Characteristics of gradually doped LWIR diodes by hydrogenation

The hydrogenation effects on HgCdTe diode performance are presented and the mechanism of hydrogenation is revealed. By the hydrogenation, R₀A is increased by 30 times and photo-response is also improved. It is supposed that these are explained by the increased minority carrier lifetime by the hydrogenation. However, it is found from LBIC measurements that the minority carrier lifetime doesn’t increase by the hydrogenation. An important clue that explains the hydrogenation effects is found from Hall measurements. It is found that, after the hydrogenation, the doping concentration of Hg-vacancy doped substrate decreases and the mobility increases. For the heavily hydrogenated bulk substrate, it is also found that the hydrogen passivates the whole Hg-vacancy and reveals the residual impurity and p-type doping concentration is exponentially graded. From these measurements, the diffusion current model of gradually doped diode is proposed. This model shows that the diffusion current of the graded junction diode is 2 orders of magnitude smaller than that of the abrupt junction diode, which clearly explains the R₀A increase by the hydrogenation. Medicisimulation to investigate the change of LBIC signal by the doping grading also coincides with the measurements. From these measurements and model, the hydrogenation effects are attributed to the grading of Hg-vacancy doped p-type substrate by the diffused hydrogen.     

Analysis of leakage currents in MOCVD grown GaInAsSb based photodetectors operating at 2 µm

This paper reports the fabrication and characterization of GaInAsSb photodetectors operating at 2 μm. At room temperature, the performance of these photodiodes under reverse bias conditions is limited by the surface leakage. A model has been developed to separate the bulk (diffusion and generation-recombination (g-r)) and the surface leakage contributions toward the total leakage current. By fitting this model to the experimental data, values of material parameters such as minority carrier diffusion length and lifetime have been estimated. The highest R₀A of 55 Ω-cm² has been obtained with a responsivity of 0.44 A/W at 2 μm.     

The magnetic field dependence of R0A products in n-on-p homojunctions and p-on-n heterojunctions from Hg0.78Cd0.22Te liquid phase epitaxy films

The analysis of R₀A products as a function of magnetic field in n-on-p diodes using a simple diffusion current model has previously been shown to yield both J_ep/J_total ratio (the relative contribution of the p-side diffusion current) and μ_ep (the minority carrier, electron mobility). In this paper, we report the good agreement between the experimental and theoretical dependence of μ_ep on the hole concentration over a wide range between 1 x 10¹⁶ and 4 x 10¹⁷ cm⁻³ in n-on-p homojunction diodes fabricated on undoped p-type Hg_0.78Cd_0.22Te liquid phase epitaxial (LPE) films. The averaged J_ep/J_total ratio varied between 68 and 90% with the hole concentration. These J_ep/J_total ratios indicate that other leakage current mechanisms than the p-side diffusion current were not negligible. Also, for the first time, comparative measurements were made on p⁺/n heterojunction diodes consisting of As-doped Hg_0.07Cd_0.30Te and In-doped Hg_0.78Cd_0.22Te LPE layers. Unlike a typical change in R₀A products by a factor of 2–3 in n-on-p homojunction diodes, the R₀A products in p⁺/n heterojunction diodes at 7 kG were typically only 2–3% higher than that at the zero field. The typical J_ep/J_total ratio in p⁺/n heterojunction diodes was about 3–4 %, which confirms the general belief that the p⁺ cap layer, due to the high doping and a larger bandgap, contributes very little to the total leakage current.     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

变磁场I-V法对碲镉汞光伏器件少子扩散特性的研究

零偏压电阻-面积乘积(R0A)和反向饱和电流密度J0是决定光电二极管性能的重要参数．提出了一种对碲镉汞(Hg1-xCdxTe)光伏器件的少子扩散特性进行研究的有效方法．利用变磁场下的电流-电压(I-V)测试,得到了组分x在0．5与0．6之间的器件R0A和J0随磁场强度B变化的函数关系．由实验结果估算得到了室温工作的短波红外(SWIR)碲镉汞光伏器件的少子扩散长度．其数值与用激光诱导电流(LBIC)方法得到的相一致．     

Correlation of laser-beam-induced current with current-voltage measurements in HgCdTe photodiodes

