A Review of Minority Carrier Recombination Lifetime Measurements

Abstract

The recombination lifetime of minority carriers is a critical parameter in semiconductor devices such as photovoltaic cells since it controls the efficiency of such devices. Many techniques have been developed to accomplish recombination measurements and thereby test semiconductor devices\' efficiencies. Recombination lifetime average values differ according to semiconductor device type; thus, choosing an appropriate technique is important. This paper studies the concept of excess minority carrier lifetime and its calculations. It also investigates the advantages, limitations, and capabilities of the most common recombination lifetime measurement techniques. A chart was drawn with all known measurement methods to make it easier to understand the relation between these techniques.

Introduction

I. INTRODUCTION

Minority carrier recombination is a phenomenon that occurs in semiconductors. Semiconductors are materials that have electrical conductivity between conductors and insulators. The conductivity of semiconductors can be increased by introducing impurities into the material, which creates excess electrons or holes. These excess carriers are called minority carriers because they are present in smaller numbers than the majority carriers (electrons or holes) [1]. In a semiconductor, when an electron and hole meet, they can recombine and release energy in the form of light or heat. This process is called minority carrier recombination. The rate at which minority carrier recombination occurs depends on several factors such as temperature, doping concentration, and the presence of defects. Minority carrier recombination plays an important role in electronic devices such as solar cells and transistors. In solar cells, it is desirable to minimize minority carrier recombination to increase efficiency [1]. The lifetime of photo-generated excess minority carriers is a significant property of semiconductors, especially in photonics and photovoltaic fields. This property is mainly employed to estimate the performance of many devices such as photovoltaic cells [2]. There is always a need for effective techniques to measure this fundamental property and estimate the efficiency of photovoltaic devices [1]. It is imperative to understand the mechanism of the recombination process of minority carriers to comprehend the practical measurements of the lifetime. Recombination typically occurs on both surfaces as well as in the bulk region of a solar cell. Thus, the effective lifetime of minority carriers in photovoltaic cells mainly depends on the minority carrier lifetime at surfaces and in the bulk region. Many techniques and methods have been developed to measure the lifetime. These techniques can be generally classified as direct and indirect measurement techniques depending on their methods to measure recombination lifetime. Direct techniques are typically directly applied to give the exact measurements of the effective minority carrier lifetime by changing conductivity or reversing the characteristics of the semiconductor. Indirect techniques measure other properties of solar cells, which can be used to estimate the lifetime [1–3]. This paper investigates the concept of minority carrier effective lifetime in photovoltaic operations. In addition, it presents a review of the most common experimental methods that are implemented in recombination lifetime measurements.

II. CONCEPT OF EXCESS MINORITY CARRIER LIFETIME (EMCL)

Excess Minority Carrier Lifetime (EMCL) refers to the duration of time in which minority carriers, such as electrons or holes, remain in a semiconductor material before recombining with majority carriers. The longer the EMCL, the greater the efficiency of electronic devices such as solar cells and transistors [4]. The EMCL is affected by several factors including the quality of the semiconductor material, doping levels, and temperature. Higher-quality materials with fewer defects tend to have longer EMCLs. Doping can also play a role in increasing or decreasing EMCLs depending on whether it introduces impurities that trap minority carriers. Temperature can affect EMCLs by increasing or decreasing carrier mobility and recombination rates. In solar cells specifically, a longer EMCL means that more photons can be absorbed and converted into electrical energy before recombination occurs [4].

Recombination mechanisms are critical parameters in solar cells since they refer to the processes by which electron–hole pairs lose their energies to stabilize in lower energy positions. The recombination rate, which affects the efficiency of the solar cell, mainly depends on the number of excess carriers in a semiconductor [5].

