Recombination processes in the si/CdTe diodes I. B. Sapaev1,2

RECOMBINATION PROCESSES IN THE Si/CdTe DIODES
I.B.Sapaev^1,2, S.O.Sadullaev^1,3, M.R.Kurbanova^4
1 “TIIAME” National Research University, ²New Uzbekistan University, ³Fundamental and Applied Research Institute under “TIIAME” NRU,
⁴School №24 in Bagat region
e-mail: [e-mail]
Recombination processes in semiconductor devices play a crucial role in determining their performance. Heterojunctions are widely used in semiconductor devices because they offer several advantages, including the ability to separate photogenerated carriers and enhance device performance. p-Si/n-CdTe heterojunctions are particularly promising for photovoltaic (PV) applications because of the high absorption coefficient of CdTe in the visible and near-infrared regions of the electromagnetic spectrum. However, the performance of these devices is limited by recombination processes that occur in the device.
In p-Si/n-CdTe heterojunctions, several recombination mechanisms can occur, including Shockley-Read-Hall (SRH) recombination, Auger recombination, and radiative recombination. SRH recombination occurs when a free carrier recombines with a deep-level trap, which is a defect in the material that can trap carriers for a relatively long time. Auger recombination occurs when a free carrier recombines with another carrier and releases energy to a third carrier. Radiative recombination occurs when a free electron recombines with a free hole and emits a photon.
Experimental studies have shown that the dominant recombination process in p-Si/n-CdTe heterojunctions is SRH recombination. The density of deep-level traps in CdTe is high, which leads to a significant contribution of SRH recombination to the total recombination. The carrier lifetime in CdTe is also short, which exacerbates the effect of SRH recombination. Auger recombination becomes important at high carrier densities, such as in heavily doped regions or under intense illumination. Radiative recombination contributes to the photocurrent and is desirable in PV devices.
Various techniques have been employed to reduce the impact of recombination on device performance, including passivation of surface states and defects, doping optimization, and interface engineering. Passivation of surface states and defects involves treating the surface of CdTe with chemicals that reduce the density of defects. Doping optimization involves controlling the concentration of dopants in the material to reduce the impact of recombination. Interface engineering involves modifying the interface between p-Si and CdTe to reduce recombination.
Calculating recombination processes in p-Si/n-CdTe heterojunctions involves determining the rates of various recombination mechanisms and their contributions to the total recombination rate. The recombination rate can be expressed as:

where , and represent the recombination rates due to Shockley-Read-Hall (SRH), Auger, and radiative recombination, respectively.
As mentioned above, SRH recombination is the main recombination mechanism in p-Si/n-CdTe heterojunctions. This study endeavors to investigate the ramifications of temperature and applied voltage on CdTe in the context of Shockley Read Hall recombination mechanisms. The rate of recombination can be expressed in accordance with the findings of [1]. The determination of the rate of recombination under conditions of steady-state but nonequilibrium is achieved through the imposition of an equality between the rate of capture of electrons and that of holes. The aforementioned condition gives rise to a consistent recombination rate for electrons and holes in a steady state:

Here R-is the recombination rate, n_t= n_i. n_i-is the intrinsic concentration of CdTe, which R-is assumed to be dependent on temperature and applied voltage, we look for the simplest case, the dependence of formula (1) on temperature and voltage. We obtain formula (2) from several mathematical substitutions of formula (1).
Given the temperature dependence of n_i, the Cadmium Telluride bandgap, the lifetime of electrons and holes, and the effective densities of the conduction and valence zones[2].


Figure 1. The rate of recombination depends on Temperature and Voltage.


In Fig.1-a, we can see that the recombination rate increases with increasing temperature when the voltage is varied between 0-0.5 V. In Fig.1-b, the voltage has varied from 0.5 to 1 V. We can see that the recombination rate decreases with increasing temperature.
So, we can conclude that the level of recombination at low voltages is directly proportional to the temperature, and at high voltages it is inversely proportional. And we should also mention that a peak of the recombination level is observed at a certain value of the voltage, and we think it would be appropriate to pay attention to this process in the next work. Recombination in p-Si/n-CdTe heterojunctions affects device performance greatly. Reducing SRH impact is crucial for improving device performance as it dominates the recombination mechanism. Different methods have been used to decrease recombination's effect on device performance, such as passivating surface states, optimizing doping, and engineering interfaces. Techniques improved p-Si/n-CdTe device performance; more research can boost future performance.
Reference
[1]. Chin-Tang Sah, Robert N. Noyce, William Shockley, “Carrier Generation and Recombination in P-N Junctions and P-N Junction Characteristics”, 1957.
[2]. S.M. Sze, “Physics of Semiconductor Devices”, Third Edition, 2007.