Wide Bandgap Semiconductors
AlGaN ALLOYS
The alloys of GaN and AlN are direct band-gap and cover the band-gap range from 3.45 eV to 6.2 eV. This is equivalent to the wavelength range 200 nm to 360 nm.
In our group, we carry out the growth of AlGaN thin films as well as quantum structures by the PA-MBE method. While deep UV Light emitting diodes have been developed for the last two decades, they are still significantly less efficient than their visible counterparts, mostly due to the large defect density in these materials, which act as pathways for non-radiative recombination.
We have focused on the development of growth processes and device designs that can increase the radiative recombination efficiency, thorough the control of the carrier localization processes away from defect sites.
This has been effected through several processes, including the formation of spontaneous alloy spatial fluctuations and the formation of quantum structures through optimization of the surface transport of adatoms during the deposition of the layers by MBE.
MOLECULAR BEAM EPITAXY
We work on a VEECO Gen 930 Dual Chamber system, for III-Nitrides and SiGe.
The III-Nitrides chamber is equipped with effusion cells for Ga, Al, In, Mg and Si, as well as an RF Plasma source.
ENHANCED RHEED
Reflection High Energy Electron Diffraction is a standard tool used in the monitoring MBE growth. We have developed a technique based on image capture and analysis.
The growth of III-Nitride materials are typically carried out under Group III - rich conditions, and the excess metal stays on the growth surface as a thin layer. The presence of the metallic layer causes the RHEED to become dim and diffuse (a), rather than sharp (b).
Using RHEED image time-evolution analysis, the thickness of the metallic layer can be evaluated in real-time, which is very helpful in monitoring the growth process.
AlGaN Thin Films: Compositional inhomogeneity
InGaN alloys are known to exhibit alloy phenomena such as phase segregation and long-range atomic ordering. The first is expected to be absent in AlGaN alloys due to the relative closeness in the sizes of the Al and Ga, and long-range atomic ordering of mono-layer, multiple layer as well as incommensurate periodicities have been observed.
Defect densities in AlGaN alloys grown on to sapphire are very high, the presence of compositional inhomogeneities, which lead to spatial fluctuation in the band-gap, are useful is localization of carriers away from non-radiative recombination centers associated with these defects. Our work has identified two different types of fluctuations arising from the MBE deposition process.
One occurs during growth under nearly stoichiometric conditions lead to reduced surface diffusion length of Al. The second occurs during growth under excess group III, where a metallic layer of Ga and Al stays on the growth surface. The compositional fluctuations in this metallic layer is introduced into the AlGaN film, and the amplitude can be very high.
The presence of these fluctuations can be observed from a strong red-shift of the band-gap luminescence from the absorption edge.
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GaN/AlN Quantum Wells and Dots
The group III to group V flux ratio alters the surface mobility of adatoms, which affects AlGaN growth in two ways: (a) change in the surface morphology, which translates to interface roughness, and (b) changes in the nature and intensity of compositional fluctuations.
In order to decouple the two, we have investigated growth mechanisms of binary GaN/AlN multiple quantum wells (MQWs). The internal quantum efficiency (IQE) which is the efficiency of radiative recombination, was measured by the ratio of room temperature photoluminescence intensity to that measured at 4 K.
Under conditions of growth where surface morphology was atomically flat, the interfaces are relatively diffuse. Here, the IQE is rather low typically ~10%.
Use of near stoichiometric growth conditions however lead to a reduction of the surface diffusivity of adatoms, and the formation of spontaneous nanostructures in the form of nano-dots of about 20 nm in diameter and high levels of uniformity. The IQE for GaN/AlN MQWs grown under such conditions is increased to as high as 28% even for samples with large dislocation densities.
These results point out the direction for development of bright LEDs based on GaN/AlN MQWs, even if the dislocation densities are relatively high.
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AlGaN Quantum Wells: An added process step
[Exposure to active plasma]
Structures with AlGaN wells and barriers were grown for emission in the deep Ultraviolet (~240nm) range.
We typically carry out two characterization processes to determine the quality of MQW structures. The interface abruptness is evaluated by X-Ray diffraction processes, by the presence and clarity of superlattice peaks. The Internal Quantum Efficiency (IQE) is determined by the ratio of PL intensity measured at room temperature to that measured at 4K.
Our results indicate that for MQWs grown in the traditional fashion, that is when the wells and barriers are deposited directly on to each under excess group III conditions, the Internal quantum efficiency is high, due to the presence of compositional inhomogeneities generated under these conditions of growth. However the interface abruptness is quite low, due to the presence of a metallic layer on the growth surface, which limit the usefulness of the structures due to carrier leakage.
