Optical characteristics of quantum-well heterostructures in saturation conditions

The resalts of theoretical investugation for spectral characterstics of gain, refraction index of QW heterostructures depending on quantum layer thickness the value of saturation field, as well as their field dependence are presented.     

Theoretical optimization of the characteristics of ZnO metal-semiconductor-metal photodetectors

A two-dimensional model of a metal-semiconductor-metal(MSM) ZnO-based photodetector(PD) is developed.The PD is based on a drift diffusion model of a semiconductor that allows the calculation of potential distribution inside the structure,the transversal and longitudinal distributions of the electric field,and the distribution of carrier concentration.The ohmicity of the contact has been confirmed.The dark current of MSM PD based ZnO for different structural dimensions are likewise calculated.The calculations are comparable with the experimental results.Therefore,the influence with respect to parameters s(finger spacing) and w(finger width) is studied,which results in the optimization of these parameters.The best optimization found to concur with the experimental results is s = 16 μm,w = 16 μm,l = 250 μm,L = 350 μm,where l is the finger length and L is the length of the structure.This optimization provides a simulated dark current equal to 24.5 nA at the polarization of 3 V.     

Overcoming absorption saturation with doping in p-type quantum well infrared photodetectors: modeling and experiment

Bound-to-continuum normal-incidence absorption in p-type GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) is strongest when the second light-hole (LH2) level is resonant with the top of the valence band QW. However, we found that such absorption saturates as a function of doping in the well. Using the envelope-function model (EFA), this paper shows that moving the LH2 resonance slightly deeper into the continuum avoids absorption saturation and produces optimal p-QWIP response. A suitable set of mid-IR samples was grown to test this conjecture and their photoresponse measured. The results indicate that absorption can be more than doubled through the use of the new p-QWIP designs. This result is explained by showing that the line of resonances in the continuum as a function of the in-plane wave vector eventually becomes a bound LH2 band in the well at some critical wave vector. Therefore, it is possible to avoid absorption saturation by matching this critical wave vector (i.e., well width and/or well depth) with the Fermi wave vector (i.e., doping in the well) for the desired QWIP (i.e., cutoff wavelength).     

Optical saturation in the photothermal spectroscopy of fluorescent materials

The dependence of the fundamental and harmonic photothermal (PT) signal on the intensity I ₀ of the illumination source is analyzed. It is shown that both components of the PT signal do not increase indefinitely with I ₀, but at sufficiently high power densities begin to decrease as 1/I ₀. Along with photoacoustic saturation, this defines an upper limit for the sensitivity of spectrometers based on PT detection.     

高浓度气体共振光声光谱信号饱和特性研究

采用恒流驱动耦合机械斩波技术在激光光声光谱装置上系统测量了5%—100%宽浓度范围甲烷气体的共振光声信号,发现在高浓度区共振光声信号呈现异常的饱和特征.基于气体吸收和光声光谱原理定量分析了光声信号饱和的主要原因及影响因素,研究表明,气体样本对入射光强吸收而导致的声源与本征共振模式的耦合系数改变是异常饱和的主要原因,并导出判定光声信号饱和深度的准则以用于判定高浓度气体饱和深度。     

Current-voltage characteristics simulation and analysis of 4H-SiC metal-semiconductor-metal ultraviolet photodetectors

The current-voltage (I-V) characteristics of 4H-SiC metal-semiconductor-metal (MSM) ultraviolet pho-todetector with different finger widths and spacings, different carrier concentrations and thicknesses of n-type epitaxial layer are simulated. The simulation results indicate that the dark current and the pho-tocurrent both increase when the finger width increases. But the effect of finger width on the dark current is more significant. On the other hand, the effect of finger spacing on the photocurrent is more significant. When the finger spacing increases, the photocurrent decreases and the dark current is almost changeless. In addition, it is found that the smaller the carrier concentration of n-type epitaxial layer is, the smaller the dark current and the larger the photocurrent wiU be. It is also found that I-V characteristics of MSM detector also depend on the epitaxial layer thickness. The dark current of detector is smaller and the photocurrent is larger when the epitaxial layer thickness is about 3 μm.     

Photoelectric characteristics of photodetectors based on a GaAs/CuPc heterojunction

A study is made of the volt-ampere, volt-farad, and spectral characteristics of photodetectors based on an n-GaAs/p-CuPc heterojunction. When exposed to white light (E=600 W/m²) through Ag, the elements Cu/GaAs/CuPc/Ag have the following characteristics: U_xx=0.6 V; I_sc=30.2 A/m², FF=0.49. Based on this data and with allowance for the transmission factor (τ=7%), η=18%. Quantum efficiency is 0.5 el./phot. in the UV-region at λ₁ = 300 nm, while α₂ = 0.85 el./phot. in the visible region at λ₂ = 800 nm. The limiting sensitivity of the photodetector is 10⁻¹¹ W. The following was determined from the volt-farad and spectral characteristics: C_b=8.6·10⁻⁴ F/m²; barrier width w₁=16nm in CuPc and w₂=34 nm in CaAs; exciton diffusion length L₁=2.5 nm and L₂=7.5 nm, respectively. The photodetectors undergo almost no degradation. The values of U_xx and I_sc remain constant with multiple exposures to UV-light of 140 W/m² intensity over 6 h at constant temperature. The sensitivity region of the photodetectors is from 200 to 1000 nm. It is found that an anisotropic heterojunction in the form of two series-connected Schottky barriers is created at the boundary between n-GaAs and oxygen-doped p-CuPc in the presence of charged surface states at the interface. Vologda Polytechnic Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 41–45, July, 1996.     

