| dc.description.abstract | This dissertation comprises the optical characteristic of p-i-n In0.23Ga0.77N/GaN multiple quantum wells (MQWs) and p-i-n InGaN/AlInGaN MQWs grown by metalorganic chemical vapor deposition method. The main works is divided to the following parts.
The optical property of In0.23Ga0.77N/GaN MQWs is investigated by means of electrotransmittance at room temperature. The strength of piezoelectric field is estimated to be 1.7~1.9 MV/cm for the sample In0.23Ga0.77N/GaN, and its direction is oriented against the built in electric field and applied reverse bias. Meanwhile, the e1-hh2 forbidden transition state is also observed in the spectra. Such investigation combined with theoretical calculation can furnish us with valuable information about band offsets, piezoelectric electric field and etc.
For In0.06Ga0.94N/Al0.1In0.02Ga0.88N MQWs structure, we identified that the ternary well and quaternary barrier by means of secondary ion mass spectroscopy measurement initially, and found that the high density si doping does not influenced the distribution of aluminum and indium. We investigated the InGaN/AlInGaN MQWs structure by means of electroreflectance. Fruitful results of bias-dependence ER feature such as parabolic-like energy shift, intensity variation and 180 ° are displayed. We give a simple explanation about modulation spectroscopy for these phenomenons of bias-dependence ER spectra. And then the in-well flat band is determined by the QWs signal intensity minimum. Using self-consistent calculation of Poission equation, we studied the quantum well field strength in p-i-n active region. And the polarization-field about 0.21 MV/cm is acquired by the condition of in-well flat band. We also acquired the spontaneous polarization field of Al0.1In0.02Ga0.88N/GaN structure by compared the difference of In0.06Ga0.94N/GaN and In0.06Ga0.94N / Al0.1In0.02Ga0.88N structure. The 0.73 MV/cm spontaneous polarization field is nearly consistent with the Vegard’s law prediction. This results implied that the weak bowing effect on AlGaN-like quaternary alloys.
Meanwhile, the si doping effect on polarization field is also taking into account. The ER signal intensity minimums (turning voltage) of un-doped and doped samples are -0.1 V and 1.1 V, respectively. We supposed that the dopant ion in barrier layer is the major factor of influenced the p-i-n built-in electric field but not the free carrier induced by the dopant atoms. Therefore, the doping may cause the flat-band condition happened to lower applied bias, this would benefit to the radiation recombination of the opto-devices. Then that point should be considered on the designing of opto-devices. Finally, electroreflectance measurements for well-width dependence In0.06GaN/Al0.1In0.02Ga0.88N MQWs at room temperature have performed. The indium and aluminum content in well and barrier are the same as the previous samples. The quantum wells ground state is observed at zero volts. Used the known polarization field of this QWs, we obtained the theoretical confined state by an analytic form. The consistency between the theoretical curves and the experimental results presents that the In0.06Ga0.94N/Al0.1In0.02Ga0.88N MQWs structure behaves like that from a conventional QW. We also can claim that the method of acquirement the polarization field strength from the ER signal intensity minimum is trustworthy.
After the electroreflectance measurement, temperature dependence electroluminescence for un-doped and 1x1018 cm-3 barrier doped samples is performed. The EL emission energy varying with the temperature exhibited the anomalous “s-shape” shift. In our measurement, the σ value obtained by the fitting are 9 and 10 me for un-doped and barrier-doped sample, respectively. Meanwhile, the blue-shift in the un-doped LEDs between 10 K and 130 K is as small as ~10 meV, which is reasonable consistent with that of the barrier-doped LED (~7 meV). So, the crystal quality of these two LEDs is nearly equal. Comparing the temperature dependence EL results and the ER results, we found that the threshold current is reduced by doping effect due to the flat-band condition is being enhanced. And this is the major factor for the improvement of the diode performance. This analysis makes it possible to optimize the UV-light emitting diodes (LEDs) structures, which is benefit for fabricating high-brightness nitrides-based UV-LEDs operating at room temperatures. | en_US |