Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

Donor-acceptor recombination in short-period silicon-doped GaAs/AlAs superlattices

A low-frequency band is observed along with an exciton band in photoluminescence spectra of short-period GaAs/AlAs superlattices doped with Si in the barriers or in the barriers and wells. This band is ascribed to donor-acceptor recombination on the basis of the dependence of its frequency on the excitation intensity under cw excitation and on the time delay under pulsed excitation. Mainly type-II superlattices are investigated. The estimate E _A+E _D≈120 meV can be obtained from the peak energy of the donor-acceptor band with a very weak excitation intensity. The estimates E _A≈23 meV and E _D≈90 meV are obtained from the temperature dependence of the band intensity. It is suggested that the deep donor level is associated with a DX center in the AlAs layers. Fiz. Tekh. Poluprovodn. 32, 839–842 (July 1998)     

MBE growth and characterization of high purity GaAs/AIGaAs on the (110) surface of GaAs

An improved quality of (110) GaAs has been grown by molecular beam epitaxy using As₂ in lieu of As₄. The most pronounced effect of using As₂ is a higher doping efficiency of Si δ-doped GaAs layers, resulting in a mobility of the (110) layers, comparable to the reference (100) samples.

The high quality of the (110) GaAs was confirmed by low temperature photoluminescence. The spectrum of the GaAs layer shows a single dominant free exciton line with a linewidth of 1.0 meV.     

非掺杂InP的低温光致发光研究

研究了2K低温下非有意掺杂InP单晶的光致发光谱,对近带边的辐射跃迁进行了仔细分析,报道了InP单晶在大功率光激发时束缚于中性施主的激子跃迁发光相对减弱、而束缚于中性受主的激子跃迁发光相对增强的现象,并探讨了其机制,确认了材料中存在Mg、Zn等残余受主杂质,并计算得到Mg受主的离化能为41.5meV.     

1.5 μm wavelength distributed reflector lasers with verticalgrating

A 1.5 μm wavelength distributed reflector laser, consisting of a distributed Bragg reflector rear facet and a distributed feedback region, was realised using deep-etching technology. A low threshold current of I_th=12.4 mA and a high differential quantum efficiency of η_d=42% from the front facet was achieved with a submode suppression ratio of 33 dB (I=2.4 I_th) for a fifth-order grating, 220 μm long and 6 μm wide device at room temperature     

1.3-μm AlGaInAs buried-heterostructure lasers

1.3-μm AlGaInAs-InP strained multiple-quantumwell (MQW) buried-heterostructure (BH) lasers have been successfully fabricated. InP current blocking layers could be smoothly regrown using the simple HF pretreatment, although the etched active region includes Al-containing layers. The threshold current I_th was typically 11 mA for as-cleaved 350-μm-long devices, which is about 30% lower than that of the ridge laser counterparts. A maximum continuous-wave operating temperature as high as 155°C was achieved. For the 200-μm-long device with the high-reflective-coated rear-facet, I_th was as low as 7.5 mA and characteristic temperature T₀ was 80 K. The BH lasers also provided more circular far-field patterns and lower thermal resistances than for ridge lasers     

Analysis of the relationship between defect site generation anddielectric breakdown utilizing A-mode stress induced leakagecurrent

The dielectric breakdown mechanism is studied from the viewpoint of the relationship with the generation of defect sites in the oxide film, utilizing the “A-mode” stress induced leakage current (A-mode SILC) under the constant-voltage stressing. It is demonstrated that the breakdown occurs when the A-mode SILC becomes a threshold level, I_th. In spite of that, the constant I_th for various stress fields is expected by the conventional model which assumes that each defect site is generated randomly in the oxide film, I _th, increases with the stress field. To explain this variety of I_th by the stress field, the concept of “breakdown-path creation efficiency” (γ_BPC), is proposed, which represents the amount of defect sites in the whole gate area required to create a breakdown path from one side of oxide film to the other side at a local spot. According to this concept, it is demonstrated that the efficiency becomes smaller with the increase in the stress field. These results require us to take account the nonuniform distribution of defect sites in the oxide film into the model for the breakdown mechanism. The introduction of the stress-field dependent depth profile of defect sites allows to explain the variety of I_th     

