Formation Of Dark Line Defects Research Articles

Band line-up and mechanisms of current flow in n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions

The properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions (HJ) prepared by hydride vapor phase epitaxy (HVPE) on 4H SiC substrates are reported. It is shown that the GaN/p-SiC HJ is staggered type II with the conduction bandoffset and the valence bandoffset values, respectively, ΔEc=−0.49 eV and ΔEv=0.65 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25–0.3 the band alignment becomes normal type I with ΔEc=0.2 eV and ΔEv=0.6 eV. Current–voltage characteristics of both heterojunctions bear evidence of strong tunneling via defect states. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC.

Electrical Properties of n-GaN/p-SiC and n-AlGaN/p-SiC Heterojunction Diodes

AbstractProperties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agrrement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ΔEc=-0.4 eV andΔEv=0.6 eV. Whenchanging GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ΔEc=0.2 eV andΔEv=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.

Electrical Properties of n-GaN/p-SiC and n-AlGaN/p-SiC Heterojunction Diodes

AbstractProperties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agreement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ΔEc=-0.4 eV andΔEv=0.6 eV. Whenchanging GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ΔEc=0.2 eV andΔEv=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.

Green II–VI light emitting diodes with long lifetime on InP substrate

We demonstrate that the quaternary compound ZnMgSeTe can be grown by molecular beam epitaxy in reasonable quality. For layers with energy gaps as high as 3.1 eV, nitrogen doping leads to free hole concentrations around 1018 cm−3. In combination with n-ZnMgCdSe, this material allows the fabrication of II–VI diodes lattice matched to InP substrate. Light emitting diodes containing a tensile strained ZnCdSe quantum well in ZnMgCdSe emit green light when operated in forward direction. In contrast to diodes with a comparable density of extended defects grown on GaAs substrate, these diodes show no formation of dark line defects and a lifetime which is about three orders of magnitude longer.

Transmission electron microscopy of 〈100〉 dark line defects in CdZnSe quantum well structures

The initial degradation of II–VI light emitting devices, namely the 〈100〉 dark line defect formation in CdZnSe quantum well structures formed during laser or light emitting diode (LED) operation, has been investigated. Optically degraded quantum well structures exhibiting dark line defects were investigated via transmission electron microscopy. The observable dislocation networks have been determined to be conglomerations of dislocation loops confined to the quantum well region having an associated Burgers vectors of a[100] and a[010]. The discovery and identification of initial dark line defects may prove useful in identifying the defect mechanism in II–VI light emitters.

2000 h stable operation in 0.87 μm light-emitting diode using stress-free InGaP/GaAs/Si

We have grown Al-free reliable 877 nm In0.49Ga0.51P/GaAs light-emitting diodes (LEDs) on Si substrates by metalorganic chemical vapor deposition. The conventional Al-containing Al0.3Ga0.7As/GaAs LEDs on Si substrates exhibit rapid degradations because of the formation of dark-line defects (DLDs). On the contrary, the In0.49Ga0.51P layer on the GaAs/Si substrate exhibited a 300 K electron mobility of 950 cm2/V s with the carrier concentration of 3.8×1017 cm−3, no residual stress and a lower surface recombination velocity. The In0.49Ga0.51P/GaAs LED on Si shows the stable operation for more than 2000 h, which results from the suppression of the formation of DLDs.

Stable operation in 0.87-μm light-emitting diode on Si substrate using Al-free materials

A reliable 877-nm InGaP-GaAs light-emitting diode (LED) is grown on a Si substrate by metalorganic chemical vapor deposition. A conventional Al-contained AlGaAs-GaAs LED on a Si substrate exhibits a rapid degradation because of formation of dark-line defects (DLD's). On the contrary, an Al-free InGaP-GaAs LED on a Si substrate has no significant growth of DLD's, As a result, a stable operation for more than 1500 h has been achieved in an InGaP-GaAs LED on a Si substrate. >

The development of low voltage room temperature continuous wave laser diodes

A brief review of the current status of blue/green laser diodes is presented. Use of the Zn(Se,Te) contact has enabled room temperature continuous wave (CW) operation at voltages in the 5 to 6 V range, although lifetime is limited by degradation associated with the formation of dark line defects. The paper emphasizes doping issues, particularly the recent observation of a DX-center behavior (AX centers) for nitrogen in ZnMgSSe.