A. Bulk Recombination Mechanisms

Bulk recombination mechanisms refer to the various processes that contribute to the recombination of electron-hole pairs in a semiconductor material. These mechanisms are crucial in determining the efficiency of solar cells and other electronic devices that rely on semiconductors. Understanding and controlling bulk recombination mechanisms is essential for improving device performance. By reducing defects in materials and optimizing device design, we can minimize these losses and improve overall efficiency. There are typically three mechanisms of recombination that can occur in a single-crystal semiconductor. These mechanisms are:

1. Radiative Recombination: It is a process that occurs when an ion and an electron combine to create a neutral atom, emitting radiation in the form of photons. It is also known as band-to-band recombination. The electron, which is located in the conduction band, recombines with a hole in the valence band. The emitted energy is given off as a photon [4, 6]. This phenomenon is observed in various natural and artificial systems, including stars, plasmas, and semiconductors. In semiconductors, radiative recombination is responsible for producing light emissions from LEDs or laser diodes. By applying an electric field across a semiconductor material, electrons and holes are injected into the device [6].
2. Auger Recombination: It is a process in which the energy released during the recombination of an electron and a hole in a semiconductor material is transferred to another electron instead of being emitted as a photon. This nonradiative process can result in the loss of energy and efficiency in devices such as solar cells and light-emitting diodes [5]. The Auger effect occurs when an electron fills a hole created by the removal of another electron, releasing energy. In traditional radiative recombination, this energy is emitted as light, but in Auger recombination, it is transferred to another free electron within the material. The excess energy can cause this second electron to be ejected from its original position or promote it to a higher energy level [7, 8]. This mechanism involves three particles, i.e. two electrons and a hole, to occur. One electron recombines with a hole in the valance band giving its energy to another electron in the conduction band. This second electron is pushed up to a higher energy level due to the received energy. It progressively releases its energy thermally to calm on the conduction band edge [7, 8]. This process can lead to significant losses in device performance, particularly for high-energy applications where Auger recombination rates are increased. Researchers are exploring ways to reduce these losses through materials engineering and other techniques.
3. Shockley-Read-Hall Recombination: It is also known as trap-assisted recombination. This recombination mechanism requires extra energy levels, i.e. traps, to occur. Shockley-Read-Hall recombination is a type of non-radiative recombination that occurs in semiconductors. This process involves the trapping of minority carriers by defects or impurities in the material, which then leads to their annihilation with majority carriers. The recombination rate is determined by the concentration and properties of these traps [5, 9]. This phenomenon has important implications for the performance of semiconductor devices, as it can limit their efficiency and speed. For example, in solar cells, Shockley-Read-Hall recombination can reduce the amount of energy that is converted into electricity, while in photodetectors, it can increase noise and reduce sensitivity [9]. In this mechanism, an electron moves to a trap, extra energy level, in the band-gap region emitting energy as a photon. It moves again to the valance band recombining with a hole and releasing energy as a photon or multiple photons [9, 10]. Fig. 1 illustrates the concept of these mechanisms. To overcome this issue, various strategies have been developed to reduce trap densities or passivate them with suitable materials.

III. RECOMBINATION LIFETIME MEASUREMENTS TECHNIQUES

Recombination lifetime measurement techniques are critical in understanding the electrical properties of semiconductors. The recombination process is a fundamental aspect of semiconductor materials, and it occurs when an electron and hole combine to form a neutral atom. The recombination lifetime refers to the time taken for the majority carriers (electrons or holes) to recombine with their minority counterpart. Various techniques have been developed to measure the recombination lifetime, including photoconductance decay, time-resolved photoluminescence, and open-circuit voltage decay. These methods rely on measuring changes in electrical signals or luminescence over time after exciting the sample with light pulses. Accurate measurement of recombination lifetime is crucial for characterizing the quality of semiconductor materials used in semiconductor devices such as solar cells, LEDs, and transistors [18, 19].

Since recombination lifetime is an imperative parameter in deciding the efficiency of semiconductor devices, there was an urgent need for different methods to measure the effective recombination lifetime of minority carriers. Many techniques have been developed depending on measurement methods, which can be classified as direct and indirect measurement methods [20].

Some many parameters and properties are related to the minority carrier's lifetime. Measurement techniques have been developed depending on these parameters, such as the conductivity change method, the steady-state photoconductivity method, and the diffusion method. Fig. 2 and Fig. 3 show the most common techniques that are usually applied to directly and indirectly measure the minority carrier lifetime of a semiconductor sample.

