs of particles bound to the same centre, etc. are not included here and add further Auger-type processes.     

Carrier density dependence of Auger recombination

Auger recombination (AR) is usually assumed to depend on the electron density n and the hole density p like n²p (or np²). But, there are deviations from these rules, mainly in degenerate semiconductors. Studying this case the following results were obtained: 1. Normal AR which is only possible in narrow-gap semiconductors goes approximately as np. 2. Phonon-assisted AR which predominates in “normal” gap semiconductors (E_g ≈ 1 eV) has the “usual” density dependence n²p. 3. Second order AR with two Auger electrons goes with (instead of an expected dependence n³p). Moreover, the density dependence may be influenced by the screening of the Coulomb interaction and by the strength of the excitation.     

1.3 /spl mu/m GaAs/GaAsSb quantum well laser grown by solid source molecular beam epitaxy

A highly strained GaAs/GaAs/sub 0.64/Sb/sub 0.36/ single quantum well laser has been grown on GaAs (100) substrate by using solid source molecular beam epitaxy. The uncoated broad-area laser demonstrates 1.292 /spl mu/m pulsed operation with a low threshold current density of 300 A/cm/sup 2/. The spontaneous emission of the laser was also studied. The result reveals that the Auger recombination component dominates the threshold current at high temperature.     

Theoretical study of auger recombination processes in deep quantum wells

The basic processes and mechanisms of Auger recombination of nonequilibrium carriers in a semiconductor heterostructure with deep InAs_0.84Sb_0.16/AlSb quantum wells (QWs) are analyzed. It is shown that a zero-threshold Auger recombination process involving two heavy holes predominates in sufficiently narrow QWs, and a resonant process involving two electrons is dominant in wide QWs. The range of QW widths at which the Auger recombination is suppressed in a given structure to the greatest extent (suppression region) is determined. In this case, the threshold process involving two electrons remains the basic nonradiative recombination process, with its probability being several orders of magnitude lower than those for the zero-threshold and resonant mechanisms. In turn, the zero-threshold mechanism involving two electrons is totally impossible in the heterostructure under study because of the large conduction-band offset (which markedly exceeds the energy gap). Also, the range of emission wavelengths that corresponds to the suppression region is estimated. It is shown that the interval calculated belongs to the mid-IR range.     

Size effects in Auger recombination for InGaAs quantum-wirestructures

A theoretical investigation of Auger recombination in lattice-matched InGaAs/InGaAlAs quantum-wire structures is presented. The valence band structure is calculated by using a four-band Luttinger-Kohn Hamiltonian. CHCC, CHHH, CHHL and CHHS Auger processes are considered with the excited carrier being either in a confined (bound) state of the quantum wire, or an unconfined (unbound) state. The model uses Fermi statistics as well as a revaluation of the Coulomb interaction overlap integral for the calculation of the Auger recombination rate. Bound-unbound Auger processes are proven to be important nonradiative recombination mechanism in quantum-wire systems. It is also found that the Auger coefficient is much more sensitive to the well width in quantum-wire structures than in quantum-well structures     

Structural and luminescent properties of ZnO:P films produced by annealing ZnP<Subscript>2</Subscript> wafers in atomic oxygen

The epitaxial ZnO:P films are fabricated by annealing the ZnP₂ wafers in atomic oxygen (oxygen radicals). The properties of the films are studied by X-ray diffraction analysis, atomic force microscopy, Auger spectroscopy, and photoluminescence measurements. In the X-ray diffraction spectra of the samples, the (002) peak is observed, suggesting that the ZnO:P films are oriented along the c axis. The Auger spectroscopy data show that the ZnO films contain phosphorus. The low-temperature photoluminescence spectrum observed for the ZnO: P films exhibits a 3.356 eV peak presumably corresponding to excitons bound at neutral acceptors and a peak at 3.306 eV (free electron)-acceptor transitions associated with the P_O level. The phosphorus related acceptor level is localized at 128 meV above the top of the valence band. The origin of the 3.312 eV band related to recombination at donor-acceptor pairs is discussed.     

Effect of auger recombination on the threshold characteristics of gain-guided InGaAsP lasers

Experimental and theoretical results are presented to study the effect of Auger recombination on the threshold current of gain-guided InGaAsP lasers. A comparison of theory and experiment suggests that Auger recombination should be included for a reasonable agreement between them. It is shown that a rapid increase of the threshold current for narrow stripes is due to the combined effect of index anti-guiding and Auger recombination. Our deduced values of the Auger coefficient at 1.3 and 1.55 ?m indicate that it increases rapidly with decreasing bandgap.     