Laser-beam-induced current (LBIC) is being investigated as an alternative to electrical measurements of individual photodiodes in a two-dimensional array. This is possible because LBIC only requires two electrical contacts to an array and the two-dimensional scanning of a focused laser beam across the array to image the entire array. The measured LBIC profiles, obtained from linear arrays of HgCdTe photodiodes, will be used to study the uniformity of photodiodes in the array and to extract the R₀A of the photodiodes. It will be shown that the shape of the LBIC signal is correlated to the electrical performance of the photodiode, with R₀A related to the spreading length of the photodiodes. Linear arrays of n-on-p, mid-wavelength infrared (MWIR) and long wave-length infrared (LWIR) devices were formed in liquid-phase epitaxy HgCdTe epilayers using a plasma junction-formation technique. The LBIC profiles were measured on each of the devices at various temperatures. For the MWIR devices, the extracted spreading length shows no correlation with R₀A. However, the LBIC signal does detect nonuniform devices within the array. For the case of the LWIR devices, the spreading length is extracted as a function of temperature, with the R₀A subsequently calculated from the spreading length. The calculated R₀A, obtained without requiring contact to each photodiode in the array, agrees well with electrical measurements. Asymmetry of the LBIC signals for certain devices in the arrays is shown to be a result of localized leakage at the photodiode junction or from the contact pads through the passivation layers. These results are confirmed by numerical modeling of the device structures.     

Experimental investigation of the excess charge

The open-circuit voltage of about 600 mV developed by 0.1 ohm-cm silicon solar cells under air mass zero illumination is about 100 mV less than voltages predicted from simple diffusion theory. The lower measured voltages appear to be controlled by junction current transport processes associated with the thin top diffused layer. Mechanisms such as low n⁺ layer minority carrier lifetime and bandgap narrowing due to heavy doping effects (HDE) have been suggested to explain these results. Experimental determinations of the properties of the diffused layer are required to assess which of these mechanisms predominate. While direct measurement is difficult, an indirect measurement methodology exists by which the lifetime or transit time in the diffused layer can be obtained. Nine p-type, 1×2 cm, 〈111〉 orientation silicon wafers were phosphorus diffused at 880°C for 45 minutes using P0Cl₃. Open-circuit voltages of 595-612 mV, typical of all 0.1 ohm-cm cell voltages, were obtained. From the open-circuit voltage and short-circuit current, the diffusion controlled I₀ was obtained. In addition to illuminated I-V characteristics, the time constants from the Open-Circuit Voltage Decay method, and the minority carrier diffusion lengths in the base region were measured. The base region charge was determined using the base region diffusion length measured by an X-ray method. The data from these experiments combined with simple theory can imply the minority carrier time constant and the excess charge in the diffused layer. From this, certain conclusions are drawn about the relative roles of bandgap shrinkage and recombination rates in the diffused layer.     

Numerical Simulation and Analytical Modeling of InAs nBn Infrared Detectors with p-Type Barriers

This paper presents finite-element one-dimensional numerical simulations and analytical modeling for ideal (diffusion current only) nBn detectors with p-type barrier layers. The simulations show that the current–voltage J(V) and the dynamic resistance versus voltage R _D(V) relations, both dark and illuminated, are in excellent agreement with the equations for ideal back-to-back photodiodes. We present a depletion approximation model for the nBn detector, analogous to that for the conventional p–n junction photodiode, based on new boundary conditions on the hole concentrations versus voltage at the edges of the nBn barrier layer. We show that these nBn boundary conditions are identical to those for ideal back-to-back photodiodes, justifying the applicability of back-to-back photodiode equations to describe the ideal nBn detector. The simulations for the space-charge regions show a low-bias-voltage regime and a high-bias-voltage regime. The integrated space-charge densities in the layers adjacent to the barrier layer vary linearly with bias voltage. Negative dynamic resistance occurs because the bias voltage changes the effective thickness of the thin-base layers that generate diffusion current. We present a new formulation of the model for ideal back-to-back photodiodes with a more elegant and transparent set of equations for J(V) and R _D(V).     

Semiconductor‐Based Photoelectrochemical Water Splitting at the Limit of Very Wide Depletion Region

In semiconductor‐based photoelectrochemical (PEC) water splitting, carrier separation and delivery largely relies on the depletion region formed at the semiconductor/water interface. As a Schottky junction device, the trade‐off between photon collection and minority carrier delivery remains a persistent obstacle for maximizing the performance of a water splitting photoelectrode. Here, it is demonstrated that the PEC water splitting efficiency for an n‐SrTiO₃ (n‐STO) photoanode is improved very significantly despite its weak indirect band gap optical absorption (α < 10⁴ cm^?1), by widening the depletion region through engineering its doping density and profile. Graded doped n‐SrTiO₃ photoanodes are fabricated with their bulk heavily doped with oxygen vacancies but their surface lightly doped over a tunable depth of a few hundred nanometers, through a simple low temperature reoxidation technique. The graded doping profile widens the depletion region to over 500 nm, thus leading to very efficient charge carrier separation and high quantum efficiency (>70%) for the weak indirect transition. This simultaneous optimization of the light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport by means of a graded doping architecture may be useful for other indirect band gap photocatalysts that suffer from a similar problem of weak optical absorption.     

