Section 2 High Power LEDs

## **Chapter 3**

## High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs

*Li Wang and Hideki Hirayama*

## **Abstract**

Presently, terahertz quantum cascade lasers still suffer from operations below room temperature, which prohibits extensive applications in terahertz spectra. The past continuous contributions to improving the operating temperatures were by clarifying the main thermal degradation process and proposing different designs with the optical gain demonstrating higher temperature cut-offs. Recent designs have attempted to employ a narrow period length with a simplified and clean state system, and reach renewed operating temperatures above 200 K. This study reveals how historic designs approach such narrow-period designs, discus the limitations within those designs, and show further possible designs for higher operating temperatures.

**Keywords:** high operating temperature, terahertz, quantum cascade lasers, narrow-period design

## **1. Introduction**

The terahertz (THz) electromagnetic spectrum (λ ∼ 600–30 μm, *ħω* ∼2–40 meV) remains the most underdeveloped spectral region, at least largely due to the lack of THz radiation sources with coherent or compact wave features and high output power. The potential for applications in THz spectra is included but is not limited to atmospheric science, astrophysics, biological/medical sciences, security screening and illicit material detection, nondestructive evaluation, ultrafast spectroscopy, and communications technology [1–4]. THz quantum cascade lasers (THz-QCLs) are thought to be the only solid-state THz sources that can offer average optical power levels much greater than milliwatts, which are profitable for imaging applications [5], and continuous-wave (CW) operation for the frequency stability desired in highresolution spectroscopy techniques.

These laser sources escape the inherent prevention of the working principle of conventional lasers, which naturally relies on the bandgap of semiconductor materials to realize electron-hole recombination. In comparison, the THz photon energy has only several or tens of meV, which is inaccessible to interband transitions. THz-QCLs employ the quantum process of intersubband transitions (ISBT) within repeating superlattice period units [6]. The radiation frequency can be freely tuned solely by engineering the energy separations of dispersed quantum levels, for example, only in

the conduction band area. Rapid developments in THz-QCLs have been reported since their first realization in 2002 [5], with the 1.2–5 THz frequency coverage range (operated without the assistance of an external magnetic field [7–9]). Although the initial demonstration of THz-QCLs was performed with the aid of single-plasmon waveguide structures [5], the subsequent development of double-plasmon metal-metal waveguides [10] led to higher operating temperatures (Tmax). Due to the subwavelength dimensions of the emitting aperture in metal-metal waveguides, their emission is characterized by divergent beam patterns and low output powers [11, 12]. However, unique distributed-feedback (DFB) techniques have recently been demonstrated to achieve single-mode operation with narrow beam patterns [13–15]. Until now, the main limitation for THz-QCL is that the operations suffer from serious thermal degradation, and a room temperature without additional cooling is still not reached. **Figure 1** shows a summary of the improvements in Tmax over the past decades [16].

The main thermal degradations can be sorted into two types. The first type is the reduction of population inversions (Δ*n*), which can be due to the reduction of population shares in the upper laser state (*u*, *n*u), and an increasing trend at the lower laser state (*l*, *n*l) as the temperature increases. The former can be mainly ascribed to the directly enhanced scattering of the LO-phonon. In general, the active region of the THz-QCL is based on GaAs semiconductor materials, and its LO-phonon energy (ELO ~ 35 meV) is quite small. THz radiation is mostly designed below the LO-phonon energy. At high temperatures, the electrons in the upper laser state will acquire enough in-plane kinetic energy (Ehot) to relax down to the lower laser state by emitting LO-phonons (Ehot + Eul ≈ ELO) instead of photon emission (Ep≈ Eul) [17, 18]. In addition, the up-scatterings from the upper laser state into the higher energy states or directly over the barrier (in the case of low barrier structures) by absorbing the hot phonons [19, 20] lead to efficient carrier leakages. Both of these phonon-assisted processes significantly reduce the population shares in the upper laser state. In addition, the population share of the upper laser state is closely related to the injection efficiency, which can be significantly degraded by injection at high temperatures via

#### **Figure 1.**

*Survey of the best operating temperature Tmax at various THz lasing frequency achieved based on different THz-QCL design within pulse operation mode, with the permission from Ref. [16].*

## *High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

resonant tunneling [10, 21]. This is because the resonant tunneling at the injector undergoes enhanced decoherence at high temperatures; as a result, more population is retarded at the injector, thus reducing the population share in the upper laser state. The latter can be thought of as a result of an increased lifetime of the lower laser state because the thermal backfilling from the injector areas is more serious at high temperatures [22, 23]. The second type of degradation is the thermal broadening of the radiation linewidth (*Γ*) in the active region, and the peak of optical gain is significantly decreased even when a large fraction of population inversion is maintained [24]. This broadening effect is primarily ascribed to the increased Coulomb scattering [25]. Meanwhile, Ref. [25, 26] also pointed out the importance of considering the correlation effect on those broadening for a self-consistent gain profile calculation, which can partially recover the gain peak. These degradation processes occur simultaneously, limiting the final optical gain (*g*~Δ*n*/*Γ*) below the cavity threshold.

## **2. THz-QCL designs**

The realization of high-Tmax THz-QCLs is indeed beneficial from the corresponding strategies for suppressing the thermal degradation in the active region and, simultaneously, trying to stabilize the real growth to consistently realize the designs. The multiple-quantum-well active region is usually grown using molecular-beam epitaxy in a GaAs/AlxGa1–xAs material system and is the heart of any QC laser. To obtain the gain for electromagnetic waves at frequency *ν*, energy levels, wavefunctions, and scattering rates must be properly engineered to provide a population inversion between two states separated by energy *hν*. The basic innovation for the successful realization of a QCL can be ascribed to the introduction of the injector region. The optical gain from intersubband transitions in semiconductor superlattices was first investigated early in 1971 [27]; however, early laser proposals suffered from electrical instability owing to the formation of high-field domains crossing the stacked superlattices, until the injector region was particularly defined for offering an accessible operating bias condition, thus realizing the first QCL in the mid-infrared spectrum in 1994, as shown in **Figure 2** [28]. Next, the first THz-QCL lasing at 4.4 THz was experimentally demonstrated in 2002, where most innovations can be ascribed to semi-insulating surface plasmon (SISP) waveguides in the THz spectra [5]. Consequently, the injector has remained a common region in QCLs design for THz QCLs. It should be noted

#### **Figure 2.**

*Schematic description of the three relevant levels where E3, E2, and E1 stand for the upper- laser, lower-laser, and extractor states, respectively (a), the diagram of the band profile of cascading structures with neighboring periods under the operating applied bias, the extracted electrons will relax into the graded area and then reinjected into the upper-laser state in the next period by resonant tunneling (b), Ref. [28]; The semi-insulating surface plasmon (SISP) waveguide for realizing the first THz-QCL (c), where the bottom contact layer is a thin heavy doped GaAs (n<sup>+</sup> -GaAs) layer grown on an undoped GaAs substrate, Ref. [5].*

that the performance of THz-QCLs is particularly sensitive to the injector because the small energy separation between the laser states makes it difficult to selectively inject carriers (electrons) into the upper laser state [29, 30]. Therefore, improving injection selectivity has gained much attention for different designs [31–33]. The detailed design parameters are trapped in trade-offs from one to another; a successful scheme always requires a dedicated balance of the different parameters.