Strain-Relieved Reliable Lasers Grown on Si By MOCVD

ABSTRACTConventional GaAs-based laser diodes grown on Si substrates suffer from rapid degradation, which results from the deteriorations of optical and electrical characteristics. Electroluminescence observation shows that the optical deterioration is caused by the formation of dark-line defects. The current-voltage (I-V) characteristic of the p-n junction is degraded with aging, and results in an ohmic-like under a higher ambient temperature and a larger forward current. The deterioration of the I-V characteristic is caused by defect-accelerated impurity diffusion because the growth of GaAs on Si substrates (GaAs/Si) involves the high dislocation density, the large tensile stress and the large amount of Si near the GaAs/Si interface. A significant improvement in reliability has been achieved in the strain-relieved AlGaAs/InGaAs laser diode on Si grown with the InGaAs intermediate layer. The stress relief by the InGaAs active layer and the reduction of the dark-line defect formation by the InGaAs intermediate layer are required to fabricate reliable GaAs-based laser diodes on Si substrates.

Scanning electron microscopy investigations of the initial degradation mechanism of GaAs quantum well lasers grown on silicon substrates

Cathodoluminescence and electron beam induced current are used to investigate the degradation of the graded-index separate-confining heterostructure laser devices grown on silicon substrates. By examining the evolution of the microscopic electronic properties of these devices during operation or under electron beam bombardment, a better understanding of the initial mechanism of degradation in a laser device results: it is concluded from this study that the degradation starts in the vicinity of the p-n junction before attaining the active layer or the formation of dark line defects. This starting of degradation is attributed to the point-defect migration or coupling in the space-charge region. The built-in electrical field plus the nonradiative recombination of excesses carriers seems to be related to these phenomena. It is also pointed out that the technological processes for device fabrication have a strong influence on the degradation mechanism.

Accelerated aging of 100-mW cw multiple-stripe GaAlAs lasers grown by metalorganic chemical vapor deposition

Lifetime characteristics of multiple-stripe, multi-quantum-well lasers operating in the 810-nm wavelength region at 100 mW cw and heat sink temperatures of 30, 70, and 100 °C are reported. Three failure modes were identified as dark line defect formation during early burn-in, gradual degradation at lower temperatures, and thermal resistance increases for long-term, high-temperature testing. Based upon gradual degradation data, the median operating life at 30 °C is projected to be 22 000 h, with a mean time to failure of 31 000 h. Actual extended life testing at 30 °C supports the validity of this projection.

The origin and elimination of Si pyramids in (Ga, Al)As: Si LEDs

It was already reported that graded-band gap (Ga, Al)As: Si light emitting diodes sometimes exhibit rapid degradation in accelerated aging tests. Such degradation was attributed to the formation of dark line defects at dislocation networks around silicon pyramids that were present on the last to grow p-surface of the devices. A closer examination of the epitaxial process showed that the size distribution of the pyramids decreases when the temperature of termination of the post-growth anneal, during which the melt and substrate are in contact, is lowered. At temperatures below 140°C the pyramids are not formed. The origin of the pyramids is analyzed in terms of silicon solubility in gallium and the kinetics of precipitation. The experimental observations are in qualitative agreement with the Turnbull-Vonnegut theory of heterogeneous nucleation catalysis. At a critical supercooling of silicon enriched gallium melt, the lattice mismatch between Si and GaAs appears to catalyze the precipitation of Si as a second phase in the form of pyramids on the epitaxial structure.

A unifying interpretation of dark line defects in GaAs and bright dislocation halos in GaP

The formation of dark line defects in GaAs, which lead to the degradation of GaAs-based light-emitting diodes and lasers, and the occurrence of bright halos around dislocations in GaP are explained by the same basic assumption that vacancies in the B sublattice, VB, or/and AB anti-site defects constitute luminescence centres strongly competing with and thus weakening the near-band-edge luminescence. In GaAs (GaP) during laser illumination, charge-carrier injection, etc. the concentration of these defects increases (decreases) in the vicinity of dislocations, thus producing dark (bright) line defects. This increase (decrease) of the VB or/and AB concentration results from the emission (absorption) of VB by dislocations climbing under simultaneous absorption (emission) of A-type self-interstitials, IA. In this model the decisive difference between GaAs and GaP is that the dominating defect species frozen-in from crystal growth are IA and VB, respectively. In Ga-rich GaAs (GaP) A=Ga and B=As(P), in As-(P-) rich GaAs (GaP) A=As(P) and B=Ga. It is shown that the above interpretation is supported by a considerable number of experimental results.

Rapid degradation of InGaAsP/InP double heterostructure lasers due to 〈110〉 dark line defect formation

Rapid degradation has been observed for InGaAsP/InP double heterostructure lasers. It has been found that the rapid degradation is due to 〈110〉 dark line defects generated parallel to the stripe and its occurrence depends on the thickness of the InGaAsP contact layer. It has been shown that formation of a InGaAsP contact layer of more than 0.5-μm thickness is necessary in order to realize high reliability of the lasers.

