Large Surface Recombination Velocities Research Articles

Surface recombination velocities for 4H-SiC: Temperature dependence and difference in conductivity type at several crystal faces

In bipolar SiC devices, which are promising under ultra-high voltage operation, the carrier lifetime is a highly influential parameter for the device performance. Surface recombination is one of the limiting factors for the carrier lifetime, and quantitative values of the surface recombination velocities are required for the design and development of fabrication processes of the devices. In this study, we observe carrier recombination at various temperatures for the Si- and C-faces of n- and p-type 4H-SiC samples and the a- and m-faces of n-type 4H-SiC samples with a treatment of chemical mechanical polishing or reactive ion etching by using the microwave photoconductivity decay method. From the experimental results, we estimate surface recombination velocities and bulk carrier lifetimes of the samples by using an analytical model. As a result, we found the smallest surface recombination velocity of 150 cm/s for the chemical mechanical polished surface of the Si-face of the n-type samples at room temperature. Surface recombination velocities increased with temperature for the chemical mechanical polished surfaces. The surfaces treated with reactive ion etching showed relatively large surface recombination velocities with weak temperature dependence. Based on these results, we discuss the origins of the recombination centers at surfaces of 4H-SiC.

High surface recombination velocity limits Quasi-Fermi level splitting in kesterite absorbers

Kelvin Probe Force Microscopy, Photoluminescence imaging and numerical simulations are used to study the surfaces of Cu2ZnSnSe4 absorber layers. In particular, the effect of NH4OH and annealing under ambient conditions is investigated. We observe drastic changes in the measured quasi Fermi-level splitting (QFLs) after chemical cleaning of the absorber surface with NH4OH, which is traced back to a removal of the surface inversion. Air annealing recovers surface inversion, which reduces the recombination current at the surface. Annealing above 200 °C leads to a permanent change in the work function which cannot be modified by NH4OH etching anymore. This modification makes the QFLs insensitive to surface cleaning and explains why air annealing in Cu2ZnSnSe4 is important. From numerical simulations we find that a large surface recombination velocity needs to be present in order to describe the experimental observations.

Opto-electric investigation for Si/organic heterojunction single-nanowire solar cells

Recently, silicon single nanowire solar cells (SNSCs) serving as the sustainable self-power sources have been integrated into optoelectronic nanodevices under the driver of technology and economy. However, conventional SNSC cannot provide the minimum energy consumption for the operation of nanodevices due to its low power conversion efficiency (PCE). Here, we propose an innovative approach to combine the n-type silicon nanowires (SiNWs) with p-type poly(3,4-ethylthiophene):poly(styrenesulfonate) (PEDOT:PSS) to form the p+n heterojunction, which shows superior opto-electric performances. Besides, PEDOT:PSS also acts as a natural anti-reflection coating (ARC) with an excellent light-trapping capability, especially in the short-wavelength range. Importantly, the photovoltaic performances of Si/PEDOT:PSS SNSC can be well maintained even in large surface recombination velocity, due to the efficient field-effect passivation of PEDOT:PSS. The minority carrier concentration at outer surface of shallow p+n heterojunction is greatly reduced by the electric field, drastically suppressing the surface recombination compared to the conventional p-i-n homojunction SNSC. Furthermore, larger junction area of p+n heterojunction facilitates the separation of photo-generated charge carriers. These results demonstrate that the Si/PEDOT:PSS SNSC is a promising alternative for micro power application.

Enhanced charge recombination due to surfaces and twin defects in GaAs nanostructures

Power conversion efficiency of gallium arsenide (GaAs) nanowire (NW) solar cells is severely limited by enhanced charge recombination (CR) at sidewall surfaces, but its atomistic mechanisms are not well understood. In addition, GaAs NWs usually contain a high density of twin defects that form a twin superlattice, but its effects on CR dynamics are largely unknown. Here, quantum molecular dynamics (QMD) simulations reveal the existence of an intrinsic type-II heterostructure at the (110) GaAs surface. Nonadiabatic quantum molecular dynamics (NAQMD) simulations show that the resulting staggered band alignment causes a photoexcited electron in the bulk to rapidly transfer to the surface. We have found orders-of-magnitude enhancement of the CR rate at the surface compared with the bulk value. Furthermore, QMD and NAQMD simulations show unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective CR centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying CR processes.

Twin superlattice-induced large surface recombination velocity in GaAs nanostructures

Semiconductor nanowires (NWs) often contain a high density of twin defects that form a twin superlattice, but its effects on electronic properties are largely unknown. Here, nonadiabatic quantum molecular dynamics simulation shows unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective charge-recombination centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying surface-recombination processes.