The main design schemes in the past are shown in **Figure 3a** *chirped superlattice (CSL)*, as shown in **Figure 3a**. The CSL scheme is a main early design, and it employs several quite narrowing quantum wells in which the ground states are raised. When the operating bias is reached, the ground states form "minibands," the lowest state of the pre-minibands is appointed as the upper laser state, and the topmost state in the down-minibands is set as the lower laser state. The electrons are inverted between them and perform the THz photon radiation. Owing to the tight couplings inside the miniband areas (by intra-miniband scattering), electrons always relax quickly to the bottom of the minibands, where the upper laser state is located, the upper laser state is filled, and the lower laser state is depopulated. b) *Bound-to-continuum (BtC)*: As shown in **Figure 3b**, this scheme also employs minibands, and the lower laser state is depleted by intra-miniband scattering, similar to the CSL scheme. The main difference

#### **Figure 3.**

*Conduction-band and wavefunctions of each state for different representative THz-QCL design. (a) CSL; (b) BtC; (c)hybrid BtC; (d) one-well injector RP, that permitted from Ref. [34]. (e) two-well extractor RP, from Ref. [35]. (f) three-well RP, from Ref. [36]. (g) and (h), two-well RP, from Ref. [37]and [38]; (i) SA from Ref. [39].*

### *High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

is in the upper-lower state, where it is positioned diagonally with a lower laser state. This diagonality of radiative transition contributes to an increase in the lifetime of the upper laser state and weakens the parasitic overlap of the wavefunctions between the injector and the lower laser state. Therefore, the BtC design displays an improved Tmax and output power compared with the CSL design. In addition, an improved BtC scheme with hybrid structures is proposed in which the phonon-assisted depopulation has been incorporated into the miniband areas, which can further relax the thermal backfillings and further improve Tmax (**Figure 3c**). c) *Resonant-phonon (RP)*: This scheme is specifically indicated for designs in which the injector follows the resonant tunneling mechanism based on a double-level alignment under operating bias. The depopulation process employs LO-phonon resonance, which mostly takes place intra-well. Therefore, the electrons in the lower laser state are scattered down to the injector states by emitting LO-phonons in the sub-picosecond range. There are several RP schemes with different purposes; for example, the one-well injector RP scheme for better injection selectivity by reducing the direct parasitic overlapping between the injector and the lower laser state (**Figure 3d**), or a two-well extractor RP scheme that is similar to the hybrid BtC for offering more tunneling freedom on the design parameters (**Figure 3e**). The first milestone on Tmax is based on a three-well (4-state) RP scheme (**Figure 3f**), including an adjoining-well resonant-tunneling injection and a one-well extractor where its 1st excited state forms resonance alignment with the lower laser state first, and significantly delocalizes the wavefunction of this lower laser state into the extractor well. Consequently, the lower laser state can form strong coupling with the next injector state with one LO phonon energy downward. The further renewed Tmax is also based on RP, but with further simplified quantum structures from three-well to two-well (4-state to 3-state, as shown in **Figure 3g** and **h**), where the depopulation from the lower laser state follows only one step via direct intra-well LO phonon scattering. d) *Scattering-assisting (SA)*: As shown in **Figure 3i**, scattering-assisting design is another important scheme. The main difference between SA scheme and the RP design is the injector region, where the SA employs LO-phonon scattering instead of alignment resonance. Resonant tunneling injection always leads to most electrons waiting at the injector before being injected into the upper laser state. Ideally, in the coherent transport regime of resonant tunneling, the upper laser state holds the same number of electrons as the injector state, which is half of the total available population in each period (i.e., a maximum of 50% share in the upper laser state). Owing to the thick injector barrier and the presence of multiple scattering channels, the population share in the upper laser state generally falls below 50%, even at low temperatures. In the SA scheme, by designing the injector to lie one LO-phonon energy up to the upper laser state, this injection limitation can be completely removed.

## **3. Active-region design by narrowing the period length**

Recalling past designs with improved Tmax, there is a general trend toward reducing the number of states per period (in other words, narrowing the period length by employing a simple quantum structure), and the radiation transition in the active region is strongly diagonal. As shown in **Figure 4**, we followed the structures calculated in Ref. [37] but recalculated them based on our nonequilibrium Green's function (NEGF) model, that is, two-well RP [37, 38], three-well RP [36], four-well SA [39], four-well BtC [35], and seven-well BtC [40]. This indicates an increasing population share in the upper laser state as the total number of states per period is reduced,

and the optical gain (the peak) at high temperatures is also significantly improved. Therefore, it provides evidence of the potential of a simplified state system based on the RP scheme. The two-well RP is further optimized by relaxing the shortcomings in its antecessor design of the three-well RP, a) *the two-well RP only uses the resonant tunneling process once at the injector*. Even though three-well RP has been well studied and gives the best Tmax~ 200 K, a major shortcoming in this scheme is the strict requirement for double resonance alignments at both the injector and extractor region when the bias reaches up to the operating condition. Such a condition is not easy to satisfy in real growth for two reasons. The first reason is the doping issue. High-Tmax designs generally employ strong diagonality in the active region, where the suitable oscillator strength for radiation transition is only 0.25–0.35. Because the optical gain is linearly related to this strength (g∝ *f*), this small strength should be compensated for by increasing the average doping levels [41]. In THz-QCLs, the dopant position is always selected far away from the radiation area and is generally at the center of the lower well with a doping length of only several nanometers for restraining the dopant migration into the superlattice interfaces. However, this local heavy doping can give rise to severe band bending as a result of space charges, thereby breaking the resonance alignment conditions. Second, active-region growth is based on molecular beam epitaxy (MBE). Although precise atomic controls have been achieved, it is still quite challenging to obtain thousands of thin quantum wells/barriers where the thickness of each layer is only several nanometers, and the percentage variation in the growth rate over the whole growth time (>10 h) can further worsen the double alignment condition [42]. In addition, the optical gain profile can be broadened and deformed because the applied bias can closely affect the couplings in the active region; thus, the exact laser frequencies suffer from uncertainties [21]. b). *The depopulation in a two-well RP follows an intra-well direct-phonon scattering*. The intra-well

#### **Figure 4.**

*Calculated population share the upper-laser state (ρn), the calculated optical gain peak gp, and the Tmax in experiments. The calculations are based on the designs of two-well (3-state) RP [37, 38], three-well (4-state) RP [36], four-well(4-state) SA [39], and four-well(5-state) BtC [35], seven-well(10-state) BtC [40].*

*High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

resonance phonon scattering is performed between its 1st excited and ground states in the lower-well, and there is a very strong coupling between them that guarantees a highly efficient depopulation within ultrafast times. Although the energy separation between them can affect depopulation efficiency, it maintains high robustness over a large tuning range of energy separation (30~60 meV [43]). c). *The barriers in twowell RP are much taller*. The tall barrier, by using Al0.25Ga0.75As [37] and Al0.3Ga0.7As [38], can confine the electrodes deeply at the desired states; thus, suppressing the carriers leaking from the confined states over the barriers into the continuum at high temperatures. In a tall barrier design case, careful quantum structure engineering for clean n-clean systems can further minimize the thermal up-scattering from the desired states into the high-lying nonrelevant states. d). *The two-well RP intentionally uses larger depopulation energy for the LO-phonon resonance*. The depopulation energy at the extractor in the high-Tmax two-well RP is 42 meV in Ref. [37] and 52 meV in Ref [38], which is higher than the previous designs with that exactly equal to one LO-phonon of 36 meV. Although Ref. [38] emphasized the benefits of nonrelevant high-confined state engineering, the contribution of the higher depopulation energy to suppress thermal backfilling is at least equal. In addition to the RP scheme, the narrow-period concept is also attractive for the SA scheme; however, the SA scheme based on a two-well structure has still not been realized in experiments.

