Nanosecond pulsed laser induced self-organized nano-dots patterns on GaSb surface


The self-organized nanodots patterns can be formed on the surface under nanosecond pulsed laser irradiation at room temperature in air, and the nanopatterns can be controlled by number of laser pulses.
The formation from random to periodic pattern with growth and shrinkage of the nano-dots size as a function of the laser pulses.
The critical dot size is required for growth to be about 107 nm GaSb (1 0 0) surface under the nanosecond pulsed laser irradiation with 532 nm wavelength.


We report a technique for formation of two-dimensional (2D) nanodot (ND) patterns on gaillium antimoide (GaSb) using a nanosecond pulsed laser irradiation with 532 nm wavelength. The patterns have formed because of the interference and the self-organization under energy deposition of the laser irradiation, which induced the growth of NDs on the local area. The NDs are grown and shrunken in the pattern by energy depositions. In the laser irradiation with average laser energy density of 35 mJ cm−2, large and small NDs are formed on GaSb surface. The large NDs have grown average diameter from 160 to 200 nm with increase of laser pulses, and the small NDs have shrunken average diameter from 75 to 30 nm. The critical dot size is required about 107 nm for growth of the NDs in the patterns. Nanosecond pulsed laser irradiation can control the self-organized ND size on GaSb in air as a function of the laser pulses.


Nanosecond pulsed laser irradiation;
Nanodots patterns;
Gallium antimonide
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Below bandgap optical absorption in tellurium-doped GaSb


Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1–2) × 1017 cm−3 at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.
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Researchers develop ErSb nanostructures with applications in infrared and terahertz ranges

Researchers develop ErSb nanostructures with applications in infrared and terahertz ranges

This is an artist's concept of nanometer-size metallic wires and metallic particles embedded in semiconductors, as grown by Dr. Hong Lu. Credit: Peter Allen, University of California, Santa Barbara

In a feat that may provide a promising array of applications, from energy efficiency to telecommunications to enhanced imaging, researchers at UC Santa Barbara have created a compound semiconductor of nearly perfect quality with embedded nanostructures containing ordered lines of atoms that can manipulate light energy in the mid-infrared range. More efficient solar cells, less risky and higher resolution biological imaging, and the ability to transmit massive amounts of data at higher speeds are only a few applications that this unique semiconductor will be able to support.