Effects of an anodic oxide passivation layer on mesa-type InGaAs (PIN) photodetectors

Because of the high density of surface states and large surface recombination velocity found in III–V materials, surface passivation remains a crucial processing step for the fabrication of mesa-type InGaAs photodetectors. It is required to find an easy and economical method to form an effective passivation layer on the surface of the detector. In this work, an oxide passivation layer formed by electrochemical anodization was proposed and a 256-element In0.78Ga0.22As photodetector array was fabricated. The dark current performance achieved is as low as 2 × 10−7 A (bias voltage of −0.1 V) at 300 K. The reduction of dark current indicates that the anodic oxide passivation layer is effective in reducing recombination at the exposed mesa surface.

Photocurrent and photovoltage from polycrystalline p-n junction solar cells

Analytical solutions are obtained for photocurrent and photovoltage from a polycrystalline p-n junction solar cell by modelling it as a filament with large surface recombination velocities at its lateral surfaces. While the calculation of light-generated (short-circuit current) does not require an exact knowledge of the surface recombination velocity S, the dark forward current cannot be calculated without its knowledge.

Transient establishment of the Dember field in fast photoconductive circuit elements

The time τ eef required to establish the Dember electric field E_{D}(t) following the sudden application or removal of incident light or other radiation is examined in general for large surface recombination velocity and illustrated for fast photoconductive circuit elements. The influence of surface scattering on mobility depends strongly on E_{D}(t) because E D results in electrons and holes locating themselves away from the surface. The analysis is subject to the assumptions underlying the ambipolar transport equation and to a single time-constant characterization of the recombination rate. The results show that if τ eff exceeds other times characteristic of the turn-on or turn-off transient, then it enters importantly into determining the transient observed. Specifically, if the product of the steady-state high injection lifetime, the ambipolar diffusion coefficient, and the square of the absorption coefficient of the incident light greatly exceeds unity, then τ eff enters importantly. Otherwise it does not.

Steady-state currents in p-n junction filaments or grains in case of large surface recombination velocities at lateral surfaces

Recently it has been pointed out that the saturation current of a semiconductor filament which constitutes part of a p-n junction diverges when the surface recombination velocity at the faces becomes infinitely large. Here we point out that this is to be expected on physical grounds since, whenever the carrier concentration is kept off equilibrium by an outside agent and at the same time recombination life times, in the bulk or in surface layers, tend to zero, concentration gradients tend to infinity. As also previously noted, the situation can be remedied by using realistic (finite) surface recombination velocities in model calculations. However, this procedure leads to mathematical complexities which have been circumvented recently by the introduction of a ‘‘heuristic’’ approach. It is the aim of this paper to assess the validity of the heuristic approach by means of detailed and exact calculations. OFF

Surface Band Bending Effects on Photoluminescence Intensity in n-InP Schottky and MIS Diodes

Surface band bending effects on the photoluminescence (PL-) intensity in n-InP Schottky and MIS diodes were studied. The bias dependence curve of the PL-intensity for n-InP is quite different from those obtained for Si, Ge and GaAs with large surface recombination velocity. Remarkable depression and enhancement of the PL-intensity were observed in n-InP with application of reverse and forward bias voltages. The maximum PL-intensity was achieved at the flat-band bias condition in Schottky diodes. In MIS diodes, the PL-intensity increased when the forward bias voltage was more than the flat band voltage. PL-intensity variation with surface band bending for Schottky and MIS diodes can be explained well using a surface dead layer model in the depletion condition and a surface excess carrier model in the accumulation condition.

Interpretation of scanning electron microscope measurements of minority carrier diffusion lengths in semiconductors

In scanning electron microscope (SEM) injection measurements of minority carrier diffusion lengths some uncertainties of interpretation exist when the response current is nonlinear with distance. This is significant in epitaxial layers where the layer thickness is not large in relation to the diffusion length, and where there are large surface recombination velocities on the incident and contact surfaces. An image method of analysis is presented for such specimens. A method of using the results to correct the observed response in a simple convenient way is presented. The technique is illustrated with reference to measurements in epitaxial layers of GaAs. Average beam penetration depth may also be estimated from the curve shape.

Theory of life time measurements with the scanning electron microscope: Steady state

A theoretical steady state analysis is given of the scanning electron microscope method of measuring bulk life time in diodes, where the plane of the junction is perpendicular to the surface. The current in the junction is obtained as a function of the beam power, the beam penetration into the semiconductor, the value of the surface recombination velocity, and the distance of the beam to the junction. Particular attention is paid to the injection level, and sufficient conditions are formulated for low and high injection, in terms of the beam current and voltage. A new method for measuring large surface recombination velocities is suggested.