## **4. Limitations in narrow-period active-region design (RP and SA scheme)**

#### **4.1 Two-well RP design**

#### *4.1.1 Intra-period thermal up-scattering via non-relevant states*

The significance of the high-lying nonrelevant states on high-Tmax has been demonstrated [19, 20, 44] and indicates that the up-scatterings that originate from the upper laser state into those nonrelevant states in the same period will play a significant role at high temperatures. The up-scattering activation energy Ea was extracted by fitting the Arrhenius plots from the measured device output power by [44]:

$$\ln\left(1-\frac{P\_{\text{out}}\left(T\right)}{P\_{\text{out}\left(\text{max}\right)}}\right) \approx \ln\left(a\right) - \frac{E\_a}{KT} \tag{1}$$

As shown in **Figure 5**, an Ea of 35 meV is extracted, which presents the thermal up-scattering occurring between the upper laser state and the 1st nonrelevant in the lower well (the 2nd excited state from this well). Following this up-scattering channel, owing to the relatively low activation energy, as the temperature increases, the electrons can escape into the continuum quickly because this nonrelevant state is close to the continuum, which results in a sharp increase in Jth at temperatures above 100 K (**Figure 5f**). Therefore, a tall barrier is essential to confine the upscattered electrons deeply or to increase the up-scattering activation energy. The validity of a tall barrier for suppressing such channels is shown in **Figure 5f** and **g** (As0.15Ga0.85As→Al0.3Ga0.7As), where a much higher up-scattering activation energy is extracted, that is, 35→117 meV. The corresponding Jth is much lower at high temperatures, and Tmax improves from 121 to 173 K.

**Figure 5.**

*Band diagram of the two-well RP design by tuning the barrier height. The corresponding light output at different temperatures are shown (a–d). The Arrhenius plots with the fitting activation energy and the Jth depending on the temperature is also shown (e–g), Ref. [44].*

#### *4.1.2 Depopulation energy and thermal backfilling*

Ref. [45] shows that at high temperatures, the thermal backfilling from the injector into the lower-laser state is the main source of the population in the lower-laser state; therefore, dramatically reducing the population inversion. The thermal backfilled electrons (nbf) approach 20% of the population share at 300 K, as shown in **Figure 6a** and **b**, which almost equals to the population of the upper-laser state population; thus, non-inversion can be formed. There are two reasons for this: First, the RP scheme relies on resonant tunneling for electrons injected from the injector, where most of the population resides at the injector; therefore, playing the role of an electron pool for thermal backfilling. Second, the GaAs material has a small LO-phonon of 36 meV. The RP design normally uses depopulation energy that is equal to one LO-phonon; this inherently small depopulation energy encourages the occurrence of thermal backfilling. This also indicates the significance of exploring large LO-phonon energy materials such as GaN [46] and ZnO [47] to suppress thermal backfilling.

If the designs are still based on the GaAs material, enlarging the depopulation energy is an effective method for suppressing the thermal backfilling, and large depopulation energy can further reduce the parasitic coupling of the upper-laser state

#### **Figure 6.**

*Band diagram in two-well RP design where the population distribution at each state is given at high temperatures(a); The thermal backfilled population from the injector into the lower-laser state (nbf) is particularly (b), Ref. [45].*

*High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

#### **Figure 7.**

*Averaged electron-LO phonon scattering rate between the 1st excited state and the ground state by varying the energy separation between them (a); A representative cross-section of the figure a (b), Ref. [43]; Plot of light output versus current density at different temperatures. Inset: plot of threshold current density versus temperature (Jth–T) with Jc and Jo as fitting parameters. The black line is an exponential fit (c); Plot of voltage versus current density (J–V) at different temperatures. Inset: a lasing spectrum taken at 246 K (d), Ref. [38].*

directly with the next injector; therefore, protecting the lifetime of the upper-laser state by preventing parasitic LO-phonon scattering between them. However, the depopulation energy deviating from one LO-phonon slows down the depopulation process, thus increasing the lifetime of the lower-laser state and reducing the population inversion. Ref. [43] estimated the average scattering rate for intra-well LO phonon resonance between the 1st and 2nd states in a single well based on a self-consistent Schrödinger–Poisson solver (**Figure 7a** and **b**). The exact energy separation between them was tuned by varying the well width. It is evident that at a high temperature of 300 K, the depopulation efficiency (here simply assumed by the scattering rate) is reduced by 33% when large depopulation energy of 60 meV is selected. Considering the highest Tmax in Ref. [38] (**Figure 7c** and **d**), they employed a larger depopulation energy of 52 meV and the improved characteristic temperature of Jth in this Ref. was also largely ascribed to the suppression of thermal backfilling, together with the thermal up-scatterings via high-lying nonrelevant states.

#### *4.1.3 Interperiod interactions via nonrelevant states*

The two-well configuration inherently has a narrow period length such that the parasitic interactions in neighboring periods can be more serious than in the wideperiod design. Furthermore, a narrow period also leads to a stronger electrical field that will lower the nonrelevant states in energy downstream. Ref. [48] addresses the critical effect of nonrelevant high-lying nonrelevant states on the laser threshold current by forming a resonant-tunneling-like channel between three neighboring periods. The

#### **Figure 8.**

*Jmax and Jth depend on the temperatures for only desired states [i, u, l] (black curves) and more high-lying nonrelevant states [i, u, l, h<sup>1</sup> , h2 ] (red curves) (a); The light output measured devices based on this 2-well RP design. The Jth at different temperatures are extracted from those lasing curves and are shown in figure a (b); The changing magnitude of Jmax and Jth (*∆*Jmax,* ∆*Jth) by including or excluding the nonrelevant states in the calculations(c); The laser dynamic of Jd depending on the temperatures in both the states inclusion(d), Ref. [48].*

#### **Figure 9.**

*Threshold current Jth mappings that resolved by growth direction and energy at both low and high temperatures (100 K, 270 K) for the desired states [i, u, l] and more high-lying nonrelevant states included [i, u, l, h<sup>1</sup> , h<sup>2</sup> ], Ref. [48].*

laser dynamics in current will significantly shrink to zero even at 270 K (**Figure 8**). The spatial and energy-resolved current mapping between the neighboring periods clearly shows the tunneling current along the growth directions (**Figure 9**).

## **4.2 Two-well SA design**

## *4.2.1 More serious inter-period interactions via irrelevant states*

The two-well configuration for the SA scheme shows the main difference is that its upper-well is wider for an intra-well LO-phonon scattering injection, and its lower-well is also wide enough to ensure the lower-laser state down to the upper laser state for THz radiation. Therefore, the high-lying nonrelevant states in the lower well were inherently low in energy, as shown in **Figure 10a**. The current-voltage plots in **Figure 10b** show that, at the operating bias, the peak current increases appreciably when both nonrelevant states 4 and 5 are allowed in the calculation. The inclusion of more high-energy states (nonrelevant states 6, 7, and 8) increases the current density further, but only slightly. Therefore, nonrelevant states 4 and 5, which belong to different neighboring periods, are crucial for serious current leakage. It is visible in the spatial and energy resolved current density mappings in **Figure 10c** (A, B, C, D), and the leakage channel is formed due to a strong interaction of the upper-laser states in the upstream period (state 2*n*–1) with state 4n in period *n* and sequential tunneling to state 5*n*+1 in the downstream period *n*+1 because three of these states are close both in energy and space.

