Investigation of parallel conduction in GaAs/ AlxGa1-xAs modulation-doped structures in the quantum limit

We present a detailed study of the transport in GaAs/ AlxGa1-xAs modulation-doped structures in the low field and high magnetic field quantum limit for varying amounts of parallel conduction in the AlGaAs region. We observe the apparent breakdown of quantum Hall effect behavior due to low mobility carriers in the parallel channel. The onset of conduction through the parallel channel by quantum transport measurments has been observed, along with a non-linear dose dependence due to photoexcitation.     

GaAs/AlGaAs量子阱微结构的电学性质(英文)

A novel manifestation of super-lattice properties in GaAs/AlGaAs quantum well photodiode structure grown by MOCVD was observed. When a sample was illuminated, the C-V profiles at different temperature showed an oscillation of charge conccntration.It was explained in terms of quantum well behaving as "giant trap". The mechanism of transport at different temperature was discussed. We also gave a comparison of interface of hete-rojuncion GaAs/AlGaAs and the trap-like effect of quantum well. The deep level of "giant trap" of quantum well was measured by DLTS. Detailed balance between emission and capture of free carriers in quantum well was deduced.     

Recombination dynamics in GaAs/AlxGa1- xAs quantum well structures

Time-resolved and excitation-dependent photoluminescence of GaAs/AlxGa1-xAs quantum well structures reveal that recombination takes place between free carriers, not excitons, at room temperature for carrier densities at and above the mid -10¹⁶cm^-3level. Other samples show trapping and release of carriers from traps, evidence of dynamic Shockley, Hall, and Read recombination for optically active traps. The traps can be saturated to a large extent. Further studies show that they are associated with interfaces between different materials and that they become active at a temperature around 150 K. Results from all samples indicate that the bimolecular radiative recombination coefficientBfor quantum wells is no larger than the value ofBfor bulk GaAs, and may in fact be smaller. This is one of the first studies of time-resolved luminescence of impurities in quantum well structures. Impurity decays at low temperatures are found to be quite slow. A spectral line which appeared to be longitudinal optical phonon-shifted emission is shown to be due to an acceptor impurity.     

Carrier recombination rates in strained-layer InGaAs-GaAs quantumwells

The carrier recombination rates in semiconductor quantum wells are found to be structure dependent. Under high levels of excitation they generally do not follow the recombination rule of the bulk material. Through a differential carrier-lifetime measurement in the strained-layer InGaAs/GaAs quantum wells, it is shown that in quantum wells with lower potential barrier or thinner well width, the recombination rates are smaller due to a larger portion of the injected carriers populating the confinement layers where the carriers recombine more slowly owing to dilute carrier volume density     

Energy characteristics of excitons in structures based on InGaN alloys

Samples containing ultrathin InGaN layers that emit radiation in the spectral range from the ultraviolet to yellow region are studied. The samples are grown by metal-organic vapor-phase epitaxy. The Urbach energy, the localization energy of excitons, and the activation energy of charge carriers are determined to characterize radiative and nonradiative processes in the quantum dots and barriers of the structures. It is shown that these energy parameters are linearly dependent on the photon energy in the range from 3.05 to 2.12 eV. It is established that temperature variations in the emission intensity are due to the increase in the number of charge carriers thermally activated from the quantum wells into barriers as well as due to the enhancement of scattering of free excitons at defects.     

The exciton photoluminescence and vertical transport of photoinduced carriers in CdSe/CdMgSe superlattices

The photoluminescence and photoluminescence excitation spectra, phonon-related Raman scattering, and vertical transport of photoinduced carriers and excitons along the growth direction in type I lowstrained CdSe/CdMgSe superlattices, which are grown on InAs substrates using molecular-beam epitaxy, are studied for the first time. The studies are carried out at various temperatures and excitation intensities. The vertical transport is studied by a purely optical method involving an enlarged quantum well built in into the superlattice. This well serves as a sink for the excitons and charge carriers tunneled through the superlattice. At 2–150 K, the carriers are preferentially transported by free excitons. However, in superlattices with periods of 5.9 and 7.3 nm, this transport is not of the Bloch type. A comparison of the calculated energies of the band-to-band transitions in the superlattices with the experimental data yields the relative magnitude of the valence-band offset in the range 0.4–0.5. The Raman spectra indicate that the behavior of optical phonons in CdMgSe is bimodal.     