"This is a new and exciting field," said Hong Lu, researcher in UCSB's Materials department and lead author of a study published recently in the journal Nano Letters, a publication of the American Chemical Society.
Key to this technology is the use of erbium, a  that has the ability to absorb light in the visible as well as infrared wavelength—which is longer and lower frequency wavelength to which the human eye is accustomed—and has been used for years to enhance the performance of silicon in the production of fiber optics. Pairing erbium with the element antimony (Sb), the researchers embedded the resulting compound—erbium antimonide (ErSb)—as semimetallic nanostructures within the semiconducting matrix of gallium antimonide (GaSb).
ErSb, according to Lu, is an ideal material to match with GaSb because of its structural compatibility with its surrounding material, allowing the researchers to embed the nanostructures without interrupting the atomic lattice structure of the semiconducting matrix. The less flawed the crystal  of a semiconductor is, the more reliable and better performing the device in which it is used will be.
"The nanostructures are coherently embedded, without introducing noticeable defects, through the growth process by molecular beam epitaxy," said Lu. "Secondly, we can control the size, the shape and the orientation of the nanostructures." The term "epitaxy" refers to a process by which layers of material are deposited atom by atom, or molecule by molecule, one on top of the other with a specific orientation.
"It's really a new kind of heterostructure," said Arthur Gossard, professor in the Materials Department and also in the Department of Electrical and Computer Engineering. While semiconductors incorporating different materials have been studied for years—a technology UCSB professor and Nobel laureate Herbert Kroemer pioneered—a single crystal heterostructured semiconductor/metal is in a class of its own.
The nanostructures allow the compound semiconductor to absorb a wider spectrum of light due to a phenomenon called surface plasmon resonance, said Lu, and that the effect has potential applications in broad research fields, such as solar cells, medical applications to fight cancer, and in the new field of plasmonics.
Optics and electronics operate on vastly different scales, with electron confinement being possible in spaces far smaller than light waves. Therefore, it has been an ongoing challenge for engineers to create a circuit that can take advantage of the speed and data capacity of photons and the compactness of electronics for information processing.
The highly sought bridge between optics and electronics may be found with this compound semiconductor using surface plasmons, electron oscillations at the surface of a metal excited by light. When light (in this case, infrared) hits the surface of this semiconductor, electrons in the  begin to resonate—that is, move away from their equilibrium positions and oscillate at the same frequency as the infrared light—preserving the optical information, but shrinking it to a scale that would be compatible with electronic devices.
In the realm of imaging, embedded nanowires of ErSb offer a strong broadband polarization effect, according to Lu, filtering and defining images with infrared and even longer-wavelength terahertz light signatures. This effect can be used to image a variety of materials, including the human body, without the risk posed by the higher energies that emanate from X-rays, for instance. Chemicals such as those found in explosives and some illegal narcotics have unique absorption features in this spectrum region. The researchers have already applied for a patent for these embedded nanowires as a broadband light polarizer.
"For infrared imaging, if you can do it with controllable polarizations, there's information there," said Gossard.
While infrared and terahertz wavelengths offer much in the way of the kind of information they can provide, the development of instruments that can take full advantage of their range of frequencies is still an emerging field. Lu credits this breakthrough to the collaborative nature of the research on the UCSB campus, which allowed her to merge her materials expertise with the skills of researchers who specialize in infrared and terahertz technology.
"It's amazing here," she said. "We basically collaborated and discovered all these interesting features and properties of the material together."
"One of the most exciting things about this for me is that this was a 'grassroots' collaboration," said Mark Sherwin, professor of physics, director of the Institute for Terahertz Science and Technology at UCSB, and one of the paper's co-authors. The idea for the direction of the research came from the junior researchers in the group, he said, grad students and undergrads from different laboratories and research groups working on different aspects of the project, all of whom decided to combine their efforts and their expertise into one study. "I think what's really special about UCSB is that we can have an environment like that."
Since the paper was written, most of the researchers have gone into industry: Daniel G. Ouelette and Benjamin Zaks, formerly of the Department of Physics and the Institute for Terahertz Science and Technology at UCSB, now work at Intel and Agilent, respectively. Their colleague Justin Watts, who was an undergraduate participant is now pursuing graduate studies at the University of Minnesota. Peter Burke, formerly of the UCSB Materials Department, now works at Lockheed Martin. Sascha Preu, a former postdoc in the Sherwin Group, is now assistant professor at the Technical University of Darmstadt.
Researchers on campus are also exploring the possibilities of this technology in the field of thermoelectrics, which studies how temperature differences of a material can create electric voltage or how differences in electric voltages in a material can create temperature differences. Renowned UCSB researchers John Bowers (solid state photonics) and Christopher Palmstrom (heteroepitaxial growth of novel materials) are investigating the potential of this new semiconductor.
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Compositional mapping of GaSb wafers from as-grown crystals and after post-growth annealing treatments


Spatial compositional analysis has been carried out on single and polycrystal wafers of GaSb grown from stoichiometric and non-stoichiometric melts. In crystals grown from stoichiometric melt, the ratio of Ga to Sb is slightly more and remains uniform throughout. At the grain boundaries in polycrystals, the Sb content is more than in the other regions of the crystal. Crystals grown from either Ga- or Sb-rich melts exhibit inclusions of the excess component. Post-growth annealing treatments in vacuum and Ga-rich atmospheres have been performed. Heat treatments in vacuum atmosphere produce very little effect on the local composition of the crystal. On the other hand, localized crystallization at grain boundaries and inclusions takes place in the presence of excess gallium. It has been shown that annealing treatments in Ga ambient can produce defect-free wafers with extremely homogeneous composition. It is concluded that the excess Sb which is liberated from the crystal during growth resides at the grain boundaries and other extended defect centers. The vacant Sb sites are then responsible for the formation of the native acceptor centers like VGa and GaSb.
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Anisotropic interface induced formation of Sb nanowires on GaSb(111)A substrates


The growth of Sb nanowires on GaSb(111)A substrates is studied by in situ azimuthal scan reflection high-energy electron diffraction (ARHEED). Bulk and layer contributions can be distinguished in the ARHEED transmission pattern through the Sb nanowires. The three-dimensional structure of the growing Sb nanowires is identified by post-growth atomic force microscopy (AFM) and x-ray diffraction (XRD). The lattice match of the Sb crystal along the $\langle \bar {2}10\rangle $ and the GaSb crystal along $\langle \bar {1}10\rangle $ directions lead to a preferential orientation of the Sb nanowires. The Sb adsorption and desorption kinetics is studied by thermal desorption spectroscopy.
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Enhanced hole mobility and density in GaSb quantum wells