#### **Figure 10.**

*Conduction band diagram and the tight-binding states of two-well SA design with three neighboring periods (n-1,n,n+1) (a); The voltage-current plots at high temperatures of 300 K when the different number of the highlying nonrelevant states are included for calculations(b); The current mapping resolved by the growth direction and the energy when the nonrelevant states are included(c), Ref. [49].*

#### **Figure 11.**

*Band diagram of the two-well SA design in cases of the desired states and the critical nonrelevant state. The optical gain spectra under two different number of stated inclusions. The spatial and energy resolved gain clearly shows the overlapping of the parasitic absorption and the gain area, Ref. [50].*

### *4.2.2 Parasitic absorption overlapping gain*

In the two-well SA design (**Figure 11**), there is a strong decoupling between population inversion and optical gain, in which the population inversion does not change significantly when the high-lying nonrelevant states are included, but the peak of gain is significantly limited by those high-lying states. The reduction of the gain peak is obvious at a low temperature of 50 K. This was ascribed to the emergence of interperiod parasitic absorption, which was caused by transitions between the injector and the first nonrelevant states in the lower well (**Figure 11**). This parasitic absorption unavoidably overlaps with the optical gain due to the engineering limit permitted in a simple quantum structure. This overlap was more severe when the lasing frequency exceeded 3 THz in the two-well SA design.

## **5. Further narrow-period designs**

### **5.1 Split-well three-state RP design**

The split-well direct-phonon concept described in Ref. [51] was proposed by using only the ground states to push up the nonrelevant states for a clean-state system, while keeping the depopulation energy almost equal to 36 meV for ultrafast extraction. In this design, each period contained three wells and employed the ground states in each well. Due to all three wells being narrow, the total periodic length is still quite

*High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

#### **Figure 12.**

*Band profile and the wavefunctions in the "split-well" design (a); Pulsed light output versus current measurement(b); The threshold current vs temperature(c); Activation energy extracted from the laser's maximum output power (Pmax) vs temperature, and the current dynamic range ∆Jd=(Jmax–Jth) vs temperature (d), Ref. [51].*

small as a "narrow-period" design. Compared with the "all-ground" design from Ref. [52], the main advance in this split-well design is the very strong depopulation couplings between the lower-laser state and the injector, where the "all-ground" design follows a diagonal depopulation process and suffers from limited laser dynamics. As shown in **Figure 12**, the results demonstrate that "split-well" lasers profit from both eliminations of up-scatterings via high-lying nonrelevant states and resonant depopulation of the lower laser states. A negative differential resistance was observed at room temperature beyond the operating bias conditions, indicating that the state system performs as a clean 3-state system, where a high characteristic temperature of Jth is fitted out. However, the difficulty of this design is that the splitting at the extractor closely relies on the alignment, and the ultrathin extractor barrier, which is only 3.5 monolayer, is not easily obtained consistently during the long-time MBE growth.

#### **5.2 Two-well double-phonon RP designs**

The design in Ref. [53] mainly considers the effect of thermal backfilling on the lower-laser state by using large depopulation energy of more than one LO phonon. This is different from previous designs; the lower-laser state is appointed by the 2nd excited state in the lower well. Simultaneously, the 1st excited state from the lower well is also a depopulation destination, and as a result, three transitions are responsible for the depopulation, that is, 3→2, 3→1, and 2→1, as shown in **Figure 13a**. The tuning of the energy separation between 3 and 2, E32, and also between 2 and 1, E21, was studied, revealing that only by maintaining the total energy separation

#### **Figure 13.**

*Conduction band diagram and the wavefunctions in a double phonon intrawell depopulated based on a two-well structure(a); The calculated optical output versus temperature by tuning the depopulation energy with singleand double-phonon(b), Ref. [53].*

between the lower laser state and the next injector, E31, more than a double phonon, the thermal backfilling can be well suppressed and thus shows a cut-off temperature above 300 K (**Figure 13b**). The challenge with such large depopulation energy is that it requires a much taller barrier, and the operating electric field is very strong, which causes many difficulties in device realization.

### **5.3 Two step-well SA design**

As described above, in a two-well SA design, the high-lying nonrelevant state (the 1st excited state in the lower well), *h*, as shown in **Figure 14a**, is naturally close to the desired states and forms a serious parasitic absorption overlapping the gain

#### **Figure 14.**

*Improved two-well SA design by employing ministeps (Al0.05Ga0.95As) where the tall barriers of Al0.05Ga0.95As are used(a); Comparison of optical gain spectra at 300 K in this improved design (red curve) and the regular two-well SA design. Both spectra are predicated by considering the high-lying nonrelevant states (u, l, i, h) (b), Ref. [50].*

## *High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

areas. Consequently, the peak gain was largely limited even at low temperatures. Ref. [50] attempted to narrow the upper well, where the lower well can be correspondingly narrower. Therefore, the irrelevant state *h* moves upward. To satisfy the energy separation between injector states *i* and the upper laser state *u* close to the LO-phonon, a ministep was intentionally introduced into the upper well. By simply tuning this ministep, the energy separation of the injector state *i* and the upper-laser state *u* can be maintained at 36 meV. To achieve the same THz frequency of 3.7 THz, the upper well is narrowed by 13% compared with the regular two-well SA design (16.2→14.2 nm), and the lower well is also narrowed by 33% (10.6→8.18 nm). The irrelevant state *h* is thus effectively moved upward, and the energy separation Eih is significantly increased from 21 to 59 meV. The optical gain spectra in **Figure 14b** show complete suppression of overlapping from this parasitic absorption and recovery of the peak gain.

## **6. Strict request on MBE growth controls for the narrow-period designs**

The simple structure requires more precise control of the growth because each layer performs multiple functions as the design parameters. The recent highest Tmax emphasizes high-quality growth on repeating periods and interfaces. It is very challenging to grow thin layers, especially in the case of tall barriers with several monolayers. The interface flatness, doping controls, and the period repeated during the long-time growth are tough topics for start-of-art designs. Meanwhile, uniformity across a 3-inch substrate is also critical, even for optimizing the rotations carefully, and an arbitrary thickness error can still be approximately 2%. Reliable and repeatable growth is essential for dedicated design parameters for a higher Tmax (i.e., even within a very small range of parameter balances in such designs).

## **7. Summary**

Despite the extensive application potential of the THz spectrum, light radiation sources remain the most urgent, where the most expected is a compact solid-state device analogous to conventional semiconductor lasers. The THz quantum cascade laser has been treated as the most attractive candidate to fill the THz gap and was soon realized in the THz range after the experimental demonstration in the midinfrared range. However, this type of THz laser suffers from thermal degradation that cannot work at room temperature until now. Different models attempt to describe the quantum transport in THz-QCLs and clarify the thermal limitations, then predict the designs with high-temperature tolerances. The design for hightemperature operation follows a path by simplifying the containing quantum well structure in each period; however, such a narrow period requires careful engineering of the barrier and requires the control of the high-lying nonrelevant states to suppress any inter-period parasitic channels (for example, creating a clean state system). Different strategies are needed depending on the injection method (resonant tunneling method or scattering-assisted method). In addition, to realize these narrow-period structures, precise control of the MBE growth is essential to ensure the accurate thickness of each layer and the flatness of interfaces with uniform alloyed barriers.