应力层多量子阱InGaAs/GaAs的光电流谱研究(英文)

The band configurations of the undoped strained-layer InxGa1-xAs(8 or 15nm)-GaAs(15nm) MQW with x = 0.1,0.15 and 0.2, respectively, have been investigated by photocurrent spectroscopy at the temperature range of 10- 300K. Both intersubband transitions and transitions between confined level and continuum are observed. The photocurrent peak related to the 2s or other excited states of heavy-hole exciton is also observed and the exci-tonic binding energy thus obtained is about 8meV. The valence band offset △Ev is determined to be 0.38 and 0.40 by means of two different methods. Owing to the strain effect, both electrons and heavy holes are confined in the InGaAs layers while light holes in the GaAs layers.     

High-efficiency and high-power vertical-cavity surface-emittinglaser designed for cryogenic applications

We report the first AlGaAs-GaAs vertical-cavity surface-emitting laser (VCSEL) that has been optimized for cryogenic applications near 77 K, with superior characteristics that include a high-output power (P_out=22 mW at I=25 mA), high power conversion efficiency (ηd=32%), low threshold voltage (V_th=1.75 V) and current (I_th=1.7 mA), and low power dissipation (9 mW at P_out =2.0 mW) for a 20-μm-diameter device     

Study on the dominant mechanisms for the temperature sensitivity ofthreshold current in 1.3-μm InP-based strained-layer quantum-welllasers

We study the basic physical mechanisms determining the temperature dependence of the threshold current (I_th) of InP-based strained-layer (SL) quantum-well (QW) lasers emitting at a wavelength of 1.3 μm. We show that I_th exhibits a different temperature dependence above and below a critical temperature T_c. It is indicated that T_c is the maximum temperature below which the threshold gain exhibits a linear relationship with temperature. We demonstrate that below T_c the Auger recombination current dominates the temperature dependence of I_th. On the other hand, above T_c a significant increase in both the internal loss and radiative recombination current in the separate-confinement-heterostructure region, which is mainly due to electrostatic band-profile deformation, is found to play a major role in determining the temperature sensitivity of I_th. On the basis of the comparison between the theoretical analysis and the experimental results, we conclude that the temperature dependence of the threshold current in 1.3-μm InP-based SL-QW lasers is dominated by different mechanisms above and below T_c     

Antiguided array diode lasers fabricated with modulated cap thinp-clad structure

Antiguided array lasers were fabricated in thin p-clad, InGaAs-GaAs single quantum-well material. The required lateral refractive index variation was achieved by precisely modulating the thickness of the GaAs cap layer using a novel pulsed anodization/etching technique. Edge-emitting arrays having 20 lasers on 7-μm centers with 5-μm-wide gain regions were characterized. At 1.2 times the pulsed current threshold (I_th), the central lobe of the lateral far field of the arrays contained about 75% of the beam power and was about twice the diffraction limit (FWHM=0.8°). At 10×I_th, the central lobe contained about 60% of the beam power and was about 1.6° wide     

Characteristics of laser diodes with a partially intermixedGaAs-AlGaAs quantum well

Laser diodes (LD's) with a partially intermixed quantum-well (QW) active layer are fabricated by Zn out-diffusion from a p-cladding layer to the QW region. The dependencies of the degree of intermixing, measured by the photoluminescence (PL) shift, on Zn concentration of the p-cladding layer (P_clad) and the Al content of the guiding layer (X_g) in a separate-confinement-heterostructure (SCH) are investigated. P_clad changes in the range from 1×10 ¹⁸ cm^-3 to 4×10¹⁸ cm^-3 and X_g changes in the range from 0.21-0.37. When P_clad is 2×10¹⁸ cm^-3 and X_g is 0.37, large bandgap energy shift of 96.1 meV is observed. The lasing wavelengths of the LD's, with the partially intermixed QW, are blue-shifted linearly with increasing P_clad and X_g. For the bandgap energy shift of 66.8 meV by PL, the threshold current density is increased by 33% from that of the nonintermixed LD. Reliability of LD's with the partially intermixed QW is investigated for the first time. In spite of a large degree of intermixing the reliability of the LD with the partially intermixed QW of 66.8 meV energy shift by PL is the same as the nonintermixed one, which is confirmed by the aging test of 2500 hours at 45°C with the output power of 1 W under CW operation     

