Optical characterization of radiative semiconductors

This paper describes a direct photoluminescent method of characterizing high quantum efficiency semiconductors in terms of an “effective absorption coefficient” which is a function of the absorption coefficient and of the diffusion length of excess minority carriers injected optically.The results obtained by this method indicate that the diffusion lengths in unintentionally doped (pure) and n and p-type GaAs can be in excess of 0.03 cm at room temperature, and that surface and bulk radiation processes in these materials can be separated.     

Effects of NEA GaN photocathode performance parameters on quantum efficiency

Using research on the negative electron affinity GaN photocathode photoemission mechanism, we obtained the reflective-type and transmission-type GaN photocathode quantum efficiency formulas. The influence on quantum efficiency and sensitivity of integral of cathode performance parameters such as electron surface escape probability P, electron diffusion length L_D, absorption coefficient α, back-interface recombination rate S_v and cathode thickness T_e, were analyzed using these formulas. It was found that to obtain negative electron affinity GaN optoelectronic cathodes with high quantum efficiencies, we must constantly improve cathode activation technologies and the surface escaping probability of cathode. Also, we must increase the electronic diffusion length, reduce the rate of compounding, and find the optimal thickness of the cathode transmit layer based for the specific electronic diffusion length.     

Simulation and design consideration of photoresponse for HgCdTe infrared photodiodes

We report on 2D numerical simulations of photoresponse characteristic for long-wavelength HgCdTe infrared photodiodes. Effects of thickness of absorption layer on the photoresponse have been investigated. Optimal thickness of absorption layers at different absorption lengths and diffusion lengths are extracted numerically. An empirical formula is proposed to predict reasonable optimal thickness of absorption layer by theoretically analyzing its correlations with absorption lengths and diffusion lengths.     

Self-adjusting formation of a lateral confinement potential in Si/SiGe heterostructures with compensatingpnlayers

We have developed a new concept to create a lateral confinement potential in a two-dimensional electron gas at the interface of a Si/SiGe heterostructure. The method is based on the compensation of then-type doping layer by an additionalp-type layer in a structure similar to a modulation-doped field-effect transistor. A numerical calculation shows that an initially depleted quantum well at zero temperature can be populated by reducing the top-layer thickness. A lateral quantum well is created below a graded step preferentially etched into thep-type layer. Beneath an etched V groove a lateral confinement is obtained with level separation in excess of 8 meV. Similar confinement is achieved in a δ-doped structure with an etch-stop controlled shallow groove.     

Effects of absorption layer characteristic on spectral photoresponse of mid-wavelength InSb photodiodes

We report on two dimensional numerical simulations of photoresponse characteristic for mid-wavelength InSb infrared photodiodes. Effects of thickness of absorption layer on the photoresponse have been investigated for both front-side and back-side illuminated devices. Optimal thickness of absorption layers for different diffusion lengths is extracted theoretically. Our work shows that the optimal thicknesses of absorption layers strongly depend on the minority carrier lifetimes. An empirical formula is proposed to predict reasonable optimal thicknesses of absorption layer by numerically analyzing its correlations with the diffusion lengths. It is also found that the positive interface fixed charge can reduce the spectral photoresponse due to the charge-induced p-n junction.     

Transport parameters in illuminated layers of semiinsulating GaAs

Measurements are reported on semiinsulating p-type Gallium Arsenide specimens illuminated with photons of energy greater than the energy gap.An excited layer is defined of thickness d given by the sum of the radiation penetration depth and the ambipolar diffusion length.Concentrations n and mobilities μ of the carriers in this layer are determined from galvanomagnetic effects in light and in darkness. The concentration n shows a slow decrease with increasing wavelength out to the absorption edge, where it falls abruptly; μ falls with rising photon energy. As a function of increasing intensity, the mobility remains constant, while n rises linearly with photon-excitation rate. The results are discussed in terms of the variations of d. Further an attempt has been made to find the dependence between electron and hole concentrations in the excited layer measuring the variations of short circuit photomagnetoelectric current (I_PME) as a function of excess conductance (ΔG).     

Quantum efficiency for degenerate p-type photoluminescence and electroluminescence in GaAs crystals2

The radiative lifetime and the internal quantum efficiency for the degenerate p-type photoluminescence and the electroluminescence in GaAs crystals have been investigated with a simplified model of degenerate semiconductors in which the recombination constant B is approximately proportional to the ?$\text{5}\text{8}$power of the hole concentration. It is suggested that the radiative lifetime reaches a minimum at some hole concentration, in good agreement with the prediction of Dumke. At 77 K, the internal quantum efficiency exhibits a maximum, found to be 100% at 5 × 10¹⁸ cm^?3 for p-type GaAs crystals, in perfect agreement with experiments of Cusano, and for p-n GaAs junction crystals η_int, _max = 60% at 3 × 10¹⁸cm^?3. Finally, it is noted that in degenerate p-type GaAs crystals, the internal quantum efficiency increases linearly with increasing temperatures.     

