Introduction
Due to the steady development of GaN-based semiconductor light sources, conventional light sources of the visible spectral range are already replaced by semiconductor light sources in various applications. Whereas LEDs of the blue spectral range have an electro-optical efficiency of up to
The emitters of the ultraviolet spectral range can be processed by using composite crystals based on the binary III-V-compound semiconductors InN, AlN and GaN [8]. UV LEDs below
For UV-A semiconductor emitters with a peak emission wavelength above
Furthermore, photons generated within the active region could be absorbed in surrounding GaN layers of the semiconductor structure, significantly reducing the radiant flux of the device [14]. As a consequence, the reduced efficiency results in an increased self-heating of the semiconductor structure affecting the lifetime of the emitters [13].
Unlike the numerous publications analyzing the degradation behavior of blue and phosphor-converted LEDs, only a few papers were published regarding the degradation mechanisms and behavior of In1-xGaxN–based UV-A LEDs [13], [15], [16], [17], [18], [19], [20], [21]. Compared directly with emitters of the blue spectral range, a significantly shorter lifetime of ultraviolet or near-ultraviolet structures can be observed [3]. The decrease in optical power can be caused by the formation of point defects, which can be traced back to previously studied GaN defects such as gallium vacancies, nitrogen antisite defects, or carbon impurities [13]. Additionally, energetic ultraviolet photons could provoke degradation mechanisms in the packaging of the semiconductor devices. Especially silicone encapsulated emitters are exposed to package-related degradation, occurring by yellowing or cracking of the silicone lens [15], [17]. As a result, the ultraviolet emitters can be expected to undergo a variety of possible aging mechanisms, which can be observed in both the LED’s package and the semiconductor’s structure significantly affecting the lifetime of the device [21].
To describe the occurrence of aging mechanisms and their effect on optical power loss, it seems obvious to use established lifetime modeling approaches for LEDs of the visible spectral range. These models forx luminous flux depreciation have been investigated in numerous studies and became standardized. Therefore, the Illuminating Engineering Society of North America (IES) recommends to perform LED lifetime projections on aging data collected under conditions described in the LM-80-20 standard [22]. By defining stress test conditions, measurement intervals and test methods, a harmonization of data collection should be ensured. Subsequent lifetime predictions are performed using the standardized methods described in TM-21-19 [23]. Respectively, an exponential function is fitted to the dataset collected for a LM-80 stress test period of more than 6000 h. According to TM-21-19, the projection of current or temperature conditions not covered by LM-80 conditions, is achieved by interpolations between the exponential fits using linear functions or the Arrhenius equation. To account for scenarios in which the degradation dynamics cannot be described by the typical exponential TM-21 function, supplementary mathematical approaches are provided in the standard. However, for these supplementary functions, inter- and extrapolations between different temperature conditions or operating currents are not given. To mitigate such weaknesses of the standard, additional approaches were introduced in literature [24], [25] e.g. double exponential functions [26], [27] or modified logarithmic functions [11].
In addition to simplified curve fitting methods, stochastic methods were established allowing to get probabilistic estimates regarding the device lifetime with variable aging parameters e.g. the IEC 62717:2014 standard [28], Gamma process models [29], [30], [31], Wiener process models [32], Filter and Brownian motion process approaches [33], [34].
Since LM-80 stress tests have to be performed for more than 6000 hours, accelerated test procedures have been developed [34], [35], [36]. These accelerations allow a significant reduction of total testing time and thus their accompanying costs. By performing the tests with higher operating conditions, the degradation mechanisms are expected to be accelerated and thus, a calculation to normal operating conditions can be performed.