Optical Properties of AlGaAs/GaAs Resonant Bragg Structure at the Second Quantum State

Photoluminescence, optical reflectance and electro-reflectance spectroscopies were employed to study an AlGaAs/GaAs multiple-quantum-well based resonant Bragg structure, which was designed to match optical Bragg resonance with the exciton-polariton resonance at the second quantum state in the GaAs quantum wells. The structure with 60 periods of AlGaAs/GaAs quantum wells was grown on a semi-insulating substrate by molecular beam epitaxy. Broad and enhanced optical and electro-reflectance features were observed when the Bragg resonance was tuned to the second quantum state of the GaAs quantum well excitons manifesting an enhancement of the light-matter interaction under double-resonance conditions. By applying an alternating electric field, we revealed electro-reflectance features related to the x(e2-hh2) and x(e2-hh1) excitons. The excitonic transition x(e2-hh1), which is prohibited at zero electric field, was allowed by a DC bias due to brake of symmetry and increased overlap of the electron and hole wave functions caused by electric field.     

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。     

Effect of the electric field in the space-charge layer on the efficiency of short-wave photoelectric conversion in GaAs Schottky diodes

A study is reported of the quantum efficiency of the short-wave photoelectric effect as function of the reverse bias applied to GaAs Schottky diodes when the light absorption length is much shorter than the width of the space charge region. The quantum efficiency of photoelectric conversion is found to depend strongly on the contact electric field and the photon energy. The field independence of the quantum efficiency is interpreted in terms of a fluctuational trap model. The model can also be used to determine the loss factor for hot photocarriers, which is found to increase in a stepwise manner with increasing photon energy. This effect is explained in terms of the formation of excitons in X-and L-valleys of the semiconductor. Fiz. Tekh. Poluprovodn. 31, 1225–1229 (October 1997)     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

Light-emitting tunneling nanostructures based on quantum dots in a Si and GaAs matrix

InGaAs/GaAs and Ge/Si light-emitting heterostructures with active regions consisting of a system of different-size nanoobjects, i.e., quantum dot layers, quantum wells, and a tunneling barrier are studied. The exchange of carriers preceding their radiative recombination is considered in the context of the tunneling interaction of nanoobjects. For the quantum well-InGaAs quantum dot layer system, an exciton tunneling mechanism is established. In such structures with a barrier thinner than 6 nm, anomalously fast carrier (exciton) transfer from the quantum well is observed. The role of the above-barrier resonance of states, which provides “instantaneous” injection into quantum dots, is considered. In Ge/Si structures, Ge quantum dots with heights comparable to the Ge/Si interface broadening are fabricated. The strong luminescence at a wavelength of 1.55 μm in such structures is explained not only by the high island-array density. The model is based on (i) an increase in the exciton oscillator strength due to the tunnel penetration of electrons into the quantum dot core at low temperatures (T < 60 K) and (ii) a redistribution of electronic states in the Δ₂-Δ₄ subbands as the temperature is increased to room temperature. Light-emitting diodes are fabricated based on both types of studied structures. Configuration versions of the active region are tested. It is shown that selective pumping of the injector and the tunnel transfer of “cold” carriers (excitons) are more efficient than their direct trapping by the nanoemitter.     

GaInNAs/GaAs量子阱的光致发光谱和光调制反射谱

研究了GaInNAs/GaAs多量子阱在不同温度和激发功率下的光致发光(PL)谱以及光调制反射(PR)谱.发现PL谱主发光峰的能量位置随温度的变化不满足Varshni关系,而是呈现出反常的S型温度依赖关系.进一步测量,特别是在较低的激发光功率密度下,发现有两个不同来源的发光峰,它们分别对应于氮引起的杂质束缚态和带间的激子复合发光.随温度变化,这两个发光峰相对强度发生变化,造成主峰(最强的峰)的位置发生切换,从而导致表观上的S型温度依赖关系.采用一个基于载流子热激发和出空过程的模型来解释氮杂质团簇引起的束缚态发光峰的温度依赖关系.     

势阱形状对电场下量子阱子带和激子能移的影响

在束缚态近似下,用级数解法求解电场下各种不同形状GaAs/GaAlAs量子阱中电子和空穴子带,进一步采用变分方法得到激子结合能.由此,我们首次得到电场下由抛物阱至方位阱子带和激子峰能移的变化图象.在考虑GaAs/GaAlAs量子阱形状影响的基础上,我们计算所得结果与实验吻合得很好.     

