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The Polarization Effects Of Ⅲ-nitrides And Its Application In Photoelectronic Devices

Posted on:2014-04-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:K X DongFull Text:PDF
GTID:1228330467487905Subject:Microelectronics and Solid State Electronics
Abstract/Summary:PDF Full Text Request
Due to large tunable direct band-gap energy, superior thermal conductivity, high electron saturated velocity, high physical and chemical stability in harsh environments, Ⅲ-nitrides based semiconductors have numerous applications as promising materials for high electron mobility transistors, high bright light-emitting diodes (LED), high power laser diodes and high sensitivity solar-bland and visible-bland ultraviolet photodiodes. However, with the development of science and technology, the semiconductors material and device with improved performance are required. One of the unique characteristics of the wurtziteⅢ-nitrides is its spontaneous and piezoelectric polarization, which lead to potential barriers, band bending, and have been confirmed to play an important role in the design of semiconductor material and devices. Hence it can be applied constructively to open a new area for the photo-electron device. But, at present, the influence of polarization of Ⅲ-nitrides on the carriers doping, distribution, transport mechanisms in quantum wells, and device characteristic are still unclear. Therefore, it is important for us to research the physical properties of the materials and understand the mechanisms of polarization effect and polarization-induced doping.In this paper, we have designed a N-face polarization-doped In0.2Ga0.8N multiple-quantum-well light-emitting diodes and a back-illuminated separate absorption and multiplication (SAM) AlGaN solar-blind avalanche photodiodes (APDs) using polarization-induced p-doping and the polarization electric field. Meanwhile, the effects of polarization-doped and polarization electric field on band bending, electric field distribution, carriers transport, and improvement of device characteristics have been discussed detailedly. The main achievements obtained in this paper are as followings:(1) In N-face polarization-doped In0.2Ga0.8N multiple-quantum-well LEDs, polarization-doped p-type AlxGai.xN layer have been designed, the Al composition in the polarization-doped AlxGa1-xN is linearly graded from0to0.3along N-face. When the doping concentration is1×1019cm-3in p-type layer, the hole concentration increases from2.5×1016cm-3for thermal ionization to1.2×21018cm-3for field ionization, an increasing of two orders of magnitude, which enhances significantly the p-type conductivity and hole injection efficiency, hence, results in a large light emitting efficiency.(2) We firstly utilize the polarization effect to adjust the total electric field in quantum wells of N-face polarization-doped LEDs. The direction of polarization field inside the quantum-wells for the N-face LEDs is opposite to the forward bias field, so the total electric field inside the wells decreases quickly with increasing forward bias voltage. The decrease of the total internal field in the well will flatten energy-band diagram of the wells and hence increases the overlap of electron and hole wave functions, which results in an increase of Electroluminescence (EL) intensity. On the other hand, the direction of polarization-induced electric field in the barriers of the N-face LED is consistent with that of the forward bias field, which assists electron/hole injection into the quantum wells. As a result, compared with metal-face polarization doped LED and conventional LED, N-face polarization doped LEDs can provide larger improvements in the efficiencies of carrier injection and EL emission at high forward voltages, even at voltage of45V, the light output power don’t appear saturated or decreased trend. However, the light output power for conventional LED and metal-face polarization doped LED appears saturated and decreased trend when applied voltage is over8V due to carrier saturation, screening, and the quantum confined stark effect.(3) Impact ionization engineering and polarization effects of Ⅲ-nitrides in back-illuminated SAM APDs based on AlGaN have been exploited by adjusting the Al composition of the p-AlGaN layer on the basis of device simulation. The avalanche breakdown voltage of the AlGaN APDs can be reduced significantly by inducing a polarization electric field with the same direction as the reverse bias in the multiplication region, whilst the multiplication gain increases pronouncedly by the increased hole ionization coefficient. Moreover, the performances of APDs can be also improved furtherly by polarization doping effect in the p-type AlGaN layer. The APD structure increases by225%in comparison with the reference APD structure.(4) In AlGaN SAM (p-il-nl-i2-n2) APD, we found that the parameters of each layer play an important role on the performance of device, especially the parameters of the nl interlay. If the electron concentration of nl layer is over2×1018/cm3or the thickness reaches the150nm, the SAM structure will degenerate to be a simple p-i-n structure. This weakens the performance of APDs due to electron taking part in the initial ionization. Moreover, the trap density in absorption and multiplication layer is sensitive to the performance of APDS. When the trap density in absorption layer is added, the photo-generated holes and electrons are recombined largely by high defect states in the absorption layer and results in fewer holes that can enter into multiplication layer. When the defect density distributed in multiplication layer is increased, the ionized holes are scattered by high density defect states, and hence lower the kinetic energy of holes and reduce the impact ionization efficiency. Therefore, the increased trap density in the absorption layer and multiplication layer can significantly degrade the performances of APD. The analysis of the parameters of AlGaN SAM APD paves the way for optimization design of APDs.
Keywords/Search Tags:Ⅲ-nitrides, polarization effect, polarization-doping, energy-bandadjustment, LED, SAM structure, avalanche photodiodes
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