An additional annealing step, where the growth is interrupted after the well and the barrier layer, and the metallic layer is consumed, leads to a better interface quality, without significantly reducing the IQE.
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AlN BUFFER LAYERS
We employ AlN buffer layers for growth of AlGaN layers by PA-MBE. This is carried out at a substrate temperature of 800 deg C.
The Al to active nitrogen arrival ratio is critical in determining the nature of the AlN buffer layer, which has many functions, including stress relaxation, defect mitigation as well as acting as a UV-transparent window for bottom-emitting LED or bottom illuminated UV-Photodetectors.
Our results indicate that there is a relatively narrow window of growth conditions that lead to a transparent and flat AlN buffer layer. Excess Al flux can lead to a metallic buildup on the surface, while a reduced Al-flux leads to rough films.
We employ a modified Migration Enhanced Epitaxy process, where the growth is carried out for short duration at excess Al, and then annealed under active Nitrogen plasma to consume the excess metal, the whole process monitored by the RHEED pattern,
However, if the Al flux employed during the deposition step is too high, or the time duration too long, then spontaneous nano-rods can be developed.
AlN nano-rods are well aligned structures with diameters of 200 nm and lengths of 1 micron or more. They show very smooth sidewalls and a hexagonal cross-section, indicating growth along the [0001] direction.
The width distribution is relatively narrow, even though the heights show a large variation,
AlGaN Multiple Quantum Wells (MQWs)
[use of Indium as a surfactant]
AlGaN quantum wells are part of the active region of ultraviolet emitters, and need to be efficient in recombining injected electrons and holes in generation of photons.
The presence of alloy fluctuations in AlGaN thin films cause the presence of multiple peaks, as the carriers can recombine from both the larger band-gap as well as the narrower band-gap regions. This is not desirable for most UV emitters.
These fluctuations can be controlled by use of Indium as a surfactant during growth of MQWs. The Indium does not incorporate into the film at high substrate temperatures, but modify the surface mobility of adatoms, and thereby reduce the compositional inhomogeneity. The result is a single sharp luminescence peak.
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Vertical Nanorods with Quantum Dots
We have also carried out studies on the growth of AlGaN films and AlGaN/AlN MQWs on to well-oriented vertical AlN nanorod clusters.
FESEM studies indicated that during the growth, spontaneously formed nanostructures were deposited on top of the hexagonal top planes. These planes show a step flow growth mode, and for nearly stoichiometric conditions, 20 nm diameter AlGaN nanodots were formed selectively at the step edges generated.
For growth under excess group III conditions, lateral nanowires were formed perpendicular to the edges of the vertical nanorods.
An enhancement of at least 15 times was obtained in the Cathodoluminescence (CL) emission peak intensity (290 nm) from the top of the nanorod structures.
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Wavelength-selective UV Photo detectors
For many applications, it is critical to detect ultraviolet radiation within a very narrow wavelength range, while cutting out all other background light.
Since the AlGaN alloys are sensitive exclusively to UV radiation, wavelength selective device such as solar blind photodetectors have been developed over the last two decades. These devices are P-I-N type, and are bottom illuminated, with the N-layer acting as the shorter wavelength cut-off window, while the longer wavelength cut-off is defined by the absorption edge of the intrinsic layer.
While imagers based on these devices are already available, they are limited by the fact that p-type doping of high Al-content AlGaN layers is very difficult, and they can only be grown on to transparent substrates, even if the lattice mismatch is high.
We have developed wavelength selective photodetectors based on the lateral transport in AlGaN MQWs (Fig below, left). The sharp nearly Gaussian sensitivity (Fig below, right) is due to the formation and dissociation of excitons in the quantum wells. This was established by the temperature and field dependency of the photocurrent spectrum.
These devices can be fabricated onto any suitable substrates, and are of a top-illuminated geometry. Since no p-type doping is necessary the wavelength selectivity can be shifted to any suitable wavelength range by appropriate choice of alloy composition of the wells and barriers.
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ULTRAVIOLET Light Emitting Diodes
The use of compositional inhomogeneity for carrier localization away from defect sites provided a pathway to brighter ultraviolet light emitting diodes deposited on to sapphire, where the dislocation densities are very high. The generation of such fluctuations was carried out by using specific growth conditions, by controlling the surface diffusivity of Ga and Al adatoms.
