Microscopic defect induced slow-mode degradation in II–VI based blue–green laser diodes

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PACS 71.55. Gs, 72.20.Jv, 78.55Et, 81.05.Dz, 81.15.Hi, 85.60.Jb This paper presents slow mode degradation mechanism of ZnSe-based white LEDs, which is induced by point defect. The white LEDs are very high quality devices, of which the optical power at a forward current of 20 mA is 8.3 mW and the luminous efficiency is 28 lm/W, which is comparable to GaN-based white LEDs in the market. It is proven that microscopic point defects in the devices, detected as deep hole traps of the H1 center (E v + 0.6 eV) and H0 center (E v + 0.8 eV) in p-type ZnMgSSe cladding layer, are responsible for the slow-mode degradation. These point defects, related to nitrogen complex centers, are very electrically active under device operation. An important insight of the H0 center are found that the mobility of the defect is extremely high, and then it induces marked interaction with macroscopic defects. IntroductionThe recent commercialization of InGaN-based white light emitting diodes (LEDs) has led to wide spread expectations for their potential applications in areas such as general purpose indication, back light for liquid crystal displays (LCDs) and plug-in replacements for incandescent lamps. The next generation of mobile full-color displays such as a cellular phone will require white LEDs with low power consumption. ZnSe-based white LEDs have several attractive advantages compared to the InGaN-based one. One of the significant advantages is the operating voltage, which is about 2.7V. This value is approximately 1 V lower than that of the InGaN-based LEDs in principle. This result leads to the advantage of power efficiency, that of the ZnSe-based one will be higher than that of the GaN-based one if their luminous efficiencies are equal.The group of Sumitomo Electric Industries have developed the ZnSe-based white LEDs for the application as the back light source of the LCDs [1,2]. The devices utilize a phenomenon unique to ZnSe homo-epitaxial structures grown on conductive n-type ZnSe substrates. A portion of the main greenishblue emission from the ZnCdSe active layer of a pn junction diode is absorbed by the conductive n-ZnSe substrate, which in turn gives off a strong broad yellow emission centered on 585 nm due to the excitation to so-called self-activated center. These two emission bands combine to give a spectrum, which appears white color to the naked eye. This phenomenon was found during the development of ZnSe-based blue laser diodes [3,4]. Wenish et al. also reported the broadband orange emission centered on 600 nm when the conductive ZnSe substrate is excited with the light of photon energy lower than the ZnSe band-gap energy [5].The present white LEDs have several advantages in practical use. One is the low operating voltage ∼2.7 V as described above. And another one is the small scattering of color tone and a large portion of the white

Section: Optical Electrical and Color Characteristics Of Znse-based mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Slow mode degradation mechanism of ZnSe based white LEDs

Adachi

Ando

Abe

et al. 2004

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“…These devices showed such degradations even though macroscopic crystal defects such as stacking faults and dislocations had been reduced to be 10 3 cm -2 level by improving the growth technology of II -VI/III-V interfaces, i.e., it could be realized that no such defect existed in the emitting region. By investigating the degradation mechanisms, it was recognized that the degradation of the ZnCdSe/MgZnSSe devices with no or very low macroscopic defects was ascribed to pre-existing microscopic point defects and their diffusion induced by nitrogen doping in MgZnSSe p-cladding layers [8,9] and compressive crystal strain in the ZnCdSe active layers [10,11]. Also relative weakness of the crystal bonds of these II-VI crystals was a potential cause of the degradation.…”

Section: Originalmentioning

confidence: 99%

Long life operations over 5000 hours of BeZnSeTe/MgZnCdSe visible light emitting diodes on InP substrates

Nomura

Nakai

Hayami

et al. 2006

Aging characteristics of BeZnSeTe yellow light emitting diodes (LEDs) fabricated on InP substrates by molecular beam epitaxy were investigated under direct current injections at room temperature. It was shown that the decay speed of the light output during the aging was slower than that of conventional ZnCdSe/MgZnSSe LEDs. A long lifetime more than 5000 h and a half lifetime of 5180 h were obtained at a current density of 130 A/cm 2 . The half lifetimes of the BeZnSeTe LEDs were about three orders of magnitude greater than that of the ZnCdSe/MgZnSSe LEDs. These results proved high reliability of the BeZnSeTe LED. Investigating the aging characteristics of the applied voltage, the injection current, and the emission spectra showed that the output decay was caused by degradation of the active layer of the LEDs.

“…This high quality device with no macro-defect is found to reveal slow-mode degradation, which involves generation and migration of microscopic point defects [1,[5][6][7]. In order to overcome the slowmode degradation, it is required to accumulate a detailed understanding of the slow-mode degradation mechanism so as to find an artificial-control of the active microscopic centers.…”

Section: Introductionmentioning

confidence: 99%

Slow‐mode degradation mechanism and its control in new bright and long‐lived ZnSe white LEDs

Adachi

Ando

Abe

et al. 2006

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This paper presents slow-mode degradation mechanism of ZnSe-based white LEDs. A systematic study has been made from a viewpoint of microscopic point defect reaction such as generation and migration in both device active layer (ZnCdSe/ZnSe MQW) and p-type ZnMgSSe cladding layer utilizing DLTS/ ICTS, SSRM (scanning spreading resistance microscope), and EL (electroluminescence)-imaging techniques, coupled with device aging experiments. We have found two different degradation stages (1st and 2nd stages) in the slow-mode degradation, which are caused by quite different microscopic point defect species. The 1st stage is induced by the long-diffusion of H0-center (nitrogen-complex deep hole trap in p-cladding layer), forming high-density dark-spots in the MQW active layer. This active center is generated only in the stress-stimulated condition such as thermal or device fabrication process. After controlling the initial concentration of the H0 center, we have observed no detectable new dark-spots during device operation, leading to fairly long device-lifetime (~1000 h). This 2nd stage has appeared as a carrier (hole) reduction in the p-type cladding layer. This final degradation stage is found to take place by an increase of shallow compensating donor-like centers in p-type cladding layer (ZnMgSSe). Based on these insights on the microscopic point defect reaction, we have developed (new) double cladding i-ZnMgBeSe/ p-ZnMgSSe white-LEDs, which has exhibited long device lifetime of over 10 000 h.