Modulation-doped quantum wells (QWs) of GaSb clad by AlAsSb were grown by molecular beam epitaxy on InP substrates. By virtue of quantum confinement and compressive strain of the GaSb, the heavy- and light-hole valence bands in the well are split and the hole mobility is thereby significantly enhanced. Room-temperature Hall mobilities as high as 1200–1500 cm2/V s were achieved for 5–10 nm QWs and biaxial strains of 1–3%. This contrasts with earlier work on GaSb/AlGaAsSb QWs on GaAs substrates in which the mobilities were found to fall off above 1% strain. Moreover, unlike in comparable InGaSb and InSb QWs, the high mobilities were maintained out to sheet densities of 3.5 × 1012/cm2. As a result, the sheet resistivities observed in the GaSb/AlAsSb wells reached record levels as low as 1500 Ω/□. Modeling indicates that this performance gain is due to the larger valence band offset of the GaSb QWs and the consequent reduction in scattering because of the better confinement and the lower doping levels needed for a given sheet charge.


► GaSb quantum wells were grown on InP substrates with AlAsSb buffer layers. ► Compressive strain enhances hole mobility in the GaSb. ► Hole mobilities of 1500 cm2/V s at 300 K were achieved. ► Sheet resistivities as low as 1500 Ω/□, world record for p-type III–V quantum well. ► Could lead to better performance in p-channel FETs and applications in III–V CMOS.

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Reflectivity modulator based on GaSb/GaAs heterostructure


A structure of gallium antimonide (GaSb) and gallium arsenide (GaAs) wafers is built to modulate light reflectivity at CO2 laser wavelength. A quantum well composed of GaSb/GaAs heterojunction with highly doped GaAs up to 3×1018 cm-3 is inserted inside a layer structure. A grating of periodic structure of GaAs and gold layer is added just below the substrate. Gsolver software is used to determine the reflectivity of incident light with the existence of free carriers. A voltage is applied to the doped layer to deplete the free electrons and the reflectivity is determined again. The significant difference in reflectivity between the two cases can be used to build a light reflectivity modulator device.
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Composition and optical properties of dilute-Sb GaN1−xSbx highly mismatched alloys grown by MBE-3

Figure 4.
Figure 4. Etched Ga-rich GaN1 − xSbx WDX GaSb mole percentage versus Sb growth flux.