*Light-Emitting Diodes - New Perspectives*

## **Author details**

Li Wang\* and Hideki Hirayama Research Center for Advanced Photonics, RIKEN, Sendai, Japan

\*Address all correspondence to: li.wang@riken.jp

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*High-Temperature Operating Narrow-Period Terahertz Quantum Cascade Laser Designs DOI: http://dx.doi.org/10.5772/intechopen.108317*

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## **Chapter 4**

## Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs

*Abdul Shabir and Cher Ming Tan*

## **Abstract**

Applications of LEDs have increased significantly, and increasing outdoor applications are observed. Some outdoor applications require high reliability as their failure can lead to hazardous consequences. Examples are their applications in automotive, street lamp lighting etc. To ensure the reliability of LEDs in outdoor applications, reliability test that include humidity on the LEDs must be done. However, it is found that accelerated life test of LEDs at high humidity level cannot be extrapolated to standard condition of lower humidity as the mechanism of degradation depends critically on humidity level. In fact, the degradation of LEDs in outdoor applications is mainly due to the degradation of their encapsulation and housing materials (or called packaging material as a whole) instead of the semiconductor chip itself. The decrease in lumen is the results of crack and discoloration of the LED packaging material. Detail understanding of the failure physics of the packaging material for LED under humidity is needed for extrapolation performed at accelerated stress condition so that LED luminary reliability can be predicted. This chapter reviews the different types of degradation physics of the packaging material using ab-initio simulation with excellent verification from experiments. The method of extrapolation is therefore derived from the physics-based model after detailed understanding of the degradation physics of LEDs. The model also provides strategy for industry to prolong the usage of LEDs in outdoor applications, either through materials or operating conditions selection.

**Keywords:** light emitting diodes, failure analysis, LED degradation, silicone, PDMS, density functional theory

## **1. Introduction**

Light is one of the most essential natural elements that has helped humans since early ages, and our dependency on light is very high. With the development of artificial modes of electrically powered lighting and heating devices for indoor as well as outdoor applications, the demand of electrical power now more than ever. About onefifth of the total power is used for lighting [1]. To reduce power usage while maintaining the standard of complexity in lifestyle as well as advancement of technologies, the search for reliable and cost-effective light sources is in demand more than ever.


#### **Table 1.**

*Lighting efficiency and lifetime of some light sources [1, 2].*

With the discovery of high power light emitting diode (LED), LEDs have become the go-to sources for applications in lighting systems. Due to their prolonged lifetime, low maintenance, small sizes and energy efficient properties, LEDs have replaced most of the conventional sources of lighting such as incandescent and fluorescent lamps all over the world. The technology used in LEDs is solid-state lighting (SSL) which provides more energy saving than conventional fluorescent lamps. The comparative study between efficiency and power output of LEDs and other conventional sources of lighting is shown in **Table 1** [1, 2].

Apart from energy efficiency, LEDs also have a longer lifetime compared to fluorescent and incandescent lamps. These devices are compact and have higher mechanical resistance compared to conventional sources. The report by Guan et.al showed nanowire LEDs that were able to bent for −2.5 mm to +3.5 mm radii of curvature while the light intensities was consistent throughout the bending cycle tests [3]. If all the conventional light sources in the world were converted to the LEDs, energy consumption could be reduced by 230 typical 500-MW coal plants, and greenhouse gas emission can then be reduced by about 200 million tonnes [4]. Owing to these benefits, high-power LEDs are now not only used to replace incandescent and halogen lamps in houses, they are also used in outdoor lighting applications such as displays, automobiles, traffic signals, lamp posts, emergency medical services, marine applications, etc. [4–12]. Unfortunately, degradation in LEDs will not only exhibit reduced light output but also change in color which can affect some of their applications. For example, a dimmed LEDs in traffic light might cause road accident to happen. Sudden blackout due to the failure of LEDs in lamp posts can create danger to the pedestrians, and these have indeed resulted in lawsuits against the manufacturers [13–16]. Thus, alongside with energy efficiency and conversion, the reliability of LEDs is of the utmost importance [17], especially our dependence on the lighting is so high.

The environmental stresses for LEDs in outdoor applications, such as humidity and temperature can lead to degradation of output light from LEDs. While reliability tests of LEDs can help to evaluate if they are suitable for various applications, a detailed understanding of the degradation physics of LEDs can provide the root causes of the unexpected degradation of LEDs so that early detection monitoring can be possible, and their lifetimes can also be enhanced for increased effectiveness in energy saving.

The LED lifetime varies somewhere between 50,000 and 70,000 hours [1, 2, 17]. The LED lifetime is measured by means of luminous flux (lumen) maintenance,

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

which shows the fall of lumen of LEDs over time. The Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has defined LED lifetime based on the time to 50% light output degradation (L50: for the display industry approach) or 70% (L70: for the lighting industry approach) light output degradation at room temperature [18, 19].

However, evaluation of lumen degradation at standard operating conditions for LED lifetime assessment is not feasible as the costs of the test and time are too high. To expedite the reliability test of LEDs, accelerated life tests are used to predict the LEDs lifetime and reliability while maintaining exact failure mechanism between operating and accelerated conditions so that extrapolation can be performed. The most common industrial standards used as qualification tests for LEDs are JEDEC and JEITA standards [20]. These standards include reflow soldering test, thermal shock test, temperature cycle test, moisture resistance cyclic test, high temperature storage test, temperature humidity storage test, low temperature storage test, vibration test, and electro-static discharge test.

The degradations of high-power LEDs are mostly due to three external stresses, namely temperature, humidity and electrical stress [21]. A large number of investigations are done to understand and examine their degradation physics under thermal and electrical stresses. On the contrary, the investigations on the effect of humidity related degradation mechanisms is quite low as reported by Singh & Tan [21]. Previously, it was believed that thermal and electrical stress are the most dominant factors in lumen degradation of LEDs. However, when the applications of LEDs are in outdoor or harsh environmental applications, it was found that instead of the LED chip, the materials of LED encapsulation and molding which is comprised of different silicone polymers degrade in presence of humidity that render major lumen degradation of LEDs in their applications. In this chapter, a detailed study of the dynamics of the degradation physics of silicone polymers in LED encapsulation and molding, and their effect on total lumen degradation of LEDs will be presented.

## **2. Identification of humidity related failure mechanisms in high-power LEDs**

The failure mechanisms induced due to humidity or penetration of moisture into LED packages can be classified mainly into two categories: a) Degradation of the Chip which is further split into semiconductor dice and phosphor; b) degradation of the LED package which includes encapsulation of the LED chip or LED chip + phosphor, LED molding and the die attach with heat sink. A schematic diagram of an LED structure is shown in **Figure 1**.

### **2.1 Degradation of the chip including phosphor layer**

To characterize the chip level degradation in the case of white phosphor LEDs, the computation of the blue to Yellow ratio (BYR) of the output light, and ideality factor (n), Reverse saturation current (Is) and series resistance between bond wires with chip and bond pads (Rs) are considered [22]. A method of analyzing the n, Is and Rs of the LEDs from the LED forward characteristics curve was developed by Tan et al. which was adopted for all such analysis mentioned hereafter [23]. The analysis of BYR is done in such a way that if the BYR decreases with time, it means the blue light of the LED chip has degraded indicating semiconductor dice degradation. On the other hand, if BYR increases, the yellow light conversion from phosphor decreases

**Figure 1.**

*Schematic of moisture penetration into a typical LED structure. The red arrow represents the possible moisture diffusion paths into the LED package.*

indicating the phosphor degradation. This phenomenon is further analyzed by observing the percentage change in the ideality factor (n) where we expect n to go out of the normal range of 2.0–7.0 for chip related degradation [23]. For Blue LEDs without phosphor layer, all the above-mentioned characterizations except BYR are done.