Synthesis and photoluminescence properties of the 4H-SiC/SiO2 nanowires

4H-SiC/SiO2 nanowires are synthesized and the temperature-dependent photoluminescence （PL） properties of the nanowires are studied. Their structure and chemical composition are studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and Raman spectra. At room temperature, an ul- traviolet PL peak and a green PL band are observed. From the PL spectrum measured in the temperature range from 80 K to 300 K, the free excition emission, donor bound excition emission and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation en- ergy values extracted from the temperature dependence of the free exciton and bound exciton peak integral intensity are about 40 meV and 181 meV, respectively.     

Evidence for large monomolecular recombination contribution tothreshold current in 1.3 μm GaInNAs semiconductor lasers

The spontaneous emission, L, through a window in the substrate electrode of 1.3 μm GaInNAs MQW lasers was studied as a function of current, I, and temperature, T Close to room temperature, a characteristic temperature at threshold T,(L) T was observed as expected for band-to-band recombination in ideal quantum well devices. However, T ₀(I_th)≃T/3 indicating other processes occur. Analysis of the variation of L with I, reveals that monomolecular recombination contributes more than 50% to the total current at threshold and also that some Auger recombination may be present     

掺铒硅光致发光激子传递能量机制

铒离子在硅中呈现弱施主特性 ,O、Er双掺杂可提高施主浓度两个数量级 .氧杂质与铒离子形成复合体 ,其施主能级可能是铒离子发光能量转换的重要通道 .提出了掺铒硅光致发光激子传递能量模型 ,建立了发光动力学速率方程 ,并进行了详细推导 .发光效率与光激活铒离子浓度、激发态寿命及自发辐射寿命等因素有关 .指出铒离子 -束缚激子复合体的热离化和激发态铒离子能量反向传递是引起铒离子发光温度猝灭的主要原因 .拟合 PL测量实验结果表明 :它们对应的激活能分别为 6.6me V和 47.4me V.     

Width of the excitonic absorption line in Al x Ga1 − x as alloys

Experimental data on the width of the absorption line corresponding to the exciton ground state in high-quality samples of Al _x Ga_{1 ? x} As (x = 0.15) quasi-binary alloy, obtained by Seisyan et al. (2005), are analyzed. The line corresponding to the 1s state is divided into separate components taking into account strain-induced splitting; Lorentzian- and Gaussian-shape contributions to the absorption curve; and overlap with the continuum absorption band, which broadens with increasing temperature. Analysis of the integrated absorption, relying on the fact that the temperature dependence of the absorption is typical of exciton polaritons in media possessing spatial dispersion, was carried out with the following parameters: critical value of the dissipation damping of the exciton Γ_c = 0.32 meV and maximum value of the absorption coefficient K _max = 89.5 eV/cm. This analysis makes it possible to determine the homogeneous component of the line broadening, which, up to the critical temperature T _c = 155 K does not exceed 0.2 meV. It is found that the “natural” width of the 1s-exciton line does not exceed 2.6 meV at T = 1.7 K, which is in agreement with theoretical estimates. At T = 1.7–60 K, the inhomogeneous broadening of the exciton peak related to the exciton scattering and localization by fluctuations of the alloy composition exceeds contributions related to phonons and ionized impurities by more than an order of magnitude and is dominant, but does not affect the integrated absorption by excitonic polaritons.     