Mechanisms accelerated by temperature are typically modeled using the Arrhenius equation [37]. Additional aging parameters, such as current, can be taken into account by Black’s equation, which was initially developed to describe the process of electromigration in semiconductors [38], [39]. Therefore, the time to failure (TTF) is given by the two assumed independent variables of junction temperature
Model parameters such as activation energy
Due to the small number of available reports discussing the degradation behavior of emitters of the violet/ultraviolet spectral range, the degradation dynamics and the physical degradation mechanisms of silicon-encapsulated LEDs with a peak wavelength of
Experimental Setup and Samples
The experiments were carried out on commercially available UV-A high-power LEDs with a rated peak wavelength of
To perform the stress tests, the silicone-encapsulated surface mount devices were assembled on a metal core printed circuit board each. In order to analyze the effect of forward current and temperature on device degradation, the thermal structure functions of the samples were determined with a thermal impedance measurement system (T3ster - Mentor Graphics).
Taking into account the thermal resistances, two different test designs were realized. An iso-thermal test design was used to draw conclusions about the effect of forward current at approximately identical junction temperatures. Therefore, three different forward currents were applied to the devices (
To study the effect of temperature at constant forward current (
The absolute maximum ratings specified by the manufacturer are given with a junction temperature of
The stress tests were carried out on separate temperature-controlled heating plates, whose temperatures were controlled measuring the LED boards’
The optical measurements were carried out using a spectroradiometer (CAS140CT - Instrument Systems) in combination with an integrating sphere (PTFE 30 cm). The system is absolutely calibrated and provides information of the electroluminescence (EL) spectrum and the absolute emitted spectral radiant flux. Measurements above
To study the operating point dependent degradation of the structures, the spectrum was taken at nine different forward currents. A current-voltage characteristic taken at each measurement interval, extends the dimensions of degradation mechanism analysis.
Results and Discussion
A. Degradation Mechanisms
In the following, the physical degradation mechanisms of silicone-encapsulated UV-A LEDs are analyzed before their dependence on aging conditions is evaluated. The degradation behavior shown in Fig. 1 is representative for the samples stressed at a junction temperature of
The results indicate measurement current dependent degradation mechanisms. With decreasing measurement current the effect of optical power loss increases, resulting in a reduction of
Taking into account the electrical properties of the emitters, the previously described increase in optical power is accompanied by several changes in the current-voltage characteristics.
The characteristics shown in Fig. 2 can be divided in three sections. Section I represents the low-injection regime, indicating a voltage drop within the first measurement interval. The initial decrease is followed by a slower but gradual increase in parasitic conduction mechanisms, primarily affecting the low-injection regime. Operating points in Section II and III (If >0.5mA) are driven by competing mechanisms. For forward currents in Section II (0.5mA< If <100mA) an analogous, but less intense initial voltage drop is observed, indicating a maximum at
The time-dependent forward voltage for
Analyzing the radiant flux depreciation in detail, a square root time dependence can be identified for
Examining the LED package, no degradation of the silicone encapsulation can be detected, manifested by a constant peak wavelength, not shown here.
Based on the previously presented results, the degradation of the devices can be separated in two modes:
The initial increase of radiant flux accompanied by a forward voltage decrease can be attributed to an Mg acceptor activation in the p-side. The higher conductivity of the p-type region, increased hole injection efficiency and reduced Schottky-barrier at the p-contact result in an increase of optical radiant flux within the first 26 hours of operation [43], [44]. In particular, the increased hole injection efficiency leads to an increase in optical power, affecting all investigated operating points equally. Synchronously, the low-injection regime indicates an increase of point defects in and around the active region [45]. These point defects could act as non-radiative recombination centers and promote parasitic conduction mechanisms by trap-assisted tunneling processes (TAT). Especially the current-voltage characteristics shown in Fig. 2 indicate increasing parasitic currents in the low-injection regime, that could be promoted by TAT. As the defects become saturated with higher current density, operating points above