Effects of Ga(As,Sb) active layers and substrate dislocation density on the reliability of 0.87-μm (Al,Ga)As lasers

Reliability data are presented for (Al,Ga)As double-heterostructure lasers that emit light near 0.87 μm. Devices were grown with and without small additions of Sb to the active layer, with some devices grown on high dislocation density substrates. The reliability is more than an order of magnitude better for lasers with GaAs1−ySby active layers with y≊0.01 than for lasers with GaAs active layers. The rate of formation of dark line defects is reduced in the Ga(As,Sb) active layer lasers such that not all devices fail due to dark line defects. However, for Ga(As,Sb) active layer lasers grown on high dislocation density substrates, dark line defects formed very rapidly. An increase of roughly an order of magnitude in the substrate dislocation density resulted in a nearly three orders of magnitude decrease in the 70 °C lifetimes of Ga(As,Sb) active layer lasers.

Stress‐Induced Dark Line Defect Formation in GaAlAs : Si LED's

Homojunction, graded bandgap LED's have been demonstrated to be highly reliable. During accelerated aging (30 mA forward bias, 250°C), these LED's generally degrade slowly without the formation of dark lines or dark spots. It has recently been found, however, that some LED's show a rapid reduction of light output even when aged without current bias at 200°C. <110> oriented dark lines appeared in the electroluminescence image of heavily degraded devices. To determine the source of the DLD's, the material quality of degraded LED's and unaged LED's from the same wafer was assessed using etch pitting and transmission cathodoluminescence. Examination of the devices revealed the presence of small pyramids, composed mainly of silicon, on the p‐surface of some of the LED's; no unusual features were found on the remaining LED's. A dislocation network surrounding each pyramid was found to be initially present. Due to bonding and thermal stresses applied to the pyramids, these networks enlarged with aging along the {111} planes and appeared as <110> DLD's in the electroluminescence image upon reaching the p‐n junction. In degraded LED's from pyramid‐free regions of the wafer with pyramids, <110> DLD's initiate at dicing damage and regions of the p‐surface subject to high bonding stress, i.e. edges and corners. The growth of these DLD's is identical to those initiating at pyramids. A similar study of LED's from a wafer that is totally free of pyramids shows that DLD's are not formed in devices that degrade slowly.

The influence of intrisic defects on the degradation and luminescence of GaAs and other III–V compounds

A significant contribution to the degradation of GaAs-based lasers and light-emitting diodes arises from the formation of so-called dark line defects. It is shown that these defects are accumulations of non-radiative recombination centres around dislocations. The centres are identified as As vacancies, which are emitted by climbing dislocations, concomitantly with the absorption of Ga interstitials. From scanning deep-level transient spectroscopy observations it is concluded that the so-called DX centres are Ga interstitials. The driving force for dislocation climb and thus for dark-line-defect formation is a supersaturation of Ga interstitials originating from the growth of the GaAs crystals under Ga-rich conditions as a consequence of the high volatility of As. Phenomena in other III–V compound semiconductors related to the formation of dark line defects in GaAs are also discussed.

Changes in the optical properties of CW (AlGa)As junction lasers during accelerated aging

Changes occuring in selected optical properties of CW (AlGa)As double-heterostructure junction lasers during the initial stage of accelerated aging are reported. The characteristics selected for investigation are relevant to the operation of the laser in an optical communication system and include the current dependence of the lasing emission and the orthogonally polarized (nonlasing) emission, the symmetry of the emission with respect to the two output faces, the time stability of the light intensity, and the frequency of the internal laser resonance. In spite of the variability among the aging behaviors of individual lasers, characteristic changes did emerge as statistically significant. For example, a substantial decrease in the emission symmetry was commonly observed. A second important effect was the creation of a self-induced intensity pulsation, at a frequency typically between 300 and 600 MHz, in lasers which initially exhibited no intensity modulation. A statistical analysis of the data was made to correlate these instabilities with asymmetries in the lasing or nonlasing emission and/or changes in the emission symmetry. No evidence was found to support previously published conjectures that the pulsations are caused by the formation of dark-line defects or by axial nonuniformities in the pumping current.

Dark-line defects induced by mechanical bending in GaAs-Ga1−xAlxAs double-heterostructure wafers

A 〈110〉-oriented dark-line defect (DLD) has been induced by mechanical bending in GaAs1−xAlxAs double-heterostructure (DH) wafers. Both the concave upward and the convex upward bending (with the DH epitaxial layer as the upward surface) of the DH wafer has been carried out using a four-point bending technique. The DLD formation has shown both the stress-direction dependence and the asymmetry of the DH wafer. Photoluminescence topography has shown that the characteristics of the DLD’s are exactly the same as those of the optically induced DLD’s. From experimental results, it is conjectured that a glide-multiplication mechanism is probably acceptable for the production of the 〈110〉 DLD dislocations.