Organic p-i-n solar cells

We introduce a p-i-n-type heterojunction architecture for organic solar cells where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled co-evaporation using organic dopants and leads to conductivities of 10^-4 to 10^-5 S/cm in the p- and n-doped wide-gap layers, respectively. The photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C₆₀ and shows mainly amorphous morphology. As a first step towards p-i-n structures, we show the advantage of using wide-gap layers in M-i-p-type diodes (metal layer–intrinsic layer–p-doped layer). The solar cells exhibit a maximum external quantum efficiency of 40% between 630-nm and 700-nm wavelength. With the help of an optical multilayer model, we optimize the optical properties of the solar cells by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. For an optically optimized device, we find an internal quantum efficiency of around 82% under short-circuit conditions. Adding a layer of 10-nm thickness of the red material N,N-dimethylperylene-3,4:9,10-dicarboximide (Me-PTCDI) to the active region, a power-conversion efficiency of 1.9% for a single cell is obtained. Such optically thin cells with high internal quantum efficiency are an important step towards high-efficiency tandem cells. First tandem cells which are not yet optimized already show 2.4% power-conversion efficiency under simulated AM 1.5 illumination of 125 mW/cm² . PACS 73.61.Ph; 78.30.Jw; 89.30.Cc     

Electroluminescence of CdSe/CdS quantum dots and the transfer of the exciton excitation energy in an organic light-emitting diode

A light emitting diode has been developed on the basis of multilayer nanostructures in which CdSe/CdS semiconductor colloidal quantum dots serve as emitters. Their absorption, photo-, and electroluminescence spectra have been obtained. The strong influence of the size effect and the density of particles in the layer on the spectral and electrophysical characteristics of the diode has been demonstrated. It has been shown that the rates of the transfer of the exciton excitation energy from organic molecules to quantum dots increase strongly even at a small increase in the radius of the core (CdSe) of a particle and depend strongly on the thickness of the shell (CdS) of the particle. The optimal arrangement of the layer of quantum dots with respect to the p-n junction has been estimated from the experimental data. The results demonstrate that the spectral characteristics and rates of the electron processes in light-emitting devices based on quantum dots incorporated into an organic matrix can be efficiently controlled.     

Quantum efficiency and responsivity of InSb photodiodes utilizing the Moss-Burstein effect

In this work a detailed analysis of the quantum efficiency of InSb n⁺-p photodetectors produced by liquid phase epitaxy is given, in the case when the n⁺ region is doped to such a level that the Moss-Burstein effect plays an important role. Our starting point was the theoretically determined coefficient of intrinsic absorption and derived expressions for the generated photocurrent in the n⁺ region, depletion layer and p-phase of the photodetector. The results are presented in the form of graphical dependence of the quantum efficiency on the wavelength, with the electron concentration in the n⁺ layer as a parameter.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Formation of p-type ZnMgO thin films by In-N codoping method

In-N codoped ZnMgO films have been prepared on glass substrates by direct current reactive magnetron sputtering. The p-type conduction could be obtained in ZnMgO films by adjusting the N₂O partial pressures. The lowest resistivity was found to be 4.6 Ω cm for the p-type ZnMgO film deposited under an optimized N₂O partial pressure of 2.3 mTorr, with a Hall mobility of 1.4 cm²/V s and a hole concentration of 9.6 × 10¹⁷ cm⁻³ at room temperature. The films were of good crystal quality with a high c-axis orientation of wurtzite ZnO structure. The presence of In-N bonds was identified by X-ray photoelectron spectroscopy, which may enhance the nitrogen incorporation and respond for the realization of good p-type behavior in In-N codoped ZnMgO films. Furthermore, the ZnMgO-based p-n homojunction was fabricated by deposition of an In-doped n-type ZnMgO layer on an In-N codoped p-type ZnMgO layer. The p-n homostructural diode exhibits electrical rectification behavior of a typical p-n junction.     

以弱p型为有源区的新型p-n结构GaN紫外探测器

提出了以弱p型（p^－-GaN）为有源区的p-n结构GaN紫外探测器.由于弱p型层的载流子浓度较低,很容易增加耗尽区的宽度,从而可以增加器件的量子效率.通过模拟计算,研究了金属与p^－-GaN层的肖特基接触势垒高度、p^－-GaN层厚度等参数对器件性能的影响.研究结果表明,降低金属与p^－-GaN层的接触势垒高度、适当减小p^－-GaN层厚度能够实现有源层方向单一的内建电场,从而提高器件的量子效率.要制备出 关键词： 弱p型GaN 紫外探测器 量子效率     

Theoretical research on optical properties and quantum efficiency of Ga1−xAlxN photocathodes

In order to design the optimal component structure of transmission-mode (t-mode) Ga_1−xAl_xN photocathode, the optical properties and quantum efficiency of Ga_1−xAl_xN photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga_1−xAl_xN photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga_1−xAl_xN photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga_1−xAl_xN layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga_1−xAl_xN photocathodes.     