Investigation of time characteristics of photodetectors based on Ge/Si nanoheterostructures

Results of investigations into the time characteristics of photosensitive layers based on Ge/Si nanoheterostructures excited by femtosecond laser pulses with a wavelength of 1.55 μm are given. It is demonstrated that the leading front duration of the photoresponse pulse for the examined specimens excited by laser pulses of 120 fs duration does not exceed 30–40 пs.     

Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.     

Optical characteristics of implanted silicon films

The first data are presented on the change (following implantation) in the refractive index and the band structure of silicon on sapphire films. The implantation was effected with phosphor ions of 40 keV and doses from 10¹² to 10¹⁶ cm^–2. An increase following implantation of the refractive index and the energy of the first direct allowed transitions E₁ is noted, indicating changes in the second coordination sphere. The profile E₁(x) is studied pointing to heterogenization effects. The films were annealed with ruby laser pulses of 0.2 J/cm². The same laser was used to study the lux dependence of the injection level _n and surface photo-emf V. Hysteresis in the V(_n) dependence (after the use of maximum intensity of the laser beam) is noted indicating irreversible straightening of the bands at the film surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–43, May, 1984.     

Optical characteristics of silver-doped polarizing glass

According to the theory of scattering and absorption of light, an anisotropic Ag colloid particle, which is small compared with the wavelength of the incident light, absorbs light differently in different directions. The underlying mechanism is regarded as electron plasma resonance. The optical anisotropy of a single Ag particle or of a group of the same particles has been analyzed. An equation for estimating the distribution of the particle shapes in space is proposed. Based on this equation and the work of other researchers, a program was designed to calculate the major principal transmittance and the minor principal transmittance of Ag-doped polarizing glass. The results show that the polarizing property of glass with particles with different aspect ratios is better than that of glass with identical particles. Also, the effective wavelength range is different if the particles' aspect ratios change, and this range tends to narrow as the parameter R (the rate of change of the aspect ratio) is increased.     

Modeling of electrical and optical characteristics of near room-temperature CdS/ZnSe based NIR photodetectors

Results of modeled photodetector characteristics in (CdS/ZnSe)/BeTe multi-well diode with p–i–n polarity are reported. The dark current density (J–V) characteristics, the temperature dependence of zero-bias resistance area product (R₀A), the dynamic resistance as well as bias dependent dynamic resistance (R_d) and have been analyzed to investigate the mechanisms limiting the electrical performance of the modeled photodetectors. The quantum efficiency, the responsivity and the detectivity have been also studied as function of the operating wavelength. The suitability of the modeled photodetector is demonstrated by its feasibility of achieving good device performance near room temperature operating at 1.55 μm wavelength required for photodetection in optical communication. Quantum efficiency of ∼95%, responsivity ∼0.6 A/W and D^* ∼ 5.7 × 10¹⁰ cm Hz^1/2/W have been achieved at 300 K in X BeTe conduction band minimum.     

Optical characteristics of rounded silver nanoprisms

We analyzed numerical optical characteristics of silver nanoprisms with rounded corners using the three-dimensional finite-difference time-domain method. The enhancement of the electric field was decreased from 240 to 13 times by introducing a large radius of curvature at the nanoprism corners such that it became a cylinder. This caused the optical multi-mode to change to single dipole mode. In the largest local electric field enhancement using the bowtie structure, which consisted of a pair of nanoprisms with rounded corners (the curvature radius and the gap distance were 8.66 and 1 nm, respectively), the electric field was enhanced by a factor of 360 at the hotspot. The bowtie structure that has non-zero curvature radii produces a larger electric field enhancement than does the single nanoprism without a curvature radius. Furthermore, the numerical simulation elucidates that the change of the curvature radius and the change of the gap distance have the same influence on the electric field enhancement.     

Optical characteristics of porous silicon structures

Morphology, composition, and optical properties of porous silicon on single-crystal-silicon substrates and p-n junctions are studied. Substrate orientation, type of conduction, and composition of etching agent are varied to obtain nano-, meso-, and microporous silicon and multilayer porous structures. A correlation of the photoluminescence intensity and intensity of the IR absorption band peaking at 616 cm^?1 is related to the presence of Si-Si bonds.     

Optimum load and saturation characteristics of an infrared helium-neon laser

An experimental study is reported for optimum mirror transmission in a helium-neon laser for a wavelength of 1.15 and generation in several longitudinal modes. The results agree closely with calculations if homogeneous saturation of the line is assumed. This is also confirmed by radiated power for distances of 0.5, 1.0, and 1.9 m between the mirrors.In conclusion the author expresses thanks to V. V. Reshetnikov and A. K. Maslov for help in the experiments.