“Exciton” photoconductivity in GaAs crystals

The spectral dependence of the photoconductivity in epitaxial GaAs of high purity and crystalline perfection grown on a semiinsulating GaAs substrate are measured at 1.7 and 77 K and analyzed theoretically. The developed theory of photoconductivity spectra takes into consideration the analytical shape of the exciton absorption edge in semiconductor crystals, the transport of photogenerated charge carriers, the finite thickness of the crystal, and the presence of surface recombination centers. It is shown that, when the excitation photon energy is lower than the band gap, the photoconductivity spectrum is determined by elementary excitations of the exciton type, while at higher excitation energies the spectrum is determined by surface states as well as by processes associated with optical-phonon emission. It is found that, in the samples under study, the probability of the direct formation of excitons upon the absorption of light can be comparable to the probability of their formation from a thermalized electron-hole gas.     

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

Extremely strong and sharp photoluminescence lines from nitrogen atomic-layer-doped GaAs,AlGaAs and AlGaAs/GaAs single quantum wells

We have performed nitrogen atomic-layer doping into GaAs, AlGaAs, and AlGaAs/GaAs single quantum wells using atomic nitrogen cracked by a hot tungsten filament. While the atomic-layer-doped GaAs layers show a series of sharp and strong photoluminescence lines relating to excitons bound to nitrogen atoms at 8K, atomic-layer-doped AlGaAs layers show several broad nitrogen-related lines. For the atomic-layer-doped single quantum well at the center of the GaAs layer, the quantum well luminescence itself disappears and a dominant and sharp luminescence is observed at a longer wavelength. It is found that the As pressure during the atomic-layer doping greatly affects the luminescence characteristics.     

Effects of impact ionization on I-Vcharacteristics of GaAs n-i-n structures including hole trap

Numerical simulations of GaAs n-i-n structures with Cr deep acceptors (hole trap) in the i-layer are performed by considering the impact ionization of carriers. At low voltages, I-V curves show sublinear or saturated features, because the voltage is entirely applied along the reverse-biased n-i junction. When the deep-acceptor density is low, a steep rise of current occurs due to trap filling, whereas when the deep-acceptor density becomes high, the steep current rise occurs due to impact ionization of carriers at the reverse-biased n-i junction. In this case, the voltage for current rise becomes lower as the acceptor density becomes higher     

Possibility of coherent light emission from Bose condensed statesin quantum-well systems

The possibility of coherent light emission in systems with a pairing of electrons and holes in a double quantum well spatially separated by a thin barrier [separated electron hole pairs (SEHPs)] is predicted. Bose condensation of excitons formed from SEHPs in GaAs/Al _xGa_(1-x)As double quantum wells of ~10 Å size are possible at relatively high temperatures. Coherent recombination emission due to the transfer of coherence from the Bose-condensed SEHPs to the photon field is expected     

Negative charged excitons in double barrier diodes

We have studied the effects of excitonic complexes formation, such as excitons and trions, on the optical and on transport properties of GaAs-GaAlAs n-i-n double barrier diodes, by measuring the current-voltage characteristics and the photoluminescence emission, as function of bias. The observation of a pre-resonance shoulder in the I(V) curves, under high laser intensities, and a negative charged excitons in the photoluminescence spectra, under the same bias conditions, were associated to the dissociation of these complexes either by thermal excitation or by scattering with ‘free’ carriers in the quantum well layer. A simple rate equation model allows us to explain the kinetics of the excitonic complexes in double barrier devices.     

GaNAs/GaAs量子阱的静压光致发光研究

在 1 5 K和 0～ 9GPa静压范围下测量了 Ga N0 .0 1 5As0 .985/ Ga As量子阱的光致发光谱。观察到了 Ga NAs阱和 Ga As垒的发光 ,发现 Ga NAs阱发光峰随压力的变化比 Ga As垒发光峰要小很多。当压力超过 2 .5 GPa后还观察到了与 Ga As中的 N等电子陷阱有关的一组新发光峰。用二能级模型及测得的 Ga As带边和 N等电子能级的压力行为计算了 Ga NAs发光峰随压力的变化 ,但计算结果与实验结果相差甚大 ,表明二能级模型并不完全适用。对观察到的 Ga NAs发光峰的强度和半宽随压力的变化也进行了简短讨论。