Auger and radiative recombination of acceptor bound excitons in semiconductors

We report on a theoretical and experimental study of acceptor bound exciton recombination. We present calculations of phononless Auger and radiative recombination in direct and indirect band gap materials. We consider hydrogenic acceptors in the direct band gap material Hg_1−xCd_xTe in which the band gap can be varied by changing alloy composition. We present calculations of the Auger transition rate and no-phonon oscillator strengths for the common acceptors in Si and Ge. We have measured the bound exciton lifetimes and no-phonon oscillator strengths for the acceptors in Si and find reasonable agreement with the calculated values.     

Auger recombination in long-wavelength strained-layer quantum-wellstructures

A model calculation of Auger recombination in strained-layer InGaAs-InGaAlAs and InGaAs-InGaAsP quantum-well structures is presented as an extension of an empirical Auger theory based on the effective mass approximation. The valence band effective masses around k_∥=0 are calculated by using a six-band Luttinger-Kohn Hamiltonian and the quasi-Fermi levels are determined with a self-consistent Poisson-Schrodinger solver under the effective mass approximation. Three basic Auger processes are considered with the excited carrier being in a bound state of the quantum well, as well as an unbound state. The empirical model includes Fermi statistics as well as a revaluation of the Coulomb interaction overlap integral in the Auger recombination rate. Bound-unbound Auger transitions are proved to be an important nonradiative recombination mechanism in strained-layer quantum-well systems. Our calculations of Auger coefficient are in reasonable agreement with the experimental data     

Temperature dependence of the threshold current of QW lasers

The temperature dependence of the threshold current in GaInAs-based laser structures has been studied in a wide temperature range (4.2 ≤ T ≤ 290 K). It is shown that this dependence is monotonic in the entire temperature interval studied. Theoretical expressions for the threshold carrier density are derived and it is demonstrated that this density depends on temperature linearly. It is shown that the main contribution to the threshold current comes from monomolecular (Shockley-Read) recombination at low temperatures. At T > 77 K, the threshold current is determined by radiative recombination. At higher temperatures, close to room temperature, Auger recombination also makes a contribution. The threshold current grows with temperature linearly in the case of radiative recombination and in accordance with T ³ in the case of Auger recombination.     

Explaining the different efficiency behaviors of PHOLEDs with/without a hole injection barrier at the hole transport layer/emitter layer interface

In this work we investigate the different efficiency behaviors of the devices with and without hole injection barrier, utilizing in our investigation the archetypical 4,4′-bis(carbazol-9-yl)biphenyl:Tris(2-phenylpyridine)iridium(III) host–guest PHOLEDs system. The results show that the recombination of electrons and holes on the host material generally leads to higher device efficiency in comparison to the case where recombination happens on the guest material. The results also show that in devices where a hole injection barrier between the HTL and the host material in the EML exists, the emission mechanism gradually changes from one based on host e–h recombination to one based on guest e–h recombination as the guest concentration is increased. When host e–h recombination is dominant, although it tends to produce higher device efficiency, host e–h recombination is generally also associated with significant efficiency roll-off; the latter arises from quenching of the host triplet excitons primarily due to host–host TTA. As the concentration of the guest molecules increases and the creation of host triplet excitons subsides (since most e–h recombination occurs on the guest) host–host TTA decreases, hence also the efficiency roll-off. In such case, quenching is mostly caused by polarons residing on guest sites. At optimum guest concentrations (∼8% Vol.), a balance between host e–h recombination and guest e–h recombination is reached, and thus also minimal TTA and Triplet-Polaron Quenching. On the other hand, in devices where hole injection barrier between the HTL and the host in the EML is insignificant, emission mechanism is always based on host e–h recombination irrespective of the guest concentration, and therefore have higher efficiency and the efficiency does not depend on guest concentration. The absence of the injection barrier in these devices results in a wider recombination zone, and hence a lower exciton concentration in general, which in turn reduces host–host TTA and thus lowers efficiency roll-off. In contrast, guest–guest TTA is not found to play a significant role in device efficiency behavior.     