The complete doping profile using MOS CV technique

Methods for determining the doping profile from the semiconductor surface to the deep bulk using the high frequency CV technique are presented. The analysis includes: (i) surface analysis using an improved Ziegler's method, (ii) depletion region with the surface state detection, and (iii) deep depletion region with the minority carrier generation correction. Experimental data shows precise bulk doping concentration can be obtained despite significant minority carrier generation.     

Stationary room temperature MOS-C deep-depletion characteristics

A room temperature experimental technique which yields time-independent MOS-C C?V_G deep depletion characteristics for both n- and p-bulk devices is described, explained and illustrated. The stationary deep depletion characteristics are obtained by making use of lateral charge transfer in a standard ring-dot structure after having controllably charged the interelectrode ambient-oxide interface. Technique limitations associated with the magnitude of the interelectrode charge, surface states, and minority carrier dynamics are also illustrated experimentally and analyzed theoretically.     

Boundary conditions for pn heterojunctions

Boundary conditions for the excess minority carrier concentrations at the space-charge region (SCR) edges are developed and used to evaluate the current-voltage characteristics of pn heterojunctions. Carrier transport by drift and diffusion within the SCR, by thermionic emission across the interface, and by minority carrier diffusion within the quasi-neutral bulk are included. When transport across the interface is slower than that across the SCR, the results reduce to the emission-diffusion model of Perlman and Feucht. Alternatively, when carrier transport within the SCR is the limiting process, a result analogous to the diffusion theory of metal-semiconductor diodes results. Finally, if the current is limited by minority carrier diffusion within the quasi-neutral bulk, the quasi-Fermi levels are constant within the SCR and the results are similar to those for pn homojunctions.     

Gain-bandwidth product of AlGaSb avalanche photodiodes analyzed byusing equivalent multiplication region method

An analysis presented for an AlGaSb diode in which it was approximated by a p-i-n diode with an equivalent multiplication region having a length of one-eighth of the whole depletion region and was combined with R.B. Emmon's p-i-n avalanche photodiode analysis (1967). A relationship between the gain-bandwidth product and the carrier concentration is derived, and it is shown to agree well with the experimental data     

偏压量子效率测试在薄膜太阳能电池特性分析中的应用

薄膜太阳能电池在不同偏压下的量子效率(QE)会呈现非常不一样的结果.对不同波长范围内偏压量子效率的分析可以研究薄膜太阳能电池窗口层区域杂质补偿情况、主结势垒高低、背势垒高度等,还可以得出耗尽区宽度以及少子扩散长度等重要参数.通过实验测量与理论分析,给出了薄膜太阳能电池耗尽区宽度(W)和少子扩散长度(Ln)与偏压量子效率的关系,提出了一种新的拟合耗尽区宽度(W)和少子扩散长度(Ln)的方法,探讨了偏压量子效率测试在薄膜太阳能电池特性分析中的应用.     

Electrical characteristics of an epitaxial high barrier Schottky diode

A Schottky barrier diode with high barrier height injects minority carriers at the forward biased condition. With injection of minority carriers the current density–voltage characteristics are altered significantly from that of the conventional exponential relationship. A model incorporating drift and diffusion currents for both holes and electrons, carrier recombination within the drift region and also the blocking properties of the low–high (n^- n⁺) interface is developed. The previous works on high barrier Schottky diodes used empirical expressions to combine the high injection model with the low injection model in order to study its behaviour at intermediate levels of injection. Whereas in the present work, a boundary condition is applied to combine the high injection model with an intermediate injection model. To combine an intermediate level model with a low injection level model, another boundary condition is introduced. The present work provides solutions for important physical quantities such as the minority carrier profile and current within the drift region, injection ratio and storage time.     

