Interface Control Of GaN/Si Nanoporpous Pillar Arrays And Study On Their Electroluminescence Properties

As a wide bandgap(~3.4 eV)direct semiconductor with high thermal stability, chemical stability and electron mobility, GaN has been widely used in fabricating light-emitting diodes(LEDs), laser diodes(LDs), ultraviolet(UV) detectors, high electron mobility transistors(HEMTs) and solar cells. Especially, GaN shows high internal quantum efficiency and insensitivity to dislocations. Up to now, GaN is thought as the most important semiconductor material which can realize high efficient blue LEDs and LDs in mass-production scale. Si is the most important and widely used semiconductor in modern electronic industry for its technical maturity, ease to integrate,element abundance in earth’s crust and low prices. In past decades the integration level of super-large-scale integration based on Si has been improved continually according to Moore’s law. But Si has an indirect bandgap which makes it an inefficient light emitter and other direct-bandgap semiconductors are usually hired concerning the fabrication of optical or photoelectric devices. The integration of GaN with Si, has the potential of combining the high light-emitting efficiency of GaN with the high integration density of Si on one chip. And this could shorten the distance between photo- and electric part and speed up the information process, leading to superior or novel functionalities. Due to the large lattice and thermal mismatch between GaN and Si, An intermediate layer like metal alloy or AlN buffer is usually needing to obtain strong bonding or good crystal quality when the contenvtional wafer-bonding or hetero-epitaxy are adopted to realize integration of GaN and Si. This results that undirect contact between GaN and Si. By growing GaN nanostructures on silicon nanoporous pillar array(Si-NPA), the direct contact of GaN with Si was realized and a novel prototype device constructed with GaN/Si nanostructured heterojunction was obtained. The device showed rectification and yellow and near infrared electroluminescence(EL). Based on these previous results, the GaN growth mechanisms, the construction of GaN/Si-NPA devices, the transport and EL mechanism of GaN/Si-NPA are studied in details in this dissertation. Furthermore, the Si-NPAs are treated by thermal oxidation and ammoniation under different temperatures to realized structure and surface modification. And the regulation effects of Si-NPA treatment on the interface structure and device performance of Ga N/Si-NPA are also studied. The mean researches conducted are list as follows.(1) Study on the GaN growth mechanism and properties of GaN/Si-NPA.The roles of Pt catalyst in growing GaN on Si-NPA using Ga and ammonia as precursor by Chemical Vapor Deposition(CVD) technique were studied thoroughly. The results shows that GaN could not be deposited when there is no Pt catalyst. By analyzing the samples which undergo the deposition process in which Ga source was used but no ammonia was inlet, it is found the Pt can turn into Ga-Pt alloy droplet by incorporating Ga vapor. When the ammonia is inlet, the GaN crystal nucleus will be formed in Ga-Pt alloy droplet under the catalysis of Pt. The effect of temperatures, ammonia pressure on the morphology and structure of the obtained GaN were also studied. The results showed that different nanostructures such as GaN nanoparticles, nanorods, stacked nanocones and nanowires can be realized by changing the growth conditions. After GaN growth process the Ga source often changed its appearance. In some cases the Ga in crucible could shrink into a metal globule, when the supersaturation is low. while in other cases the Ga could creep up along the internal wall of crucible, even out of crucible, when the supersaturation is high. Both can alter the Ga evaporation area and lower the growth repeatability. The phenomenon was explained and solutions were proposed to improve growth repeatability. The photoluminescence(PL) spectra measured at varied temperature was conducted to study the properties of the GaN/Si-NPA. The results showed that the PL spectrum of GaN/Si-NPA is the superposition of that from GaN and that from the beneath Si-NPA. Neither variation of PL from GaN nor Si-NPA were found caused by interdiffusion between GaN and Si-NPA. This proved that Si-NPA might be an ideal substrate for preparing Si-based GaN nanodevices.