Matthew Zervos

Tin Oxide Nanowires: The Influence of Trap States on Ultrafast Carrier Relaxation

We have studied the optical properties and carrier dynamics in SnO2nanowires (NWs) with an average radius of 50 nm that were grown via the vapor–liquid solid method. Transient differential absorption measurements have been employed to investigate the ultrafast relaxation dynamics of photogenerated carriers in the SnO2NWs. Steady state transmission measurements revealed that the band gap of these NWs is 3.77 eV and contains two broad absorption bands. The first is located below the band edge (shallow traps) and the second near the center of the band gap (deep traps). Both of these absorption bands seem to play a crucial role in the relaxation of the photogenerated carriers. Time resolved measurements suggest that the photogenerated carriers take a few picoseconds to move into the shallow trap states whereas they take ~70 ps to move from the shallow to the deep trap states. Furthermore the recombination process of electrons in these trap states with holes in the valence band takes ~2 ns. Auger recombination appears to be important at the highest fluence used in this study (500 μJ/cm2); however, it has negligible effect for fluences below 50 μJ/cm2. The Auger coefficient for the SnO2NWs was estimated to be 7.5 ± 2.5 × 10−31 cm6/s.

Ultrafast time-resolved spectroscopy of In2O3 nanowires

Ultrafast carrier dynamics in In2O3 nanowires with an average diameter of ≈100±20 nm grown by the vapor-liquid-solid method have been investigated in detail using differential absorption spectroscopy with femtosecond resolution. Measurements revealed that state filling is important for states above the band gap and states just below the band edge, thus demonstrating the critical role that shallow traps play in the relaxation of the photogenerated carriers. Furthermore, time-resolved intensity measurements revealed the importance of Auger recombination in the relaxation of carriers in the In2O3 nanowires and provided the maximum fluence (∼3 μJ/cm2) where this recombination mechanism may be considered negligible. Transient measurements in this low-fluence regime for carriers above the band gap revealed single exponential recovery (∼1.5 ns) associated with recombination of the photogenerated carriers. Similar behavior has been observed for the photogenerated carriers distributed within the shallow traps just...

Investigation into the charge distribution and barrier profile tailoring in AlGaN/GaN double heterostructures by self-consistent Poisson–Schrödinger calculations and capacitance–voltage profiling

The two-dimensional electron gas (2DEG) distribution and conduction-band profile tailoring of the AlxGa1−xN/GaN/AlyGa1−yN/GaN double heterostructure (DH) has been studied in detail by self-consistent Poisson–Schrodinger (SCPS) calculations. We show that a 2DEG is always created at the AlyGa1−yN/GaN interface beyond the GaN quantum well but the latter will not be occupied if the lower barrier thickness or Al content exceed those of the top barrier. These findings were confirmed by capacitance–voltage profiling of a 5 nm Al0.3Ga0.7N/5 nm GaN/AlyGa1−yN/GaN DH grown by molecular beam epitaxy on n+ GaN, where the lower barrier thickness was varied between 10 and 40 nm. A maximum 2DEG density of 1.0×1013 cm−2 was achieved for a 40 nm lower barrier and y=0.2. We found good agreement between the integrated carrier concentration versus depth curve and the calculated equilibrium 2DEG density. The bias required to bring about a flatband condition at the lower AlyGa1−yN/GaN interface increased with the thickness of t...

Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy

THz spectroscopy has been applied to investigate the photo-induced and intrinsic conductivity in SnO2 nanowires using the Drude-Smith model. The refractive index of the nanowires was found to decrease from 2.4 to 2.1 with increasing THz frequency and the dc mobility of the non-excited nanowires was determined to be 72 ± 10 cm2/Vs. Measurements reveal that scattering times are carrier density dependent, while a strong suppression of long transport is evident. Intensity-dependent measurements provided an estimate of the Auger coefficient found to be γ = (7.2 ± 2.0) × 10−31 cm6/s.

Ultrafast hole carrier relaxation dynamics in p-type CuO nanowires.

Ultrafast hole carrier relaxation dynamics in CuO nanowires have been investigated using transient absorption spectroscopy. Following femtosecond pulse excitation in a non-collinear pump-probe configuration, a combination of non-degenerate transmission and reflection measurements reveal initial ultrafast state filling dynamics independent of the probing photon energy. This behavior is attributed to the occupation of states by photo-generated carriers in the intrinsic hole region of the p-type CuO nanowires located near the top of the valence band. Intensity measurements indicate an upper fluence threshold of 40 μJ/cm2 where carrier relaxation is mainly governed by the hole dynamics. The fast relaxation of the photo-generated carriers was determined to follow a double exponential decay with time constants of 0.4 ps and 2.1 ps. Furthermore, time-correlated single photon counting measurements provide evidence of three exponential relaxation channels on the nanosecond timescale.