On the determination of the minority carrier lifetime from the reverse recovery transient of pnR diodes

The determination of the minority carrier lifetime of pn diodes with abrupt junctions and recombination (R) contacts is discussed. It is confirmed that the base transit time of diodes with sufficiently small base widths (less than approximately three minority carrier diffusion lengths) affects the reverse recovery transient and that large error can be introduced if the minority carrier lifetime of a diode with a small base width is computed using an expression which was derived for diodes with an infinite base width. Approximations to the general expression for the minority carrier lifetime as a function of the storage time and base width are developed, and the errors associated with their use are analyzed. In addition the effect of measurement errors on the determination of lifetime in short base diodes is discussed.A procedure is described which can be used to compute the minority carrier lifetime of pnR diodes which the ratio of the base width to the minority carrier diffusion length is larger than some small number. The applicability of the procedure and the approximations which are developed are a function of the ratio of the diode base width to the minority carrier diffusion length $\text{(}\text{W}\text{L}\text{)}$, and the forward to reverse current ratio $\text{(}\text{I}{}_{\mathrm{f}}\text{I}{}_{\mathrm{r}}\text{)}$ at which the experimental measurements are made. An example is given which shows that less than 1 per cent error is introduced in the computed lifetime by the approximations if the lifetime is computed from measurements made at $\text{I}{}_{\mathrm{f}}\text{I}{}_{\mathrm{r}}\text{= 4}$ and 8 for diodes such that $\text{W}\text{L}\text{> 2 × 10}{}^{\mathrm{?2}}$.     

Perimeter governed minority carrier lifetimes in 4H-SiC p+n diodes measured by reverse recovery switching transient analysis

Minority carrier lifetimes in epitaxial 4H-SiC p⁺n junction diodes were measured via an analysis of reverse recovery switching characteristics. Behavior of reverse recovery storage time (t_s) as a function of initial ON-state forward current (I_F) and OFF-state reverse current (I_R) followed well-documented trends which have been observed for decades in silicon p⁺n rectifiers. Average minority carrier (hole) lifetimes (τ_p) calculated from plots of t_s vs I_R/I_F strongly decreased with decreasing device area. Bulk and perimeter components of average hole lifetimes were separated by plotting 1/τ_p as a function of device perimeter-to-area ratio (P/A). This plot reveals that perimeter recombination is dominant in these devices, whose areas are all less than 1 mm². The bulk minority carrier (hole) lifetime extracted from the 1/τ_p vs P/A plot is approximately 0.7 μs, well above the 60 ns to 300 ns average lifetimes obtained when perimeter recombination effects are ignored in the analysis. Given the fact that there has been little previous investigation of bipolar diode and transistor performance as a function of perimeter-to-area ratio, this work raises the possibility that perimeter recombination may be partly responsible for poor effective minority carrier lifetimes and limited performance obtained in many previous SiC bipolar junction devices.     

Minority carrier traps in epitaxial gallium arsenide phosphide

The optical injection of minority carriers in the depletion region of a reverse biased Schottky barrier has permitted the study of deep levels in vapour phase epitaxial n-type GaAsP (Te) structures. The deep level transient spectroscopy (DLTS) experiments have yielded two hole traps at (E_v + 0.60)eV and (E_v + O.36)eV. Additional subsidiary transient capacitance, photocurrent and capture experiments are analysed, the details of which give electron and hole capture cross-sections, trap densities and activation energies. Derived values of minority carrier lifetime (~1 ns) agree with experimental values and with injection-level dependent LED efficiency.     

High-low junctions for solar cell applications

A new theoretical model to calculate the effective surface recombination velocity (S_eff) of a high-low junction with an arbitrary impurity distribution is presented. The model is applied to erfc-diffused pp⁺ junctions using experimental data of bandgap narrowing, lifetime and mobility. Bandgap narrowing is shown to degrade the minority carrier reflecting properties of the high-low junction. Computer results are applied for the design of BSF solar cells and to study other solar cells structures based on high-low junctions.     

Effects of chemical and electrochemical etching on polycrystalline thin films of CuGaSe2

Electrochemical and, especially, chemical oxidative etching drastically improves the photoresponse of liquid electrolyte/CuGaSe₂-on-Mo junctions. This is expressed in decreased effective doping levels and increased effective minority carrier diffusion lengths. It is accounted for by removal of highly defective surface layers, which also leads to an increase in the barrier height, as judged from a positive shift of the flat band potential (on the electrochemical scale). The etching effects are seen clearly in Zn/CuGaSe₂ devices, by electron beam-induced current. This last method also reveals a supra-grain structure, which is tentatively explained by thermal stress-induced strain at the Mo-CuGaSe₂ interface.