Radiative recombination in the CdMgTe matrix with ultrathin narrow-gap CdMnTe layers

Exitonic emission from three structures consisting of CdMnTe layers separated by thick wide-gap CdMgTe layers are studied in the temperature range from 13 to 300 K. The thickness of the CdMnTe layers corresponds to the 0.5, 1.5, and 3.0 monolayers. To calculate the excitonic density-of-state spectra in these structures, a simple model that takes into account variations in the elemental composition of the alloy inside the exciton sphere under variations in its position with respect to the narrow-gap layer is used. It is shown that the energy extent of the emission and density-of-state spectra and the positions of the characteristic features of the spectra are in agreement. On low-intensity excitation (1–10 W/cm²), heating of the samples from 13 K yields a relative decrease in the emission intensity of excitions localized in the CdMgTe layers, so that this emission at 80 K is much lower in intensity than the emission of excitions bound at the narrow-gap layers. In the conditions of high-intensity excitation (10³–10⁵ W/cm²) at 80 K, the excitonic emission from the CdMgTe layers prevails; however, at room temperature, the emission from the narrow-gap layers dominates over the emission from the wide-gap layers. The intracenter luminescence of the Mn²⁺ ions in the sample with three monolayers CdMnTe presents a doublet structure, with the components corresponding to the internal and external layers of the three-layer planar inclusions containing manganese.     

High-temperature operation of InGaAs/InGaAsP compressive-strainedQW lasers with low threshold currents

We investigated the influence of the band gap wavelength of barrier layers and separate confinement heterostructure (SCH) layers λ_SCH on the high-temperature operation of InGaAs/InGaAsP compressive-strained quantum-well (QW) lasers. The optimum λ_SCH was 1.2 μm, at which carriers were sufficiently confined into quantum wells. The QW laser with λ_SCH = 1.2 μm exhibited low threshold currents of 2.3 mA at 20°C and 9.7 mA at 100°C and CW lasing up to 150°C     

On the problem of lasing in traps for the bose condensation of dipolar excitons

Polariton-mode lasing in traps for the Bose condensation of dipolar excitons is considered and conditions under which this type of lasing takes place are examined. The width of the spectrum of the polariton modes involved in lasing and the role of the spatial and spectral inhomogeneity of the exciton distribution are discussed. The possibility of lasing in a system close to the exciton Bose condensation threshold is investigated in detail. The effect of inhomogeneous broadening of the exciton spectral line on the stability of steady-state lasing is analyzed. Experiments that help to reveal contributions from different physical processes to polariton-mode lasing in semiconductor structures designed for the Bose condensation of excitons are proposed.     

Investigations on indium phosphide grown by chemical beam epitaxy

In this paper, we present a systematic study of the properties of indium phosphide (InP) layers grown by chemical beam epitaxy (CBE). Trimethylindium (TMIn) and phosphine (PH₃) are used as source materials. The relation between the phosphine cracker temperature and the cracking efficiency has been studied by mass spectroscopy during growth. The growth rate and morphology of the layers have been studied by varying the TMIn and phosphine flow rates as well as the substrate temperature. We have found that, under a wide range of growth conditions, the deposition rate is only determined by and proportional to the TMIn flow rate. This is in agreement with literature. Additionally, we observe that the growth rate decreases below a certain phosphine to TMIn flow rate (V/III) ratio and becomes phosphine flow limited. From investigations of the growth rate as a function of temperature, it is concluded that the desorption of indium species from InP starts at a temperature slightly below 540°C. For this desorption process, we have found an activation energy of (217 ± 20) kJ/mol. Further characterization of the InP layers has been carried out by photoluminescence and Hall measurements. From both methods, the optimum growth conditions have been established. Under these conditions, we reproduc-ibly obtain InP layers showing linewidths of the donor-bound exciton transition at 5K around 0.25 meV and a mobility at 77K of about 7.0·10⁴ cm²/Vs. From the analysis of the mobility in the temperature range from 20 to 300K, we conclude that, additionally to shallow donors and acceptors, deep-donor centers with an activation energy of about 150 meV are present in all layers.