It has been seen that moisture penetration from ambient through the LED package has caused serious lumen degradations. This happened either due to the degradation of the chip or degradation of phosphor or both, and degradation of the silicone encapsulation and molding which will be discussed in the next section. An example of the degradation of the chip due to moisture penetration that reduces the lifetime of the LED chip where its BYR drops to half of its original value at 20% rapidly was reported in Ref.s [22, 24]. The intensity of the emitted blue light degrades over time. Upon further analysis of the cause of failure of the semiconductor dice, it was found that the moisture trapped in the die attach or in the electrical contact pad vaporized and the increase in vapor pressure resulted in the crack of the GaN based die [24, 25].

An example of degradation due to phosphor is the work by Tan et al. [24–26]. The BYR was found to increase for some samples which indicated the degradation of phosphor in LEDs. It was found that moisture trapping inside the LED package has caused the dissolution of phosphor which resulted in the overall lumen degradation.

#### **2.2 Degradation of LED package**

It is noteworthy that the humidity test reported until now has a metal reflector below the chip and the effect of moisture on the LED degradation can be identified as drastic [24, 25]. However, with the maturation of chip technology against humidity and the replacement of reflector molding material by the mixture of silicone polymers in high-power LEDs, question on its humidity reliability is re-visited.

To address this question, high temperature-humidity tests at 85°C/85%RH was conducted on OSRAM LEDs according to IPC/JEDEC standard J-STD-020D (2007) [19, 20]. Such test was performed by Tan and Singh [26–28], but they included one more test parameter, namely the drive current of the LEDs, and such reliability tests were termed as moisture-electrical thermal (MET) tests.

The test was performed on two sets of white (with phosphor) and blue (without phosphor) LEDs where one set of each type was kept ON with 350 mA constant current and the other set was turned OFF.

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

A significant difference in degradation between the ON and OFF sets of each type of LEDs was found as seen in **Figure 2**. The white LEDs in the OFF condition did not experience any lumen degradation which is in agreement with reports from Tan et.al [22, 25, 26] where phosphor acts as a protective layer for the LED. However, in the ON condition, white LEDs experience 33% lumen degradation in a span of 144 hours of testing. On the other hand, a difference of 5% lumen degradation was observed between the ON and OFF blue LEDs and the difference increase to 18% after the tests was concluded for each case. It was also found that the BYR of the fresh and degraded LEDs exhibited insignificant changes which omits the possibility of chip degradation as mentioned in the previous section.

Upon closer examination of **Figure 2**, a sharp degradation of lumen is observed, and it is attributed to the moisture entrapment in the silicone encapsulation of the LEDs. After moisture penetrates through the LED encapsulation, two phenomena are most likely to happen viz. trapping of moisture in silicone encapsulation which causes output light scattering and reduced lumen output; and the diffusion of

#### **Figure 2.**

*Percentage lumen degradation vs. test time for blue and white LEDs tested under different operating conditions. (a) Blue LEDs under power-on condition; (b) blue LEDs under power-off condition; and (c) white LEDs under power-on condition. Red line (dotted line) denotes the initial degradation; green line (dash line) indicates the lumen recovery and the blue line (solid line) indicates the final degradation for the LEDs [26].*

moisture to the LED chip including phosphor, thereby causing damage in the chip level and resulting in reduced lumen output as discussed in the earlier section. However, such rapid degradation might also have followed by a recovery if the above two damages were not happened. Furthermore, upon further investigations, it was concluded that in order to prevent such moisture penetration, the pore size of the silicone encapsulation must be lower than 50 nm. A detail description of the work can be found in Ref. [22].

## **3. Failure analysis of humidity reliability of LEDs at OFF state during MET test**

Singh et al. identified three stages of degradation mechanisms during the MET test which has been characterized in **Table 2**. The amount of moisture adsorbed by the LED encapsulation and die attach has also been studied by Singh et.al as shown in **Figure 3** which provide good insights in understanding the degradation mechanism of the LEDs. As the degradation mechanisms of LEDs due to moisture for LEDs at ON and OFF conditions are different as evident from the experimental finding, let us discuss the failure mechanism for the LEDs at OFF condition.

It was found that at the beginning of the test, moisture adsorption in the LED encapsulation is much higher than the die attach area and saturates around 132 hours of testing as shown in **Figure 3**. This initial degradation stage is attributed to moisture adsorption in the encapsulant which causes a "cloudiness" or light scattering effect as mentioned in Ref. [25]. The moisture trapped in the silicone encapsulation causes discoloration of the encapsulant material attributing to the "cloudiness" effect which cause lumen degradation. However, for white LEDs the phosphor layer prevents moisture penetration into the die attach region as well as the heat generated from the phosphor during lumen measurements help to evaporate some moisture in the encapsulant. Hence, no significant lumen degradation was observed.

As the test proceeds over 132 hours in OFF conditions, moisture entered the die attach material that caused subsequent die attach delamination which in turn increased the thermal resistance of the die, renders degradation in the overall lumen output of the LEDs. Furthermore, the moisture in the die attach cannot be evaporated during testing [22, 25], and this renders significant lumen degradation. It was found that moisture penetration is 40% higher in blue LEDs as compared to their white counterparts and this excess moisture is housed in the die attach material, therefore when the blue LEDs were tested for 336 hours, die attach delamination has already occurred [26].

For better understanding of the diffusion of moisture in LED degradation, finite element method (FEM) simulation was performed. The three most important


**Table 2.**

*Stages of degradation in LEDs during 85/85 test at ON and OFF conditions [26].*

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

#### **Figure 3.**

*Amount of moisture penetration into LED package and die attach during testing under OFF condition. The results are computed using finite element analysis as described in the text [26].*

parameters considered for such modeling are the moisture diffusivity D which is a function of temperature, moisture saturation concentration Csat and moisture absorption and desorption times. The moisture diffusion in water permeable body is given in Eq. (1) as (**Table 3**) [30].

$$\frac{\partial \mathbf{C}}{\partial t} = D(\frac{\partial^2 \mathbf{C}}{\partial \mathbf{x}^2} + \frac{\partial^2 \mathbf{C}}{\partial \mathbf{y}^2} + \frac{\partial^2 \mathbf{C}}{\partial \mathbf{z}^2}) \tag{1}$$

A term W, called normalized concentration or wetness was introduced so that thermal modeling can be applied for moisture modeling as moisture concentration may not be continuous across an interface of two materials, unlike temperature. W is given in Eq. (2) as follows [29]:

$$\mathcal{W} = \mathbb{C} / \mathbb{C}\_{\text{sat}} \tag{2}$$

where C is the moisture concentration (kg/m <sup>3</sup> ) in a material.

It was found that during the absorption cycles, the moisture concentration was highest near the die-attach region and during the desorption cycle, the moisture concentration was high in the silicone than ambient. FEM results also indicated that moisture can reach the die attach under 24 hours of test [31, 32].

Tan et.al [28] used FEM to understand the experimental phenomena of initial rapid light output degradation followed by a recovery. The "recovery" is the result of moisture absorbed by die attach material that suck the moisture from the silicone. Thus the "recovery" of lumen degradation is actually associated with the degradation in the internal structure of the LED package which is not reversible.


**Table 3.**

*Correspondence table for thermal/moisture analogy [29].*

## **4. Failure analysis of LEDs at ON state during MET test**

From **Table 2**, it is obvious that the LEDs under power-on condition degrade at a faster rate than the one under power-off condition as expected. Since the LEDs at ON state generate heat and are able to reach a temperature of 135°C, the silicone in the encapsulation and molding start to expand due to excess heat. As the material compositions for encapsulation and molding materials (even though both of them are silicone) are different, their difference in thermal co-efficient of expansion led to interfacial void, cracks and delamination as found by Singh et.al [26]. These delaminated interfaces produce gap for moisture to penetrate preferentially into the LED packaging, causing lumen degradation for both white and Blue LEDs.