Figure 4 shows the weak but directly proportionate relationship between GaSb incorporation and Sb growth flux. It can be seen that the GaSb mole % is significantly reduced in the Ga-rich samples compared to the N-rich samples most likely due to the large quantities accumulated on the surface.
RBS data were measured from etched and pre-etched samples. For pre-etched material a value for the GaSb content in the bulk was established from plateau regions, which are clearly visible beyond the accumulated surface metal droplets, as shown in figure 5(a). Similar traces from etched samples gave a very close match for the GaSb composition in this region. The RBS data show a similar trend to the WDX data, but with larger GaSb content, shown in figure 5(b).
Figure 5.
Figure 5. RBS measurements showing (a) Ga-rich sample, before and after removal of the metal drops and (b) GaSb mole percentage against Sb growth flux.
To compare the difference in measured composition between the WDX and RBS, more samples were analysed which had higher measured GaSb fractions, represented by the triangular data points shown in figure 6. The solid line is a guide to the eye of the relationship between the WDX and RBS measurements and shows the techniques agree well, diverging only in their estimates of the very dilute-Sb samples composition. The dashed line is a one to one correlation between RBS and WDX. The square and circular data points are the measured Ga-rich and N-rich results, respectively. The most dilute, Ga-rich, samples show a divergence where RBS predicts higher GaSb mole concentrations. The exact reason for this is unknown, however it is possible unknown factors are affecting the WDX ZAF iterative routines due to the large difference between the composition of the GaSb standard relative to these very dilute GaN1 − xSbx samples.
Figure 6.
Figure 6. Comparison of the WDX and RBS technique's measurement of GaSb composition. Three sample series are shown including the Ga-rich (squares) and N-rich (circles) samples, as well as samples from Army research labs (ARL) which have a higher GaSb mole fraction (triangles). The solid line is a guide to the eye and the dashed line is the one to one correlation between RBS and WDX.
Absorption measurements from pre-etched Ga-rich samples were performed on samples with various GaSb mole percentages, determined by RBS. Figure 7 shows the absorption coefficient (α) as a function of the energy for a series of GaN1 − xSbx samples grown with 0 ≤ x ≤ 0.1%. The figure presents the clear observation of sub-gap absorption (<3.4 eV) for Ga-rich samples which had Sb present during growth. For the sample with no measured GaSb there is no observed sub-gap absorption. The sub-gap absorption can be seen at very low GaSb contents, which increases as the GaSb content increases.
Figure 7.
Figure 7. Absorption coefficient (α) against energy for Ga-rich, GaN1 − xSbx samples grown with and without Sb.
Room temperature PL and CL spectra were measured for Ga-rich GaN1 − xSbx samples, as seen in figure 8. The CL samples were fully etched and the PL samples were unetched. For these Ga-rich samples strong luminescence was observed. Using a 5 kV, 20 nA, focused electron beam and 5 s acquisition time, point CL was performed at a number of points for each sample. Monte–Carlo simulations show the 5 kV electron beam deposits 90% of its energy within ≈100 nm of the surface for the compositions measured for Ga-rich samples.
Figure 8.
Figure 8. (a) Typical room temperature CL and PL spectra for samples with Ga flux = 2.3 × 10−7 Torr, with and without Sb and (b) room temperature CL spectra for samples with various Ga flux and fixed Sb flux = 3 × 10−8 Torr.
The CL measurements in figures 8(a) and (b) show a strong GaN band-edge luminescence peak with 3.4 eV centre energy. Excitation studies, where the intensity of the electron beam excitation source was increased, show this peak to increase proportionately with beam intensity. The PL spectrum for the sample in figure 8(a) does not show a GaN band-edge peak but it should be noted that most other samples in this series did show a PL peak at 3.4 eV. A broad luminescence peak near 2.2 eV was also observed in Ga-rich samples (Ga flux > 2.3 × 10−7 Torr) where there was Sb present during growth. There was no 2.2 eV peak observed in samples grown under the same conditions, but with no Sb, however in this sample there is still a strong 3.4 eV peak. As discussed above the substitutional Sb is expected to introduce a localized energy level at ≈1.1 eV above the VBM, providing a possible explanation as to the origin of the broad 2.2 eV peak that could be attributed to the optical transitions from the CBM to the Sb level. It should be noted however that the observed peak energy coincides with the yellow luminescence peak very often observed in GaN. Figure 9 shows the plot of normalised 2.2 eV peak height versus Sb flux. The clearly observed increase of the peak intensity with Sb content support the notion that the localized Sb levels contribute to the emission in the 2.2 eV range.
Figure 9.
Figure 9. Normalised CL (Pre and Post-etch) and PL (Pre-etch) 2.2 eV peak heights against Sb flux, with fixed Ga flux = 2.3 × 10−7 Torr.
There is a strong relationship between the Ga growth flux and the peak intensity, which increases linearly. Point CL observes a small variation (≈10 meV) of the centre energy of the 2.2 eV peak with position probed. There is no observed correlation between this and the growth conditions, possibly due to a small degree of lateral compositional inhomogeneity. Point CL also showed a large variation of the peak height with probing position, therefore CL maps were performed to see the extent of the luminescence inhomogeneity, seen in figure 10.
Figure 10.
Figure 10. CL intensity map, showing the peak height in the range 2.0–2.4 eV, taken using an 8 kV, 10 nA, focussed beam for a sample containing Sb.
The CL map shown in figure 10 was performed with a 8 kV, 10 nA, focused electron beam, with 2 s acquisition time per pixel. The mapping area was 50 × 50 µm2. The map shows several bright features brighter than the mean value. Due to the very dilute nature of GaSb within these samples, the characteristic x-ray intensities are very low, which precludes a simultaneous map of Sb x-ray intensity

4. Conclusion

The compositional and optical characterisation of three series of dilute-Sb GaN1 − xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, were presented. WDX and RBS measurements show that for the same growth conditions more GaSb is incorporated during the growth under the N-rich rather than the Ga-rich conditions. The optical properties of the Ga-rich samples were measured using room temperature CL, PL and absorption measurements, on etched and pre-etched samples. The strong Sb content dependent luminescence with a peak at 2.2 eV is attributed to the optical transition from the conduction band to the localized Sb levels. CL mapping revealed a large spatial variation of the peak intensity of this 2.2 eV peak. The strong luminescence from these samples continues to suggest that dilute-Sb GaN1 − xSbx alloys are an excellent material system for use in solar energy conversion devices.