Intersubband optical absorption of holes in quantum wells

The problem of hole energy spectrum and interlevel optical absorption in p -type quantum wells is considered theoretically. To obtain analytical results, terms in the Luttinger Hamiltonian containing in-plane momentum are treated as a perturbation. In this approximation energy spectrum, wave functions, hole statistics and interlevel optical matrix elements can be found for an arbitrary shape of quantum well. Finally, optical absorption spectra are calculated for different interlevel transitions.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Numerical analysis of In0.53Ga0.47As/InP single photon avalanche diodes

A rigorous theoretical model for In 0.53 Ga 0.47 As/InP single photon avalanche diode is utilized to investigate the dependences of single photon quantum efficiency and dark count probability on structure and operation condition.In the model,low field impact ionizations in charge and absorption layers are allowed,while avalanche breakdown can occur only in the multiplication layer.The origin of dark counts is discussed and the results indicate that the dominant mechanism that gives rise to dark counts depends on both device structure and operating condition.When the multiplication layer is thicker than a critical thickness or the temperature is higher than a critical value,generation-recombination in the absorption layer is the dominative mechanism;otherwise band-to-band tunneling in the multiplication layer dominates the dark counts.The thicknesses of charge and multiplication layers greatly affect the dark count and the peak single photon quantum efficiency and increasing the multiplication layer width may reduce the dark count probability and increase the peak single photon quantum efficiency.However,when the multiplication layer width exceeds 1 μm,the peak single photon quantum efficiency increases slowly and it is finally saturated at the quantum efficiency of the single photon avalanche diodes.     

Determination of electron-diffusion length from photocurrent characteristics of the structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd

In this study the electron diffusion length L _n is determined from the relative spectral response of the photocurrent characteristics of the p/i/n sandwich structure ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd. The techniques used for the preparation of the a-Sic:H and a-Si:H amorphous films were glow-discharge and rf magnetron sputtering, respectively. The thickness of the p-type, intrinsic and n-type layer were 400 Å, 7000 Å and 600 Å, respectively. The response of the short-circuit current density J _sc was measured versus the photon energy hv at both constant light intensity and constant temperature. The electron diffusion length was found to be 0.31 m by means of the method of Agarwala and Tewary. Although, in the case of single crystals many diffusion length measurements have been made, there are only few papers for amorphous silicon this films [1]. As it is well-known, the diffusion length of the charge carriers is the most important parameter from the point of view of solar cell applications [2]. In order to obtain a high efficiency in a solar cell all carriers created under illumination in the intrinsic layer should reach the electrodes [3]. In the case that the thickness of the intrinsic layer is much larger than the diffusion length, not all carriers can reach the electrodes and, accordingly, a low efficiency results [4]. On the other hand, carriers which reach the electrodes without thermalizing do not contribute to the photocurrent and finally the efficiency of the solar cell is negatively affected. In order to avoid such an effect to a large extent, the thickness of the amorphous layers in a p/i/n solar cell must be conveniently chosen compared to the diffusion length of the carriers.Here it is aimed to determine the electron diffusion length. In order to achieve this goal, the photocurrent characteristics of an ITO/a-SiC:H(p-type)/a-Si:H/a-Si:H(n-type)/Pd structure was measured versus the photon energy at constant light intensity and constant temperature. In order to determine the electron diffusion length, the method of Agarwala and Tewary [5] was utilized.     

Emission of positronium in a nanometric PMMA film

Positron beam experiments have been performed for the first time on a self-supporting polymethyl metacrylate (PMMA) film of 310 nm-thick made by spin coating. The positronium (Ps) emission from the PMMA surface is studied as a function of the positron implantation energy by using Doppler profile spectroscopy and Compton-to-peak ratio analysis. When the sample and the Ge-detector are perpendicular to the positron beam, the emission of para-positronium (p-Ps) is detected as a narrow central peak. By rotating the sample 45° with respect to the beam, the emission of p-Ps is detected as a blue-shifted fly-away peak. The bulk Ps fraction, the efficiency for the emission of Ps by picking up an electron from the surface, and the diffusion lengths of positrons (thermal and or epithermal), p-Ps and ortho-positronium (o-Ps) are obtained.     

Intersubband absorption in strained AlxGa1−xN/GaN quantum wells with InyGa1−yN nanogroove layers

The intersubband absorption in a four-energy-level system consisting of a strained Al_xGa_1?xN/GaN quantum well with an In_yGa_1?yN nanogroove layer is calculated by considering the strain modification on the material parameters and polarization effect. It is found that the In_yGa_1?yN nanogroove layer in the middle of quantum well can enhance the confinement of electrons and their energy levels which consequently affect the intersubband absorption. With increasing the In composition and the groove thickness or applying a moderate compressive stress, an inflexion of energy levels appears when the lowest energy potentials of the left well and the groove are equivalent. The intersubband absorption spectrum exhibits multiple peaks contributed by different transitions. The position and height of absorption peaks are sensitive to the structural parameters (i.e., In composition and nanogroove thickness) and the strain induced by the groove layer.