The Blue LEDs however were found to undergo a recovery phase of light output after 24 hours of the test and the lumen started degrading again after 48 hours. This phenomenon was studied extensively using FEM simulations and experimental verification through confocal scanning acoustic microscopy (c-SAM) [28]. It was found that in the first 24 hours the moisture trapped at the LED encapsulation caused its discoloration and cloudiness which led to a sharp lumen drop of 12%. However, as the LEDs were turned ON, moisture seeping into the die attach occurred thereby exhibiting a recovery stage during the LED test. After 48 hours, delamination in the encapsulation-molding started to occur due to the heat generated by the LED chip. It led to a higher flux of moisture inside the LED package causing constant lumen degradation hereafter.

In case of White LEDs, the phenomenon of reliability recovery was not found as in the case of blue LEDs, and a total degradation of 33% occurred continuously till 144 hours. To further understand the root cause of this degradation, optical microscope imaging was used to observe changes in LEDs after every 24 hours of testing. The optical images revealed heavy discoloration of the LEDs which was suspected to be in the silicone encapsulation and molding part of the LEDs as shown in **Figure 4** [33].

The discoloration in the molding part and encapsulation of the white LEDs were found to be more severe as compared to blue LEDs due possibly to the excess heat generated by the phosphor layer during the light conversion process [33]. Destructive analysis of the LED encapsulation and molding parts revealed that cracks were observed in both components. However, the cause of such cracks in both encapsulation and molding parts of the LED package were different as reported by Singh et.al [33, 34].

#### **4.1 Analysis of encapsulant degradations**

The surface of the fresh LEDs was found to be smooth, while after degradation, cracks started to appear and flakes or tube like structures were observed on the

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

encapsulation of the blue LEDs. The depth of cracks on the encapsulant was quite minimal where average depth was around 3 μm for blue LEDs and that for white degraded LED encapsulant, it is almost negligible and not able to measure using optical microscope [33].

Scanning Electron Microscopy (SEM) was employed to further investigate the surface of the blue and white degraded LED's encapsulant and compared to a fresh LED encapsulant. SEM images also found cracks on the degraded encapsulants which was consistent with optical imaging results. The flake like structures were more evident on the blue degraded LEDs surface as compared to the white degraded LEDs. Both SEM and optical micrographs shows that the crack density and crack depth are higher in blue LED's encapsulant than the white LED [33].

#### **Figure 4.**

*Optical micrograph of silicone discoloration in (a) white LEDs tested for 144 hours, (b) blue LEDs tested for 356 hours respectively when compared to (c) fresh LEDs. (d) a closer look of the top view of the degraded white LED. Encapsulant and molding parts are also detached from one another for detail examination. Optical micrographs at 500 X magnification of (e) encapsulant and (f) molding part of degraded white LEDs tested under 85°C/85% RH and ON condition for 144 hours [33]. (reproduced with permission from [33]).*

However, if the total rise in temperature of both blue and white LEDs are compared when LEDs are powered on at the same drive current, it can be easily found that the total temperature of the white LEDs are more. The higher temperature of white LEDs is attributed to mainly two reasons; the junction temperature of the LED junction in the die and the heat generated when phosphor layer is activated for conversion of blue to yellow light. Therefore, it is expected that, in comparison to the percentage lumen degradation of both blue and white LEDs, and taking into account the heat generation in white LEDs, the crack in the encapsulation of the white LEDs should be more than blue LEDs. However, this was not the case, and as reported above, the encapsulants of blue LEDs experienced severe cracks than the white LEDs.

Energy dispersive system (EDS) was employed to examine the crack surface of the degraded and fresh encapsulants to provide a better understanding of the nature of crack on the encapsulants. EDS results showed that the percentage increase in oxygen is higher in the case of the blue degraded LED encapsulant as compared to that of the white LEDs. A higher percentage in oxygen element could indicate that the surface of the material has undergone oxidation which can be through a series of chemical reactions. To narrow down the type of chemical reaction which has occurred in the encapsulant surface, FTIR spectroscopic analysis was employed. FTIR analysis indicated that the degraded encapsulant of both the LEDs contained –OH peaks which indicates degradation by hydrolysis. However, the OH peaks concentration was higher in case of blue LEDs than its white counterpart. This means that moisture accumulation in white LED encapsulants was lower compared to that in blue LEDs, and thus less severe hydrolysis of the white LEDs than blue. The moisture concentration of the white LEDs was lower due to excess heat generated by the phosphor layer that drives away the moisture from the surface of the LED encapsulant [33]. The degradation mechanism related to cracks in encapsulation was thus found to be hydrolysis of the silicone polymers of the LED encapsulant [33, 34].

### **4.2 Analysis of molding part degradations**

The degradation mechanisms of the molding part of the LEDs have not been extensively investigated, to the best knowledge of the authors, and we now present detailed investigation of the white LED molding degradation in this section. It was found that alongside with the yellowing of the molding part, cracks also started to develop as the test progressed from 45 to 150 hours. Optical micrograph investigation of the crack depths revealed that the average crack depth was found at 105 hours of testing corresponding to 25–27 μm. When the test progressed towards 150 hours, depths began to decrease by an average of 10 μm. This indicates that different types of chemical changes in the silicone molding must have taken place to observe such changes [34].

This hypothesis was later confirmed by the reports from FTIR analysis where it revealed two sets of exothermic and endothermic reactions in the silicone molding part. It was found that in the initial phases of the test (0–45 hours), the moisture penetration towards the molding part of the LEDs caused the hydrolysis of the silicone polymers which is an endothermic reaction [34–36]. The endothermicity of hydrolysis was further confirmed by the drop in temperature in the LED packages as reported by Singh et al. [34]. The endothermic nature of the hydrolysis reaction was further confirmed by Shabir et.al using density functional theory (DFT) calculations where it was found that 460 kJ/mol of energy was absorbed during hydrolysis of the silicone polymers under study [35]. Hydrolysis reaction was immediately followed by condensation of the silicone polymers which is an exothermic reaction where a

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

raise in temperature was also observed by Singh et.al and others [34, 37]. Shabir et al also verified that condensation is exothermic as DFT calculations show 545 kJ/mol of energy release during condensation reaction [35].

As seen from **Figure 5**, it is evident that in the initial 48–72 hours of test, discoloration of the molding part was prominent, and the development of cracks was negligible. DFT calculations on the hydrolysis and condensation of silicone polymers found that condensation reaction is highly spontaneous as compared to hydrolysis during this time. Also, due to the exothermic nature of condensation reaction, the energy released from condensation is continuously re-absorbed by the moisture molecules in the outer part of the molding, thereby increasing the rate of hydrolysis exponentially after some point of time [35]. It was hence found that the sudden increase in lumen degradation was due to simultaneous hydrolysis and condensation reactions as reported by Shabir et al. [35].

As the temperature rises in the sites of the molding part where condensation has occurred, the decrease in Si-O peak and the increase in H2O peaks indicates that a different chemical reaction occurs where the involvement of H2O is reduced. It was found that silicone network crosslinking can occur producing silicone ring-like structure due to thermal oxidation at temperatures of 0-200°C which is exothermic in nature [34, 38, 39]. The occurrence of thermal oxidation in those sites after condensation at 48 hours of test is believed to lead to crack depth formation which is the dominant degradation mechanism until 105 hours.