This work was undertaken with support from the EPSRC UK, under grant numbers EP/I004203/1 and EP/I00467X/1. The MBE growth at Nottingham was also supported by the US Army Foreign Technology Assessment Support (FTAS) program (grant W911NF-12-2-0003). The characterization work performed at LBNL was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract No. DE-AC02-05CH11231. Data associated with research published in this paper can be accessed by contacting the corresponding author.
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Composition and optical properties of dilute-Sb GaN1−xSbx highly mismatched alloys grown by MBE-2

3. Experimental results

The GaSb-incorporation in the samples grown under N-rich conditions was studied using RBS and WDX. Figure 2 shows the Sb profile measured by RBS from the sample with an Sb flux of 3.4 × 10−8 Torr and also the Monte–Carlo simulation of x-ray generation under 8 kV electron beam excitation.
Figure 2.
Figure 2. RBS measured depth profile of the GaSb mole fraction for a typical N-rich GaN1 − xSbx sample (solid line) and Monte–Carlo Sb x-ray generation with depth (dashed line).
Due to the WDX surface sensitivity, Monte–Carlo simulations were used to estimate the x-ray generation rate with depth. A weighted average was then performed between the Monte–Carlo x-ray intensity with depth curve, and the measured RBS GaSb depth profile, shown in figure 2. This allowed a weighted average GaSb percentage to be determined for direct comparison of the RBS and WDX results. Figure 3 shows the WDX and RBS measurements of GaSb mole % incorporation with Sb growth flux for N-rich samples. WDX shows the lowest measured GaSb mole% to be (0.27 ± 0.01)% and the highest measurement to be (0.66 ± 0.02)%, assuming a systematic error of 1% of the measured value.
Figure 3.
Figure 3. Plot of WDX and RBS GaSb mole percentage against Sb growth flux for N-rich GaN1 − xSbx layers.
An 8 keV, 40 nA electron beam was used to search for room temperature CL from these N-rich GaN1 − xSbx samples. No GaN1 − xSbx related luminescence peaks were observed in the range 330–850 nm.
The GaSb incorporation was found to be much lower in the Ga-rich GaN1 − xSbx samples. Due to the very small amounts of Sb extra care and analysis were required to quantify the GaSb content using WDX. To maximise the signal to noise a 7 kV electron beam, large counting times (240 s for the Sb L peak) and high currents (150 nA) were used. For each sample 10 random points were probed across the surface using a 10 µm defocused electron beam. In some cases the measured Sb x-ray counts were below the measured background for some of the data points and a negative value was then used in the calculation of the average GaSb atomic percentage. The resulting GaSb mole percentages are plotted against Sb flux in figure 4 which shows the lowest non-zero measurement to be (0.004 ± 0.002)% and the highest measured GaSb mole% to be (0.017 ± 0.001)%, assuming a systematic error of 5% of the measured value due to the large composition difference between the standards and the sample. At such low concentrations there may be additional uncertainties due to the correction procedures applied in the analysis of the WDX data.
Source: Iopscience
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Composition and optical properties of dilute-Sb GaN1−xSbx highly mismatched alloys grown by MBE-1


In this work the compositional and optical characterization of three series of dilute-Sb GaN1 − xSbxalloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux < 2.3 × 10−7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux > 2.3 × 10−7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.

