Study On The Luminescence Properties Of V-pit-containing GaN Based Blue LEDs On Si Substrates

Despite the dislocation density as high as108-1010/cm2, the InGaN light emittingdiodes (LEDs) has high external quantum efficiency. Two mechanisms have beenproposed to explain this phenomenon. The traditional theory considers that the In-richlocalized states in the InGaN multiple quantum wells (MQWs) could confine thecarriers in the potential minima, which keeps them from being nonradiativelyrecombined. The recent theory proposed that the V-shaped pits (V-pits), which form atthe dislocations, contain sidewall MQWs with bandgap several meV higher than thatof c-plane MQWs. This higher bandgap acts as an barrier for carriers and preventsfrom reaching the dislocations. Both of these two theories can explain theluminescence behavior of the InGaN LEDs to a certain extent. Since the proposal ofthe localized theory approximately twenty years ago, there have not been any resulton the control the localized states yet. The V-pits, however, have much bettercontrollability. Thus, this thesis chooses to study the V-shaped pits. The major workand achievements include:1. The formation and coalescence mechanisms of V-shaped pits (V-pits) arestudied.(1) To study the formation mechanism, the effects of the low-temperatureGaN thickness and the indium on the size of V-pits are investigated by atomic forcemicroscopy (AFM). It is discovered that the size of the V-pits increases with thelow-temperature GaN thickness and decreases with the incorporation of the indium.The densities of the V-shaped pits obtained by AFM are found to be the same order asthe dislocation densities, which are evaluated by FWHM of XRD rocking curves.(2)To study the coalescence mechanism, the effects of growth temperature, growth rateand growth ambient are studied. The results indicate that high growth temperature,low growth rate and H2ambient facilitate the coalescence of the V-pits.2. In the devices containing V-pits, the effect of low-temperature unintentionallydoped (UID) and heavily Mg doped (HD) Al0.2Ga0.8N electron blocking layer (EBL)on the optoelectronic properties of device is investigated by temperature and injectioncurrent dependent electroluminescence (EL).(1) A broad peak is discovered at theshort-wavelength side of the main emission in the cryogentic-temperature EL spectraof the sample with UID EBL. Analysis leads to the conclusion that the broad peak isfrom the sidewall MQWs in the V-pits.(2) The emission of the sidewall emission broadens across the short-wavelength bands with increasing current densities in thelow current density regime and stabilizes as an exceptionally broad line when currentdensity increases to a relatively high value. This phenomena is well explained by theequivalent device circuit model.3. In the sample with V-pits and UID EBL, the EL peak of the c-plane MQWs isobserved to broaden across the short-wavelength bands when the peak intensity ratioof the emission of sidewall MQWs to that of the c-plane MQWs reaches0.3. Analysissuggests that holes have been injected from the sidewall of V-pits into the nearbyc-plane MQWs. Affected by the V-pits, the nearby c-plane MQWs have slightlyshorter-wavelength emission than those relatively far away from the V-pits. The UIDEBL causes a high proportion of hole current to flow via the V-pits. This enables theemission intensity from the c-plane MQWs in the vicinity of the V-pits to becomparable to that from the MQWs distant to the V-pits, which broadens the c-planeMQW emission.4. The effect of enhanced size of V-pits and the unintentionally Mg doped P-GaNlayer filling the V-pits on the EL of the sidewall MQWs have also been studied bytemperature and current dependent EL.(1) Besides the EL peak of sidewall MQWs,the EL peak of the InGaN in the strain relief layer is also observed. It is suggested thelarger diameter of V-pits in the P-type layer increases the distance between the holesand the dislocations in the V-pits, which enhances the injection efficiency of holes andleads to the overflow of holes to the N-type layer.(2) With the unintentionally Mgdoped P-GaN layer filling the V-pits, the relative intensity of the sidewall MQW ELpeak to that of the c-plane MQWs decreases evidently, it is thus concluded that theintroduction of this layer could reduce the proportion of hole current via the V-pits.Besides, the effect of silicon doping in the barrier has also been investigated bytemperature and current dependent EL.(1) It is found that silicon heavily doped in thebarrier close to the p-type layer could cause the MQWs grown under the samecondition to have two EL peaks. It is proposed that silicon doping in the barriergenerates PN junction built-in field, which compensates the piezoelectric field in theQW(s) between the doped barrier and the P-type layer, making this (these) QW(s)emit shorter wavelength.(2) The EL spectra of samples with silicon doping in thedifferent barrier have been compared. It is found that the effect of the PN junctionfield compensating the piezoelectric field becomes more pronounced as the dopedbarrier gets closer to the P-type layer.(3) The EL spectra of different temperatures and current densities are compared. It is observed that the carriers are more inclined torecombine in the QW(s) near the P-type layer at lower temperatures and highercurrent densities, which is consistent with the conclusion obtained by othermethods.The results have been published in Journal of applied physics.