Photoluminescence measurements indicate that transitions from such fluctuations generate peaks significantly red-shifted compared to the absorption edge. A series of UV LEDs were growth with varying degrees of such nanoscale features within the active region.
The samples were grown by PA-MBE and consisted of a nucleation layer of AlN, a thick n-type doped AlGaN film, which may incorporate superlattice layer for dislocation reduction, a number of AlGaN MQWs in the active region, an AlGaN electron blocking layer, and p-layers of AlGaN and GaN. A number of device geometries were tested, including a simple square, interdigitated mesa, and a spiral mesa.
The fabrication was carried out using an initial mesa lithography, followed by reactive ion etching. Then the n-contacts were formed using Ti/Al/Ni/Au stacks. Finally, the p-type contacts were fabricated using Ni/Au metals, all evaporated using thermal and e-beam sources. The wafers were tested using our custom designed wafer-level-probing system.
Our results indicate that the incorporation of compositional
inhomogeneity led to a several improvement of LED device performance. The luminescence intensity increased by an order of magnitude. More interestingly, there was no sign of “droop”, that is the loss of efficiency for high injection currents for these devices. However, these LEDs showed a luminescence peak significantly red-shifted compared to those without the alloy fluctuations.
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Deep UV Photo detectors based on MQW/QDs
The wavelength range below 235 nm has gained prominence due to the fact that illumination at these wavelengths is expected to show high germicidal properties without having a carcinogenic effect on human skin due to low penetration depth.
We have extended our work on UV detectors based on lateral transport in quantum wells to the shorter wavelength range by employing high Al content AlGaN in the well and AlN in the barrier.
We find that for quantum well structures (S2, figure below), the photocurrent peak was in the 210–215 nm range with a width of ∼20 nm, with no other additional signatures in the entire UV–visible range.
When dot-in-well (DWELL) structures formed by droplet epitaxy was employed, a second red shifted peak was observed at ∼225 nm with significantly (up to 10×) higher responsivity (S1, S3, figure below). The quantum dots had truncated pyramidal structures with near-uniform size distribution and density of 6 × 1010 cm−2 within the quantum wells.
WAVELENGTH-SWITCHABLE ULTRAVIOLET Light Emitting Diodes
Dual-wavelength ultraviolet light-emitting diodes (UV-LEDs) exhibiting two discrete emission peaks of comparable intensities are reported in this work. Furthermore, this is the first report where complete switching between these two peaks was achieved by simply changing the duty cycle of the pulsed-mode excitation.
The dual-wavelength nature of our LEDs was brought about by the deliberate incorporation of nanometer-scale alloy fluctuations in the quantum well based active regions by modulating the relative surface diffusion rates of Ga/Al adatoms. The wavelength selectivity was linked to the variation in thermally assisted carrier delocalization, transport, and subsequent recombination processes in regions of different compositional inhomogeneities.
Sol-gel grown ZnO Thin Films
The sol-gel technique is a powerful, yet simple process for the deposition for a wide range of semiconducting thin films. We employ this process for the deposition of ZnO materials, with Mg and Al employed as co-dopants.
Zinc acetate dihydrate dissolved in isopropanol using diethanolamine acting as a stabilizer is the precursor solution. Magnesium acetate tetrahydrate, and Aluminum nitrate nonahydrate are used as Mg and Al sources respectively. Films were coated on to glass, quartz or sapphire substrates using either the dip-coating or the spin-coating technique. The initial iterative process of coating and preheating at 300 degrees was followed by a final air anneal step at temperatures ranging from 500 C to 750 C.
Undoped ZnO thin films deposited by this process show a poly-crystalline nature, which changes with doping (see below), and the photoluminescence at room temperature show a band-edge peak in the UV along with strong sub-bandgap luminescence peaks arising from various defect states. Mg incorporation shifts the peak to shorter wavelengths (see below), and strongly reduces defect-related luminescence.
In general, our results indicate that both the crystallinity as well as the optical properties can be strongly modified by the incorporation of Mg, Al or both. This gives rise to possibilities that with optimized levels of doping and annealing schemes, thin films with significantly improved electrical and optical properties can be produced.
Vapor-Liquid-Solid (VLS) grown ZnO NWs
We also work on ZnO nanowires grown by the Vapor-liquid-solid process.
In this process, the substrate, typically silicon, is coated with a ~1nm thick gold film using PVD processes. Subsequently, the coated substrate is annealed at 900 C in an inert ambiance to form gold nanoparticles.