are primarily affected by Mg acceptor activation. In this range, the resulting voltage drop increases linear with forward current and reaches a maximum atI_{\mathrm {f}}= {\mathrm {1~ \text {m} \text {A} }} , not shown here. Operating points above{\mathrm {30~ \text {m} \text {A} }} are primarily driven by an increase of series resistance, becoming the dominant mechanism at higher operating currents.{\mathrm {30~ \text {m} \text {A} }} The subsequent degradation for
>26 h is characterized by a gradual decrease in radiant flux indicating a square root time dependence. In addition, the ideality factor and forward currents in the low-injection regime also show a comparable degradation dynamics. According to various literatures, such dependencies suggests diffusion processes to be involved [44], [46], [47], [48], [49], [50], [51]. Hydrogen or Mg-dopant atoms migrate from p-doped layers to the active region resulting in an increase of non-radiative recombination accompanied by parasitic current paths. In violet LEDs stressed by Nam et al. [52] a magnesium back diffusion from the p-doped layers was observed, indicated by a broadening of the initial doping profiles. The diffusion of Mg atoms into the active region was suggested to be the major mechanism for optical power loss. Moreover, the diffusion of Mg is much more pronounced in LEDs grown on sapphire substrate compared to layers grown on free-standing GaN, suggesting that dislocations promote the diffusion of point defects. Nevertheless, it should be noted that the exponential radiant flux decrease observed by Nam et al. does not coincide with the square root dependent results shown in Fig. 5. Additionally, the observed Mg diffusion processes have never been confirmed by other studies. In high current density experiments (3 kA cm-2) performed on InGaN laser diodes, no significant change in the Mg concentration profile was observed [53].t Therefore, the migration of hydrogen accompanied by a de-hydrogenation of
-H3 defect complexes seems to be a more appropriate explanation for the generation of point defects within the active region [44].\text{V}_{\mathrm {Ga}}
As a result of the manufacturing process, the hydrogen concentration follows the magnesium profile in the p-doped layers. The incorporated hydrogen passivates both acceptors and negatively charged point defects. Various studies in (In)AlGaN LEDs revealed a hydrogen diffusion from the p-doped layers to the n-side during operation, leaving behind negatively charged point defects [44], [54]. Such point defects could act as non-radiative recombination centers and assist parasitic conduction mechanisms in the low-injection regime. According to Nykänen et al. [55], the required energy to remove hydrogen atoms from
Consequently, different interactions can be considered for the removal of H from
Temperature-induced activation of defects: Considering a junction temperature of
a thermal energy ofT_{\mathrm {j}}= {\mathrm {70~ ^{\circ}C}} can be assumed, allowing to rule out a temperature-driven de-passivation of the defects.{\mathrm {29.5~ \text {m} \text {eV} }} Interaction with hot carriers: According to Iveland et al. [57], the kinetic energy spectrum of electrons in InGaN LEDs shows a local maximum at
, supporting a de-hydrogenation of the defects by collisions with hot carriers. Due to the correlation between optical power decay and the inverse of the cube of the stress current density Ruschel et al. [11] proposed Auger recombinations to be involved in the generation of hot carriers in{\mathrm {2~ \text {eV}}} UV-B LEDs.{\mathrm {310~ \text {n} \text {m} }} Auger-driven recombination: Recent studies [58], [59], [60], [61] suggest a new type of Auger-driven recombination process. The process is different from classical Auger recombination due to its dependence on trap concentration. As a result of its quadratic dependence on carrier density and its relation with defect density, this process could drive the degradation of optoelectronic GaN structures. Since this recombination mechanism has been proposed recently, its effect on optical power depreciation has not been fully investigated [61].
Photo-induced effects: Results presented by De Santi et al. [62] demonstrated the formation of point defects in irradiated InGaN LEDs using a
laser and an irradiance of 361 W cm−2. Similar results were reported by Caria et al. [63] in open-circuit conditions. Consequently, it should be considered that{\mathrm {405~ \text {n} \text {m} }} -\text{V}_{\mathrm {Ga}} defect complexes could also be de-hydrogenated by the energetic short wavelength radiation (\mathrm {H}_{\mathrm {n}} ).{\mathrm {365~ \text {n} \text {m} }}\approx {\mathrm {3.4~ \text {eV}}}
Accordingly, a possible scenario is the
Due to the partial linear correlation between low-injection forward current increase at e.g.