These techniques mainly depend on the optical and electrical properties of semiconductor devices. Thus, the most used techniques can be classified according to these properties to be [11]:

1. Optical measurements techniques: The most common techniques that depend on the optical properties are Photo-conductance Decay, Photo-luminescence Decay, Quasi-Steady-State Photo-conductance, Steady-State Short-Circuit Current, Surface Photo-voltage, Electron Beam Induced Current, Light Beam Induced Current, and Free Carrier Absorption.
2. Electrical measurements techniques: The most common techniques that depend on the electrical properties are Pulsed MOS Capacitor, Open-Circuit Voltage Decay, Diode Current-Voltage, and Reverse Recovery.

A. Advantages and Limitations of the Most Widespread Measurements Techniques

Recombination lifetime measurement techniques are essential in determining the quality of semiconductor materials and devices. The two most widespread techniques used for this purpose are photoconductance decay (PCD) and time-resolved photoluminescence (TRPL). Both methods have their advantages and limitations [37].

The PCD method is a non-destructive, fast, and straightforward technique that provides accurate measurements. It is especially useful for measuring recombination lifetimes in heavily doped semiconductors. However, PCD requires high-quality contacts in the sample, which can be challenging to achieve in some cases [38].

On the other hand, TRPL is a highly sensitive method capable of measuring low-level minority carrier lifetimes with high accuracy. It also allows for spatially resolved measurements. However, it requires expensive equipment and is more time-consuming than PCD [39, 40].

Each technique has some unique advantages that make it the best choice in specific fields. The following points show the most interesting advantages and disadvantages of the most common techniques.

1)  Photo-conductance Decay (PCD): This technique, which is a direct measurement method, typically depends on a laser beam, which is focused on a fixed point on the front surface of the solar cell, to generate electron-hole pairs [27]. Fig. 4 shows a schematic diagram for contact PCD measurements.

The main advantages of this technique are that it is easy to implement, it has an accurate interaction with real electrical parameters, and it has a good analysis of experimental data [41]. In addition, the OCVD technique can achieve effective measurements of recombination lifetime at high and low-level injections [25]. The essential issue with using this technique is that it is not able to give accurate measurements when it is applied to structures with non-uniform carrier lifetime distribution since it depends on the position of the contacts [26].

''Small signal'' open-circuit voltage decay (SSOCVD) has been developed from the main method, which is the open-circuit voltage decay method.   ''Small signal'' open-circuit voltage decay allows controlling the injected carriers. In detail, additional carriers can be injected with pulse ''on'', and carriers start recombining with pulse ''off''. The main issue with this method is that some factors like surface recombination are neglected [42].

3)  Light Beam Induced Current (LBIC): This technique is an indirect method, and it is typically used to achieve localized characterization on solar cells. LBIC technique uses a scanning laser beam to create electron-hole pairs in tested samples [43]. Fig. 7 shows the main schematic diagram of this technique.

B. Range of Capability of the Most Widespread Measurements Techniques

Recombination lifetime is an important parameter to measure in semiconductor materials as it determines the efficiency of charge carrier generation and recombination. There are various techniques available for measuring the recombination lifetime of semiconductors, each with its range of capabilities. The most widespread measurement techniques include time-resolved photoluminescence (TRPL), time-resolved microwave conductivity (TRMC), and photoconductance decay (PCD). TRPL can measure lifetimes ranging from nanoseconds to milliseconds and is suitable for materials with high radiative recombination rates. TRMC measures lifetimes ranging from microseconds to milliseconds and is best suited for materials with low radiative rates but high mobility. PCD measures lifetimes ranging from microseconds to seconds and is ideal for materials with low mobility. Overall, the choice of technique depends on the material being tested and the desired accuracy of the measurement. It's important to understand the range of capability of each technique before selecting one for a specific application. [48, 49].

In solar cells, for example, the recombination lifetime is typically larger than that in transistors. Thus, implementing an appropriate technique is the key to achieving accurate results. TABLE II shows the ranges of measurements for the most common techniques.

TABLE II. RANGE OF MEASUREMENTS

Conclusion

Many direct and indirect measurement methods can be applied to achieve accurate recombination lifetime measurements. These techniques mainly deal with the optical and electrical characteristics of semiconductor devices. The key to choosing an appropriate method is to know its capability and its limitations. In solar cell measurements, it is desired to have methods that can measure large lifetimes for different solar cell thicknesses.

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Copyright © 2023 Idris H. Smaili, Ghazi Ben Hmida. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.