Equal amounts of ZnO and graphite powders were placed in a quartz boat maintained at 900–925 °C. The annealed Au-coated substrates were located downstream and kept at 800–850 °C. Argon was used as the carrier gas. The reaction was carried out for 15–30 min.
Variations were thus made in thickness of Au film deposited, annealing temperature of the Au film for the formation of nano-particles, Ar flow rate, ZnO deposition time and growth temperature, to optimize fabrication.
The optimized nanowires were deposited on to both bare silicon as well as silicon pre-coated with Al-doped ZnO thin films deposited by the sol-gel process.
Our results indicate the while growth on to bare sapphire leads to nanowires with very high aspect ratio, those grown on to AZO are more uniform in height and diameter. They also have stronger band-edge luminescence.
Furthermore, EELS studies indicate that ZnO nanowires grown by VLS onto AZO thin films show the presence of Al which we believe diffuses from the underlying film.
Selective deposition of ZnO nanowires on top of Silicon microneedles
Growth of ZnO nanowires on various crystallographic facets of silicon was carried out in order to promote spatially-selective growth. Spontaneous growth of these structures at specific locations of a wafer is necessary for many applications, including photovoltaics and field-emission.
Silicon micro-pyramidal arrays were fabricated using the crystallographic wet chemical etching process. ZnO nanowires were grown onto these structures using the vapor-liquid-solid technique employing Au nanoparticles generated by annealing a sputter deposited layer.
Several variations were made in the crystallographic etch process to generate different facet morphologies. Furthermore, the gold annealing step was carried out under different conditions, and its effect on the nature and density of ZnO nanowires was determined. It was observed that even for identical deposition conditions for the ZnO nanowires, modification of these two parameters strongly affected the selective deposition on various silicon facets.
For relatively low Au annealing temperatures the entire surface of the micro-pyramidal structures was coated densely and uniformly with ZnO nanowires which were mostly aligned vertical to the facet surfaces. SEM as well as Cathodoluminescence (CL) imaging studies show that annealing of the Au film at higher temperature led to deposition of ZnO nanowires on the plateau at the top of the micro-pyramidal structures, while the side facets were completely bare.
The nanowires deposited on the plateau on top the pyramids were as long as 40 µm with a diameter of ~200 nm.
Oxygen breathing in ZnO thin films
We gave investigated the dependence of photocurrent intensity and switching on ambient conditions. Comparative studies were carried out on undoped, Mg-doped and Al-doped ZnO thin films deposited by the sol-gel process.
The photocurrent generated by UV excitation was observed to increase sharply under vacuum conditions compared to that measured in an air ambiance.
The undoped and Al-doped ZnO samples also showed extremely long persistent photocurrent under vacuum conditions, which reduced only when oxygen was reintroduced.
Based on these results, we believe that the photocurrent induced by two distinct phenomena in ZnO thin films under UV illumination. First is the generation of electron-hole pairs due to photoexcitation. The second phenomena is linked to surface adsorption of ambient oxygen which traps free electrons under atmospheric conditions, reducing the photocurrent levels. On UV excitation under vacuum the oxygen is irreversibly desorbed, leading to an increase in free electrons, and hence of photocurrent.
Mg-doped ZnO thin films however did not show such effects, indicating that UV excitation induced oxygen de-trapping is retarded in these samples.
We postulate that these properties are affected by the presence of defect states linked to grain boundaries in these materials. The photocurrent transients are related to UV-induced out-diffusion of oxygen from grain boundaries, and are influenced by the relative magnitude of the O-Mg and O-Zn bonds
Zn(Mg,Al)O based Ultraviolet Photo detectors
Ultraviolet photoconductivity studies were carried out on ZnO thin films grown by sol-gel process, using silver Ohmic contacts.
The films were either undoped, or doped with various levels of Mg and Al by adding their respective salts to the precursor solution. Energy dispersive X-ray spectroscopy (EDS) studies were carried out to determine their incorporation level into the film.
Undoped ZnO films show relatively high resistances, which was increased significantly with Mg doping. Al is a well-known dopant, and with low Al doping, both the dark current and the photocurrent increase significantly. However, these films show very long photocurrent transients, which is detrimental for applications were a fast-changing optical signal must be sensed. The Mg doped samples showed fast switching speeds despite the low photocurrent levels.
Co-doping the films with both Mg and Al allows us to optimize the switching characteristics, without sacrificing sensitivity. With optimized concentration of Mg/Al co-doping in ZnO, the photocurrent increased by ~98 times compared to ZnO films doped only with Mg. Simultaneously, the photocurrent transients became ~44 times faster than ZnO films doped only with Al.
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