The negatively charged point defects left behind in the p-side would result in acceptor decompensation, explaining the increase in series resistance. Since the change of the series resistance deviates from the square root dependence of radiant flux depreciation, it can be assumed that the change of
These deviations could be explained by a worsening of metal/semiconductor interface at the p-type region, described by Meneghini et al. [65]. As a result of hydrogen diffusion from the passivation layer to the immediate proximity of the p-contact Mg acceptors are passivated, whereas acceptors of the bulk remain unaffected. Therefore, the reduced conductivity of the Schottky contact seems a plausible origin of different electrical and optical degradation dynamics being observed in the high-injection regime.
In conclusion, it is not possible to identify the underlying gradual degradation mechanisms in detail, rather, promising scenarios are presented likely explaining the observed device behavior.
Above junction temperatures of
For an aging current of
Based on the aging data, it can be concluded that lens cracking is followed by a progression of crack formation, successively contributing to a step wise reduction of the radiant flux. In detail, the observed crack formation affects the reduction of optical power to a different extend, varying with measurement current. A change in the light extraction efficiency
Fig. 8 shows the external quantum efficiency for different measurement currents. Under the premise that the operating point at
According to Fig. 8, the further progression of the crack formation manifests that the decrease of the external quantum efficiency after 8665 hours cannot be exclusively explained by a change of transmission properties of the silicone lens.
A better understanding of the device degradation can be achieved under consideration of the current-voltage characteristic shown in Fig. 9. The abrupt change in forward voltage observed here, occurs synchronously with crack formation and suggests an additional electrical degradation mechanism of the device. The additional process is characterized by the formation of a parasitic conductive path shorting the active region. With stress time the conductivity increases and indicates, due to its logarithmic progression in Fig. 9, an ohmic behavior. After 8665 hours of stress, the effect of the reduced parallel resistance
By removing the silicone lens, the chip surface can be assessed. Fig. 10 (a) shows the chip surface representative for the stressed devices. The highlighted area is close to the electrical contacts and indicates surface damage in the indium tin oxide (ITO) layer. The additional image of the structure under forward bias with
In particular, the area around the defect shown in Fig. 10 (a) suffers from a lower radiance, indicating a significant current flow within the device. Thus the electrical degradation, which occurs synchronously with crack formation, contributes primarily to the decrease in optical power.
Crack formation within the primary lenses can be attributed to bond breaking and embrittlement of the polydiphenylsiloxane used for the LED’s encapsulation [66]. While covalent Si-O bonds have an average binding energy of
For the devices studied, the metallic n-contacts are located on top of the semiconductor structure placed on a layer of indium tin oxide (ITO) [21]. According to Singh et al., the formation of ohmic conduction mechanisms is due to the migration of metal atoms into the active region [71]. The origin of the metal atoms can be both, the electrical contacts and the ITO layer itself [72]. A diffusion of moisture, oxidizing substances and corrosive gases through the silicone lens could result in a dissolved ITO layer by electrolytic corrosion. Therefore, with the appearance of the lens cracking the underlying passivation layer becomes vulnerable. Possible effects of delamination accompanying the process of lens cracking could affect the fraction of moisture prevailing at the chips’ surface. Emerging electrolytic corrosions separate metallic indium migrating through a damaged passivation layer, finally shorting the active region. The increasing conductivity of the ohmic leakage channel shown in Fig. 9, is due to the ongoing chemical reduction of ITO, resulting in an agglomeration of metallic indium atoms at the leakage path. The self-heating of the resulting shunt accelerates the degradation of the device itself.
In conclusion, the degradation of the silicone encapsulated UV-A LEDs in this temperature range is due to the occurrence of radiation-induced cracks in the primary lens, which is followed by processes of electromigration. The extent to which the device temperature and the forward current contribute to an acceleration of these degradation mechanisms will be discussed in the following.