1. Introduction

Highly Mismatched Alloys (HMAs) are semiconductor alloys where the substitutional atoms have very different atomic radii and/or electronegativity [1]. Examples include GaNAs [2], GaNBi [3] and InNAs [4]. Conventional semiconductor growth mechanisms have meant that such compounds could not be grown over a large range of compositions and were immiscible. Recently plasma assisted molecular beam epitaxy (PA-MBE) has been used to synthesise a number of HMAs over a large (or complete) range of compositions [4]. For example, GaN1 − xAsx has been synthesised over the entire composition range and GaN1 − xBix which has been grown with a GaBi concentration up to ≈11% using very low growth temperatures [23]. This allows their properties to be explored experimentally and compared with theoretical studies [45].
HMAs display a large bowing of their bandgap with composition and their electronic structure is drastically different from their constituent binary materials. The Band Anti-Crossing (BAC) model has been successfully used to describe the electronic structure of the conduction and valence bands of HMA s in the dilute alloy limit [67]. For the HMA GaN1 − xAsx the BAC model predicts a bandgap range of 3.4–0.7 eV with considerable bowing below the GaAs bandgap [28]. Even stronger modifications of the band structure are expected for more extremely mismatched GaN-based alloys, such as GaN1 − xSbx and GaN1 − xBix. The large bandgap range and controllable position of the conduction and valence bands make these materials promising systems for use in solar energy conversion devices [6].
For example, theoretical calculations predict that the addition of As or Sb to GaN at concentrations below ≈10% can substantially lift the valence band edge and thus reduce the fundamental bandgap [89]. The modification of the band structure enables the material to capture more photons from the solar spectrum while still maintaining the favourable alignment of the GaN band-edges with the redox potential of water for spontaneous hydrogen production by water splitting [410]. Such materials can be used as the photoelectrode within a photoelectrochemical (PEC) cell [9].
The electronic band structure of HMAs is determined by the anticrossing interaction between the localized level of the mismatched anion and the extended states of the host matrix (i.e.: Sb in GaN1 − xSbx). The energy of the localized state can be deduced from the known location of the state in another III–V compound and the assumption that the energy of the state remains constant relative to the vacuum level. It has been found previously that the Sb level in GaAs is located at 1.0 eV below the valence band edge [7]. This locates the Sb level at 1.0 eV above the valence band edge of GaN as the valence band offset between GaAs and GaN equals 2 eV [1112].
Transitions from the conduction band edge to this level would result in photons being emitted at an energy in the range 2.2–2.3 eV, namely 1.0 eV less than the GaN bandgap of 3.4 eV and including a Stokes' shift of about 0.1–0.2 eV. A similar situation has been observed in the GaN1 − xAsx HMA system, where the localized EAs level was determined by BAC fitting to be at 0.6 eV above the valence band edge [8]. This was associated with a blue emission at about 2.6–2.7 eV [1314] and used to explain the absorption edge shift with increasing As content [8]. Observation of the dilute Sb induced level would help to confirm the use of the BAC within the GaN1 − xSbx system and other similar alloys and would be useful in supporting the potential of these alloys for use in solar energy conversion devices.
GaN1 − xSbx is one important HMA and although there has been extensive study of the Sb-rich case [15] of GaSb alloyed with dilute amounts of N, there has been comparatively less reported on the dilute-Sb GaN1 − xSbx system. We have studied a wide range of growth temperatures for Sb doped GaN—from 10° C up to approximately 500° C [16]. The incorporation of Sb increases with decreasing growth temperature. In this paper we concentrate on low GaSb concentrations as it is better to grow GaN layers doped with Sb at temperatures that are as high as possible in order to increase the quality of the layers. In this work the GaSb contents in several series of dilute-Sb GaN1 − xSbx layers are accurately quantified, mapped and correlated with the strength of luminescence peaks observed in cathodoluminescence (CL) and photoluminescence (PL) spectra.