(2) Study on the current transport and EL mechanisms of GaN/Si-NPA nanostructured heterojunction LEDBy comparing the I-V characteristics of GaN/Si-NPA devices based on GaN/Si-NPA with different morphologies, it is found the relatively flat GaN nanoparticle film had a better electrode contact and more stable I-V characteristics. Based on the result, the growth temperature of 800?C with ammonia pressure of 500 Pa, together with growth temperature of 950?C with ammonia pressure of 500 Pa 950?C,500 Pa were chosen to construct two typical GaN/Si-NPA nanostructured heterojunction LEDs for further study. The device based on 800?C-obtained GaN/Si-NPA shows a excellent rectification with very small reverse current. The I-V characteristics follows the exponential relation of the thermionic emission model under low forward biases and approaches the quadratic relation of space charge limited current(SCLC) mechanism under higher forward biases. The device based on 950?C-obtained GaN/Si-NPA shows much less rectifying, and the reverse current is in same magnitude with that under identical forward voltage. Further analysis discloses that the large reverse current is meanly owed to the Fowler-Nordheim(F-N) tunneling of electrons in valence band of Si-NPA into the conduction band of GaN under reverse biases.The 800?C-obtained GaN/Si-NPA shows bias-independent yellowish white EL when forward biases were over 5 V. The EL spectra are near Gaussian systematic single broad peaks centered at ~540 nm with full width in half maximums(FWHM s) of ~171 nm. No EL are observed under reverse biases. The 950?C-obtained GaN/Si-NPA, on the other hand, shows EL under both forward and reverse biases. The EL spectra under reverse biases are similar to that of the 800?C-obtained GaN/Si-NPA under forward biases. The EL spectra under forward biases red-shift compared to the EL under reverse biases. Through analysis it is believed that the yellowish white EL originates from the electron transition from the conduction and/or shallow donors to the deep acceptors in GaN. While the red-shift of EL of The 950?C-obtained GaN/Si-NPA is due to the contribution of the red EL from the Si-NPA substrates.(3) Study on the aged, thermally oxidized and thermally ammoniated Si-NPA.Fresh Si-NPA shows only a broad red PL. It changes into a ultraviolet, a purple-blue and a red PL band after aging. It is replaced by a purple-blue band after oxidation and a purple-blue and a red band after ammoniation. Using steady-state PL, time-resolved PL, air annealing and wavelength-dependent PL, the PL mechanism of aged Si-NPA were systematically studied. Based on the analysis the ultraviolet(UV), purple-blue and red PL bands from aged Si-NPA are believed originated from the oxygen excess defects in Si oxide, oxygen deficiency defects in Si oxide and band-to-band transition of nc-Si under the frame of quantum confinement(QC), with surface states of nc-Si contributing partially, respectively. The purple-blue PL band of oxidized Si-NPA ammoniated Si-NPA are both assigned to a same origin, namely the oxygen-deficiency-related defects. The red PL band of ammoniated Si-NPA, which is assigned to nc-Si, red-shifts with increasing ammoniation temperature. This indicates that the size of nc-Si increases with increasing ammoniation temperature.Using 950?C,500 Pa growth condition, GaN nanostructures were grown on oxidized and ammoniated Si-NPA to studied the modulation effect of treatment of Si-NPA on performance of GaN/Si-NPA. The results show that the treatments to Si-NPA under 800?C(either oxidation or ammoniation) have no apparent effect on the rectifying and EL performance of GaN.The oxidation of Si-NPA over 900?C, will make the rectification of the GaN/Si-NPA vanish and the I-V characteristics becomes symmetrical about the origin. The ammoniation of Si-NPA at 900?C and 1000?C does not alter the rectification nature of the obtained GaN/Si-NPAs. The behaviors under forward biases are unvaried, but the reverse currents decrease dramatically and the drop is more obvious with higher ammoniation temperature. The ammoniation of Si-NPA at 1100?C will make the rectification of GaN/Si-NPA vanish.The EL spectra under reverse biases of the GaN/Si-NPAs obtained on treated Si-NPAs show no change relative to that grown obtained on fresh Si-NPA, except for the 1000 and 1100?C oxidized Si-NPAs. The EL of GaN/Si-NPA obtained on 1000?C-oxidized Si-NPA is located at 495 nm, while the EL vanishes for 1100?C-oxidized Si-NPA. The EL spectra under forward biases of some of the GaN/Si-NPAs obtained on oxidized Si-NPAs show red-shift relative to that under reverse biases. But others show no shift, no systematic pattern is found. As to the ammoniated Si-NPAs, all but the 1100?C ammoniated Si-NPA show EL shift under reverse and forward biases, which indicates that ammoniation is more beneficial for preserve the red EL emission from Si-NPA.