B. Temperature Dependency of Degradation Mechanisms
The temperature dependency of the degradation mechanisms and the associated reduction of optical power for 8665 hours of stress is shown in Fig. 11. With increasing junction temperature, the time to failure is reduced, indicating a significant temperature dependence of the catastrophic failure. In addition, further embrittlement of the silicone and electromigration after cracking is temperature dependent, manifested by a faster reduction of optical power with increasing temperature. Temperature acceleration of gradual optical degradation caused by defect generation within the semiconductor can only be analyzed to a limited extent, especially since the degradation is dominated by lens cracking and the accompanying processes of electromigration.
Defining the occurrence of crack formation as failure criterion of the device, the averaged lifetime results according to Fig. 12. Depending on junction temperature, there is an exponential decrease in time to failure, that could be described using the reciprocal Arrhenius equation. The occurrence of the failure criterion
According to Fig. 12 an activation energy of
Due to the activation energy of
C. Current Dependency of Degradation Mechanisms
For a junction temperature of
The non-thermal accelerated degradation can be described using the inverse power law, whereas the time to failure is calculated as a function of aging current
Using a linearizing transformation, the exponent
The coeffcient
Despite the fact that the transformed lifetimes can only be described to a limited extent using a linear curve fit, it must be taken into account that an equal distribution can be assumed within the uncertainty intervals. Consequently, a linear regression can be considered as appropriate.
In direct comparison with LEDs of the visible spectral range, it must be noted that general lighting white LEDs indicate
D. Lifetime Modelling
Considering the dominance of primary lens embrittlement and crack formation, the modeling of gradual semiconductor degradation is obsolete unless a working range can be defined within no damage to the silicone encapsulant is to be expected. The definition of crack formation as a failure criterion allows the modeling of the lifetime of the silicone encapsulant and thus of the entire semiconductor device. Based on the measurement data collected at different aging conditions, a lifetime model is derived, allowing to calculate the
The lifetime model is based on Black’s equation and therefore on a multiplication of temperature
Alternatively, the lifetime can be calculated using
The coefficient
Using the surface diagram shown above, the lifetime can be determined as a function of operating parameters.
According to Fig. 15, a lifetime of more than 10000 hours can only be expected for a small range of aging conditions. Therefore the LEDs should ideally be operated below
The maximum junction temperature
In order to validate the model derived from the measurement data shown in Fig. 15, an additional data set is analyzed with respect to its consistency with the model prediction. The test data set is gathered analogous to the previously used aging data, and therefore the measured lifetime represents the average of four individual samples. For an aging current of
For the aging condition at
To evaluate the effect of temperature/power cycling due to the electro-optic characterizations, two additional devices were stressed for an interval of 300 hours at
In summary, the temperature dependence of crack formation can be described using the Arrhenius equation, while the current-induced acceleration can be modeled using the inverse power law. Using the combined equation of the dependencies, also known as Black’s equation, the
To which extent the validity of the model equation can be guaranteed beyond the operating range, cannot be assessed on the basis of the current data situation. Additional data sets, currently measured at
Conclusion
Based on the analyzed data of silicone encapsulated UV-A LEDs, we conclude that the lifetime of the devices is affected by aging processes within the semiconductor structures as well as by package-related degradation mechanisms. The generation of point defects within the semiconductor structure is accompanied by radiation-induced crack formation in the primary lens. Occurring lens cracks are enhanced by temperature/power cycling due to the performed measurements, finally promoting processes of electromigration shorting the active region. The processes are accelerated by increasing thermal and electrical stress parameters, whose dependencies can be described by the Arrhenius equation and the inverse power law. A multiplicative combination of the functional dependencies, also known as Black’s equation, allows lifetime and SOA modeling, the validity of which could be confirmed with independently collected data sets. The failure mechanisms studied in the stress tests can be used by manufacturers and research groups to optimize future generations of ultraviolet devices with regard to their lifetime and reliability. To separate the observed aging mechanisms, additional experiments will be carried out on identical devices without encapsulant.
ACKNOWLEDGMENT
The work reported in this paper reflects the author’s view and that the JU is not responsible for any use that may be made of the information it contains.