Measurement of Auger recombination in silicon by laser excitation

A new experimental arrangement for the study of Auger recombination in silicon is described and analyzed. A relatively weakly absorbed YAG:Nd laser beam was used for excitation. The decay of the carrier concentration after the injection pulse was studied by measuring the recombination radiation in a direction perpendicular to the laser beam. At some distance from the injection surface the influence of surface recombination and diffusion is then negligible. It has previously been shown that in this geometry the carrier concentration distribution after the laser excitation is accurately described by an analytical expression which accounts for attentuation of the laser beam by both interband and free carrier absorption. Thus the local carrier concentration in the sample can be computed to a high degree of accuracy, which is essential in the determination of the Auger recombination coefficient from decay measurements. Furthermore, this experimental geometry eliminates the problems with laser stray light. Assumptions regarding the influence of surface recombination and diffusion are not necessary in the interpretation of the experiments. The method is usable for silicon in the temperature interval 150–400 K. Preliminary measurements of the Auger coefficient at room temperature are reported.     

The role of stacking faults and their associated 0.13 ev acceptor state in doped and undoped ZnO layers and nanostructures

An emission band at 3.31 eV is frequently observed in low-temperature photoluminescence (PL) measurements on ZnO p-doped with group-V elements, and also on nominally undoped ZnO layers and nanostructures. It has alternatively been ascribed to LO- or TO-phonon replicas of free excitons, to acceptor-bound excitons, to donor-acceptor pair transitions, to two-electron satellites of donor-bound excitons, or to free-to-bound transitions. This band frequently dominates the PL of ZnO nanostructures and layers at room temperature. Annealing leads to drastic changes in its intensity.We report on low-temperature cathodoluminescence measurements with very high spatial resolution and high-resolution transmission electron microscope investigations carried out on the same pieces of hetero-epitaxial ZnO samples with unusual layer orientation. These data allow us to correlate this emission unambiguously with c-plane stacking faults. The emission is found to be due to the recombination of a free electron with a hole bound to a relatively shallow acceptor state ≈130 meV above the valence band edge. Locally, these acceptor states occur in high concentrations of up to some 10¹⁸ cm⁻³, and thus lead to strong two-dimensional perturbations of the free carrier concentration. They have severe implications for the conductivity of layers and nanostructures in general, and on the interpretation of Hall and luminescence data in particular. Literature data are critically reviewed in the light of these findings.     

Recombination statistics and kinetics in semiconductor nanostructures

A new approach to describing the observed features of recombination in semiconductor nanostructures is suggested. In addition to radiative exciton recombination, a nonradiative channel of exciton Auger recombination involving local interface states is taken into account. Recombination statistics and kinetics in semiconductor nanocrystals are considered both for low and high densities of local interface states. The case of a low excitation level, where a statistical approach to recombination in isolated nanocrystals is no longer valid, is analyzed. It is shown that the presence of nonradiative exciton Auger recombination accounts both for the linear dependence of photoluminescence intensity on the excitation level and for the low photoluminescence quantum efficiency.     

Simulation of ultraviolet- and soft X-ray-pulse generation as a result of cooperative recombination of excitons in diamond nanocrystals embedded in a polymer film

Using numerical simulation, it is shown that the recombination of free excitons photoexcited in diamond nanocrystals embedded in a polymer film can occur in the cooperative mode. It is found that this mode can be implemented despite the fact that diamond is an “indirect” semiconductor. It is shown that the power of the generated radiation at the pulse peak during the cooperative recombination of free excitons can exceed that of the incoherent spontaneous emission of the same initial number of free excitons by more than an order of magnitude. Finally, it is shown that the process under consideration can be used to generate picosecond pulses of ultraviolet and soft X-ray electromagnetic field at a wavelength of 235 nm.     

Life time measurements in PbTe and PbSnTe

Experimental life time data of n-type PbTe and Pb_0.8Sn_0.2Te epitaxial layers are reported. Experiments were carried out with photoionized and impact ionized excess carriers between 6 and 300 K. Both materials show a strongly increasing life time with decreasing temperature. With PbTe an extremely non-exponential excess carrier decay was observed. Measurements of carrier sweep out and time resolved photo Hall effect have shown that in PbTe a strong minority carrier trapping occurs, whereas in PbSnTe, both electrons and holes, have nearly equal decay times. Comparing the experimental data with theory it can be concluded that in PbTe a Shockley-Read mechanism dominates the recombination at low temperatures. Auger recombination might become efficient only at temperatures above 200 K. The recombination behaviour in PbSnTe cannot be explained by one of the well-known models. An Auger recombination theory taking into account degeneracy might describe the observed data.