The phenomenon of crack recovery after 105 hours was seen. Silicone loses its elasticity after degradation especially in harsh environmental conditions as observed by many researchers in their studies [40]. Thus, possibility of molding part's crack

#### **Figure 5.**

*Optical micrographs of white LEDs with varying test time intervals. The blue colored circle in the fresh sample represents the inner molding part close to LED chip periphery and the green colored circle represents the outer molding part which is far from the LED chip [34]. (reproduced with permission from [34]).*

recovery due to its elastic nature is not valid. Another possibility is due to the diffusion of low molecular weight species such as Si-O (also known as Si oligomers or silica nano-fillers) from the interior of silicone to its surface as it could be invoked by the high temperature of the silicone due to thermal oxidation. The Si oligomers cover the cracks area, renders an apparent crack depth recovery [39]. This is also observed as an increase in Si-O peak in FTIR results as seen in **Figure 6**. EDS results also supports this theory showing an increase in the Si atoms after 45 h of testing appear to confirm the presence of Silicon oligomers. In other words, as cracks are generated due to thermal oxidation, Silicon oligomers can now diffuse out from the crack surfaces, filling the cracks and thus the cracks depth decrease.

As the LED package degrades, the temperature of the device also increases as shown in Ref. [34]. At higher temperature, thermal aging of silicone occurs as evidenced by the increase in Si-CH3 molecules in the FTIR results. Although thermal aging is an endothermic reaction as thermal energy is required to break the chemical cross-linking bonds in the silicone structure, the temperature of the LED does not decrease when thermal aging step in. This could be due to the degradation at either LED chip site or at the die attach as observed by many [24, 26] which can result in additional temperature rise and thus offset the decrease in temperature due to thermal aging reaction. In fact, this additional temperature rises at the LED chip or die attach sites will lead to rapid diffusion of oligomers in the region closer to the LED chip than the area far away from the chip region, and this is indeed observed by Singh et al. [34] where higher amount of crack depth recovery is observed for inner region cracks when compared with outer region cracks from 105 to 150 h duration.

However, the bond dissociation energy of Si-O-Si bond varies between 12 and 162 KJ/mol as reported in many literatures [41–46]. So, if we consider the energy of ambient temperature of 85°C, it only corresponds to 2.97 kJ/mol. If the LED temperature is also added with the LEDs powered ON, it only corresponds to 3.89–5.7 kJ/mol. It is therefore clear that the energy from temperature cannot be the sole driving force for such kinds of degradations.

#### **Figure 6.**

*Overall peaks intensity variation for different samples with varying test time [34]. (reproduced with permission from [34]).*

*Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

To provide detailed understanding of the driving force of the degradation mechanisms as mentioned above, Shabir et.al performed ab-initio calculations on the hydrolysis and condensation of silicone polymers [35]. They found that the activation energy Ea of hydrolysis of silicone was 123.2 kJ/mol while that for condensation it was −6.9 kJ/mol. The Ea of hydrolysis computed in this study was found to be in excellent agreement with the Ea of hydrolysis of silicone polymers present in the LED molding experimentally. The driving force of such reactions was found dominantly from the mixture of blue and yellow light emitted from the LEDs during the test [35]. This also agrees excellently with the test reports where white LEDs in OFF conditions exhibited no lumen degradations as mentioned earlier [26, 28].

As LEDs in outdoor applications are designed to perform for longer periods of time, lifetime prediction of LEDs is done with accelerated stress levels of temperature and humidity know as accelerated life tests (ALTs), and then lifetime at such conditions are extrapolated to estimate their lifetimes under normal operating conditions. The underlying idea of such extrapolation is that the devices under test should follow the same degradation mechanisms in both accelerated and normal operating conditions. The common acceleration models used for reliability test of LEDs are the Peck models which includes the extrapolation of test data from 85%/85°C to normal operating conditions and the Arrhenius equation is included for temperature effect therein. However, it was found by Tan et.al that the degradations are completely different for humidity level varying between 70% RH, 85% RH and 95% RH [24]. This renders the inapplicability of the Peck model for extrapolation of test data to normal operating condition for LEDs. Peck model also does not consider the effect of light energy on the degradation of LEDs. On the temperature extrapolation, Arrhenius equation is commonly used, and the assumption of constant activation energy is made, which indicate that the degradation mechanism is invariant as it is the basis of extrapolation as mentioned earlier. However, our study also found that this is not true. Hence there is a need for a new model which quantitatively covers all the factors for silicone degradation, and this model was developed by Shabir et.al from detailed understanding of DFT simulations and experimental data as well as the of physics of degradation from various reports [35]. The new model related the growing of the area of discoloration Ad with time (t) is given in Eq. (3) below.

$$A\_d\left(t\right) = A\_0 + A\_2 e^{A\_1 t} \tag{3}$$

where

$$A\_1 = \frac{\sum\_{i=1}^{2} \beta\_i P\_i}{\alpha};\tag{4}$$

P1 represents the power density of the blue light and P2 represents the power density of the yellow light. The corresponding β represents the absorption coefficient of the different lights by the discolored region, respectively, which was assumed to be constant in this work. As the discolored region is dark brown in color, it was considered that all the blue light is absorbed, i.e., β1 = 1.

$$\alpha = \left(k\_1 E\_a - \frac{k\_3 T}{V} - \alpha\_{\text{condensation}} \beta\_3\right) \delta \text{ ; Here, \delta is the effective penetration depth of } \delta$$

light. At a given ambient temperature, α is approximately a constant, Ea is the activation energy, kB is Boltzmann constant, T is temperature and V is volume of the silicone molding the meaning of other parameters can be found in Ref. [35].

A2 is related to the quality of the silicone. Specifically, if the silicone has lager pore size or trapped particles that can absorb light, the value of A2 will be larger; *A A* 0 2 = − .

The model fits very well to the experimental data at different light intensities and drive currents as shown in **Figure 7**. It also predicts no degradation if no light is present which was the case of white LEDs with power OFF during testing as reported by Singh et.al [26]. The model allows us to predict LED molding discoloration in the future and simultaneously quantify the quality of the silicone material of the LED molding, which is a factor that will affect the variation of the degradation time of LEDs.

## **5. Conclusion**

Reliability tests and causes of degradation have become important criteria of focus for large scale product manufacturing companies. Proper reliability test and analysis of the related cause of degradation shall help increase the product lifetime and market value. Hence, a descriptive review on the various degradation analysis of LEDs is presented here.

#### **Figure 7.**

*Model fitting with experimental data for 350 mA and 300 mA drive currents [35]. (reproduced with permission from [35]).*

## *Degradation Analysis of Silicone as Encapsulation and Molding Material in High Power LEDs DOI: http://dx.doi.org/10.5772/intechopen.107956*

After detailed analysis using experimental and DFT techniques, it was found that the degradation of the encapsulant and molding part material, both make of silicone, are the primary sources of lumen degradation. The inapplicability of Peck's model was also found as it failed to factor-in the primary driving force of silicone degradation which is the light emitted from the LED chip itself. A new model for prediction of silicone degradation was hence developed which determines the degradation of silicone as a function of time, considering all its driving forces. This model allows prediction of the quality of silicone used for LED packaging which in turn can improve the LED reliability. It can also help to predict the rate of discoloration of the LED which relate to its lumen degradation.

## **Author details**

Abdul Shabir1,2 and Cher Ming Tan1,2\*

1 Center for Reliability Science and Technology, Chang Gung University, Taiwan

2 Department of Electronic Engineering, Chang Gung University, Taiwan

\*Address all correspondence to: cmtan@cgu.edu.tw

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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