2. Experimental details

The dilute-Sb GaN1 − xSbx epilayers were grown at ≈500° C using plasma assisted molecular beam epitaxy (PA-MBE) in a MOD-GENII system on two-inch diameter sapphire substrates. The active nitrogen for the growth of the group III-nitrides was provided by an HD25 RF activated plasma source. Standard Veeco effusion sources were used for Ga and Sb. In order to increase uniformity across the wafer, all films were grown with substrate rotation of ≈10 rpm. In MBE the substrate temperature is normally measured using an optical pyrometer, however, because uncoated transparent sapphire was used, the pyrometer measures the temperature of the substrate heater, not that of the substrate. Therefore in this study estimates of the growth temperature are based on thermocouple readings [417].
Prior to growth the sapphire wafers were heated to ≈700° C and annealed for 20 min. After annealing, the substrate was cooled down to the growth temperature over a 20 min period under a reduced active nitrogen flux and growth was started by simultaneous opening of the Ga and N shutters. The Sb shutter was opened after a 1 min delay in order to avoid the deposition of any Sb on the sapphire surface before GaN growth. The growth time was kept at 2 h for all layers. The growth temperature was approximately 500° C resulting in low levels of GaSb incorporation. Studies of similar systems of GaN alloyed with group V anions [17], such as Bi, showed that in order to incorporate large amounts of the mismatched anions even lower growth temperatures were required, ranging from approximately 500° C to 80° C. The thicknesses of the GaN1 − xSbx layers are estimated to be approximately 500 nm.
A separate series of samples with higher GaSb contents were studied for comparison. These were grown at lower growth temperatures (275–375° C) in a second reactor. The material was grown using a Gen II Veeco solid-source MBE system equipped with Sb valved cracker sources, solid sources for Ga and a Uni-bulb plasma-source supplied the N. The samples were rotated at 5 rpm. The 2 inch sapphire substrates were outgassed at 800° C for 30 min. The growth time was 20 h.
There are three main growth regimes for PA-MBE of GaN [18]; N-rich growth (here the active nitrogen flux is larger than the Ga-flux), Ga-rich growth (here the active nitrogen flux is less than the Ga-flux) and strongly Ga-rich growth (here the active nitrogen flux is much less than the Ga-flux and Ga droplets are formed on the surface). In this study three dilute-Sb series were grown under the N-rich and Ga-rich regimes; with Sb fluxes varied up to 6.5 × 10−7 Torr. With the N supply held constant, the Sb and Ga growth fluxes for the samples in these three series are shown in figure 1.
Figure 1. Sb and Ga growth flux, representing three growth series: N-rich with various Sb flux; Ga-rich with various Sb flux; and fixed Sb flux with Ga flux which extends from N to Ga-rich. The dotted line indicates the region of the transition from N-rich to Ga-rich growth conditions.
Compositional studies were performed using Rutherford backscattering spectrometry (RBS); and by electron probe micro-analysis (EPMA) using a Cameca SX100 apparatus. A 3.04 MeV He2+ ion beam was used for RBS measurements to probe near the surface [19] and spectral fitting of the RBS data was performed using the SIMNRA [20] and SIMtarget [21] codes to obtain composition with depth information.
The EPMA has three wavelength dispersive x-ray (WDX) spectrometers, as well as the addition of an optical spectrograph and Silicon CCD array for Cathodoluminescence (CL) measurements [2223]. The samples are mounted on a precision stage which allows for micron-scale simultaneous mapping of WDX and CL signals [24]. For quantitative WDX the peak-to-background signals were compared with GaN and GaSb standards to obtain the experimental k-ratios (sample intensity/standard intensity). The k-ratios were converted to atomic percentages using standard ZAF correction iterative procedures [2225]. For each WDX measurement 10 points were probed along a length of about 5 mm. For quantitative measurements electron beam energies of 7 or 8 kV were used, with currents between 100 nA and 150 nA. Larger currents and acquisition times were needed for the very dilute samples. Monte–Carlo simulations [26] show that 90% of the energy in a 7 kV beam is deposited within a depth of 165 nm for GaN1 − xSbx(x = 0.016%), corresponding to 90% of the Sb x-rays being generated within 100 nm of the surface. A higher beam energy of 8 kV was used for the N-rich samples. Monte–Carlo simulations show 90% of the 8 kV beam energy is deposited within a depth of 175 nm, which corresponds to an Sb x-ray generation depth of 125 nm.
For point CL a 5 kV, 20 nA, focused beam was used with a 5 s acquisition time to probe multiple points. For mapping the conditions were changed to a 8 kV, 10 nA, focused beam with a 2 s acquisition time to improve image resolution. Room temperature photoluminescence (PL) measurements were performed using a ≈ 5.6 eV CW laser. The absorption spectra were measured using a LAMBDA-950 UV/vis/NIR spectrophotometer over the range 190–3300 nm [19].
For samples grown under Ga-rich conditions there was a surface layer of metal droplets, composed of Ga accompanied in some cases by Sb. These were removed by etching for approximately 20 mins using concentrated Hydrochloric (HCl) acid in an ultrasonic bath. Confirmation of the removal of the metal droplets was performed using Secondary electron (SE) and back-scattered electron (BSE) imaging and by WDX mapping.
Keywords:GaSb wafer,
Source: Iopscience
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