Defect states of chemical vapor deposition grown GaN nanowires: Effects and mechanisms in the relaxation of carriers

Carrier relaxation in GaN nanowires, grown by atmospheric pressure chemical vapor deposition, via direct nitridation of Ga with NH3 at 950 °C has been investigated in detail. Differential absorption measurements reveal a large number of defect states located within the band gap. The relaxation dynamics of the photogenerated carriers suggest three distinct regions of energy states below the band edge identified as shallow donor states, midgap states, and deep acceptor states. Measurements suggest that Auger recombination is not a contributing factor in carrier relaxation even at the highest fluence (∼1 mJ/cm2) used in this work for carriers located within the conduction band. On the contrary, Auger recombination has been observed when probing the shallow donor states for fluences above 40 μJ/cm2. Measurements at the lowest fluence reveal a biexponential relaxation for the donor states with the fast component (∼50 ps) corresponding to the relaxation of carriers into the midgap states and the slow component ...

Effects of the sapphire nitridation on the polarity and structural properties of GaN layers grown by plasma-assisted MBE

The nitridation of the (0001) sapphire surface by a nitrogen rf-plasma source used in GaN molecular beam epitaxy has been investigated. Auger electron spectroscopy measurements were used to estimate the extent of the nitridation effect. Significant nitridation occurred during 100 min at a high temperature (HT) of 700 °C and the nitridated layers usually exhibited three-dimensional islands, while weak nitridation occurred at a low temperature (LT) of 165 °C and resulted to atomically flat surfaces. Linear time dependence has been found for the amount of HT nitridation. High nitridation temperature resulted in Ga-face and low temperature in N-face polarities of overgrown GaN films. Electron microscopy observations revealed that the N-face material exhibited a superior crystalline quality although presented higher surface roughness.

An investigation into the conversion of In2O3 into InN nanowires

Straight In2O3 nanowires (NWs) with diameters of 50 nm and lengths ≥2 μm have been grown on Si(001) via the wet oxidation of In at 850°C using Au as a catalyst. These exhibited clear peaks in the X-ray diffraction corresponding to the body centred cubic crystal structure of In2O3 while the photoluminescence (PL) spectrum at 300 K consisted of two broad peaks, centred around 400 and 550 nm. The post-growth nitridation of In2O3 NWs was systematically investigated by varying the nitridation temperature between 500 and 900°C, flow of NH3 and nitridation times between 1 and 6 h. The NWs are eliminated above 600°C while long nitridation times at 500 and 600°C did not result into the efficient conversion of In2O3 to InN. We find that the nitridation of In2O3 is effective by using NH3 and H2 or a two-step temperature nitridation process using just NH3 and slower ramp rates. We discuss the nitridation mechanism and its effect on the PL.

Low Temperature Growth of In2O3and InN Nanocrystals on Si(111) via Chemical Vapour Deposition Based on the Sublimation of NH4Cl in In.

Indium oxide (In2O3) nanocrystals (NCs) have been obtained via atmospheric pressure, chemical vapour deposition (APCVD) on Si(111) via the direct oxidation of In with Ar:10% O2at 1000 °C but also at temperatures as low as 500 °C by the sublimation of ammonium chloride (NH4Cl) which is incorporated into the In under a gas flow of nitrogen (N2). Similarly InN NCs have also been obtained using sublimation of NH4Cl in a gas flow of NH3. During oxidation of In under a flow of O2the transfer of In into the gas stream is inhibited by the formation of In2O3around the In powder which breaks up only at high temperatures, i.e.T > 900 °C, thereby releasing In into the gas stream which can then react with O2leading to a high yield formation of isolated 500 nm In2O3octahedrons but also chains of these nanostructures. No such NCs were obtained by direct oxidation forTG < 900 °C. The incorporation of NH4Cl in the In leads to the sublimation of NH4Cl into NH3and HCl at around 338 °C which in turn produces an efficient dispersion and transfer of the whole In into the gas stream of N2where it reacts with HCl forming primarily InCl. The latter adsorbs onto the Si(111) where it reacts with H2O and O2leading to the formation of In2O3nanopyramids on Si(111). The rest of the InCl is carried downstream, where it solidifies at lower temperatures, and rapidly breaks down into metallic In upon exposure to H2O in the air. Upon carrying out the reaction of In with NH4Cl at 600 °C under NH3as opposed to N2, we obtain InN nanoparticles on Si(111) with an average diameter of 300 nm.

Magnetotransport of delta-doped In0.57Ga0.43As on InP(001) grown between 390 and 575° C by molecular beam epitaxy

Silicon (Si) delta- (δ-) doped In0.53Ga0.47As layers were grown by molecular beam epitaxy on InP(001) substrates between 390 °C and 575 °C. Subbands formed at the δ layer were examined with Hall and Shubnikov-de Haas effect measurements in conjunction with self-consistent Poisson-Schrodinger modeling. Below a growth temperature of 525 °C we find good agreement with modeling, but above 525 °C a decrease in active doping level suggests possible surface aggregation, or reaction with impurities in the growth chamber. Significant surface segregation spread of the Si is only found for growth above 450 °C. There is some evidence that DX-like centers may be present, since their incorporation improves slightly the quality of the fits to subband occupancies. Samples grown at 390 °C show strong persistent photoconductivity at low temperatures, attributed to defect states in the InGaAs.