Defect identification based on first-principles calculations for deep level transient spectroscopy

Abstract

Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid functional calculations for two important defects in GaN that have similar thermodynamic transition levels, and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and in some cases large in comparison to the ionization energy. By calculating quantities that can be directly compared with experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS.PACS numbers: 72.20.Jv, 84.37.+q Point defects and impurities are present in all semiconductors. They can act as recombination centers that lower the efficiency of optoelectronic devices, or as carrier traps in electronic devices such as transistors. Microscopic identification of the detrimental defects is crucial in order to mitigate their impact. Deep level transient spectroscopy (DLTS) is a powerful technique for determining the properties of defects; from an analysis of electrical measurements on a pn junction or Schottky diode, properties such as the position of the defect level within the band gap, electrical nature (donor or acceptor), density, and carrier capture cross section of specific defects can be obtained. [1][2][3] Translating this wealth of information to a microscopic identification of a given defect requires comparison with theoretical or computational models, and first-principles calculations based on density functional theory (DFT) have proven very helpful. [4][5][6][7][8] One of the key quantities measured in DLTS is the activation energy for carrier emission from a defect, ∆E a . Defect identification is often based on comparing ∆E a with values of the defect ionization energy ∆E i determined from zero-temperature first-principles calculations. However, the underlying theory of DLTS 2,3 makes clear that ∆E a and ∆E i are distinct, and the use of ∆E i can affect the correct identification of a defect.In the present study we describe a first-principles approach to explicitly determine the activation energies measured in DLTS. Recent advances have enabled the quantitative prediction of defect levels in the band gap 9 and the ability to accurately describe nonradiative carrier capture. 10 We will show that ∆E a can significantly differ from ∆E i for some defects, demonstrating the need to explicitly calculate the activation energy in order to correctly identify defects detected by DLTS. Our analysis is general, but will be illustrated with examples of deep defects in GaN...

The complex defect chemistry of antimony selenide

“…3. A recent study found that variation of ionization energies with temperature can cause disparities between defect transition levels and the actual likely activation energies seen in experimental DLTS studies in GaN (highly dependent on the defect itselffor 300 K, a variation of $60 meV was seen for V Ga -O N -2H, while a signicant $250 meV shi was seen for C N ), 31 and as such this agreement between our simulations and DLTS could be fortuitous. In both prior, PBE-based defect studies of Sb 2 Se 3 , however, no transition levels align well with these DLTS levels, even considering a large temperature effect (neither study gives a possible hole trap within 0.3 eV of the 0.71 eV hole trap position, and the closest transition levels are at least 0.1 eV from the rst hole trap)this does seem to demonstrate the necessity for a hybrid functional, as well as a comprehensive study of all possible charge states to properly describe the material's complex defect chemistry.…”

Electrical charge state identification and control for the silicon vacancy in 4H-SiC

“…An upper estimate for this barrier can be obtained by constructing a one-dimensional configuration coordinate diagram for the transition, as explained in refs. 37,38 , i.e., the activation energies for V Si observed by DLTS are predicted to occur within the highlighted ranges in Fig. 3.…”

Mechanisms of Thermal Quenching of Defect‐Related Luminescence in Semiconductors

“…First‐principles calculations predict a barrier δE ≈ 0.49 eV for the capture of holes by the C N acceptor. [ 50 ] As a result, the C p i is expected to increase by two orders of magnitude as the temperature increases from 20 to 600 K. This prediction contradicts the experimental data. Indeed, according to Equation (), the PL intensity before its quenching is proportional to the C p i , and no variation of PL intensity with temperature can be noticed for the YL1 band (Figure 2).…”