Passivation of Surface States of AlGaN Nanowires using H3PO4

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Passivation of Surface States of AlGaN Nanowires using H3PO4 Treatment to Enhance the Performance of UV-LEDs and Photoanodes Mahitosh Biswas, Vinayak Chavan, Songrui Zhao, Zetian Mi, and Subhananda Chakrabarti ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00447 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Nano Materials

Passivation of Surface States of AlGaN Nanowires using H3PO4 Treatment to Enhance the Performance of UV-LEDs and Photoanodes Mahitosh Biswas1, Vinayak Chavan1, Songrui Zhao2, Zetian Mi3, Subhananda Chakrabarti1* 1

Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

2

Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada

3

Department of Electrical and Computer Engineering, University of Michigan, Ann Arbor, MI 48109, United States Corresponding author*: [email protected]

ABSTRACT: Surface states serve as additional charge-carrier-trapping centers and create an energy barrier at the semiconductor–electrolyte interface. This in turn may severely reduce the internal quantum efficiency of AlxGa1−xN nanowire ultraviolet light-emitting diodes (UV-LEDs) and solar-to-hydrogen energy conversion efficiency of photoelectrodes used in photoelectrochemical water splitting applications. These states also cause fermi-level pinning and band bending, leading to Shockley–Read–Hall non-radiative recombination. Hence, surface states need to be passivated. In the present study, we used phosphoric acid to passivate the surface states in AlGaN nanowires. The internal quantum efficiency of the near-band-edge emission peak of the chemically treated nanowires was 7%, whereas that of the as-grown nanowires was 3%. Suppression of the oxide layers was achieved, as indicated by the reduced intensity of the O 1s peak. The higher carrier lifetime of 3.2 ns of the treated nanowires compared to the lifetime of 2.6 ns of the as-grown nanowires 1 ACS Paragon Plus Environment

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directly evidenced passivation of the surface states. Crystallinity loss at the nanowire edges was caused by strain relaxation, resulting in broadening of the A1(LO)AlGaN phonon mode. The experiments and findings could be useful in the fabrication of UV-LEDs and photoelectrodes with improved performance for water splitting applications. KEYWORDS: AlGaN, nanowires, surface sates, H3PO4 treatment, internal quantum efficiency, carrier life time, strain relaxation INTRODUCTION Nanowires (NWs) possess superior properties to planar nanostructures because of their dislocation-free growth on foreign substrates. Moreover, the reduced polarization field and high light-extraction efficiency of NWs make them suitable for fabricating AlGaN-based optoelectronic devices, which have found tremendous potential application in the fields of medicine and biosensors. For ultraviolet light-emitting diodes, a highly efficient and spectrally pure 3.65-eV (340-nm) ultraviolet (UV) emission excitation source must be integrated into nanobiosensors for disease diagnostics1,2, and a biocompatible implantable UV source is required in medical devices in order to optically control neutrons and perform photolysis of photolabile caged compounds3,4. In such cases, emission above 3.65 eV is not applicable because deep UV light induces toxicity5,6. Recently,

III-nitride

nanostructures

have

been

employed

in

photoelectrochemical water splitting applications owing to their tunable band gap and band edge potential7,8. However, III-nitride NWs have severe surface states, which causes surface fermi-level pinning and surface band bending. This leads to Shockley–Read–Hall nonradiative recombination and indirect recombination associated with the radial Stark effect9,10. For example, InGaN/GaN NWs grown through molecular beam epitaxy (MBE) exhibit poor photoelectrochemical properties such as low photocurrent, high leakage current, and low turn-on potential11. Therefore, to achieve a high solar-to-hydrogen 2 ACS Paragon Plus Environment

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energy conversion efficiency and long-term stability in NWs for water splitting applications, the effect associated with the surface states needs to be minimized. Various surface treatments have been used to passivate surface states in semiconductors to improve device performance. Sulfides are the most widely used agents for surface treatment as they strongly bond with the surface atoms of III–V compound semiconductors, such as GaP, GaN, and GaAs12,13

14

,

resulting in surface state passivation of these materials. Usually, ammonium sulfide ((NH4)2S)15 is used as an inorganic compound while thioacetamide and octadecylthiol are used as organic compounds for surface passivation. Khan et al. recently reported contact resistance and enhanced emission of passivated InGaN/GaN NW LEDs through octadecylthiol treatment16. Dilute H3PO4 solution has also been used to moderately remove the residual oxide layer from an AlN layer17. However, to our knowledge, the use of H3PO4 for surface passivation of III-nitride semiconductor nanostructures has not been reported thus far. In the present study, we investigate H3PO4 treatment for as-grown AlGaN NWs in order to reduce the pronounced effect of chemisorbed hydroxyls and dangling bonds as well as to eliminate the oxide layer formed on the surface of the NWs. By optimizing the conditions for surface passivation, the photoluminescence intensity of passivated NWs was improved more than three-fold compared to that of the as-grown sample. The carrier lifetime of the treated NWs improved significantly,

as

is

confirmed

using

time-resolved photoluminescence

spectroscopy; this indicates a reduction in the number of non-radiative recombination centers. Growth of AlGaN nanowires. An MBE (Vecco Gen II) system equipped with a radio-frequency plasma source for incorporating nitrogen was employed to grow the NWs. Native oxide desorption occurred at a substrate temperature of 770°C. An ultrathin layer (∼0.6 nm) of Ga, called the seeding layer, was first 3 ACS Paragon Plus Environment


grown. Subsequently, vertically aligned AlGaN NWs were grown at a rate of 2.5 nm/min with Si doping under N-rich conditions and at a substrate temperature of 800°C. The Si doping concentration was 1 × 1018 cm-3 (based on calibration from Si-doped AlGaN epilayer grown under the similar conditions), nitrogen flow rate was maintained at 1 standard cubic centimeter per minute, and plasma power was constant at 350 W. Si was introduced in AlGaN NWs in order to make them n-type. The as-grown NWs were cleaned with 2% HF solution and subsequently treated with H3PO4 for 5 min, as can be seen in Figure 1.

Figure 1. A scheme to illustrate AlGaN NWs and their fabrication process.

A schematic of single-layer n-doped AlGaN NWs grown on an n-Si (111) substrate is shown in Figure 2(a). For investigating the structural properties of the as-grown AlGaN NWs, we performed field-emission gun scanning electron microscopy (FEG-SEM; JEOL). Figure 2(b) shows a 45° tilted view of the ensemble of the NWs grown on the Si substrate; the figure indicates that the NWs are aligned to the substrate, and the NW ensemble has a length distribution of ∼1 µm. Figure 2(c) displays the top view of the NWs with an areal density of ∼6 × 109 cm-2. Transmission electron microscopy (TEM; JEOL) measurements were conducted on a single NW transferred on a copper (Cu) 4 ACS Paragon Plus Environment

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grid; the NW was 1.25 µm long and had a top diameter of 40 nm and a bottom diameter of ∼106 nm (Figure 2(d)). A high-resolution (HR) TEM image confirmed the nearly hexagonal morphology of the NW base, as shown in Figure 2(e). For the treated NWs, a very thin, relatively less crystalline layer (∼1 nm) was formed at the periphery of the NW (Figure 2 (f)), which was assumed to be due to the H3PO4 treatment. When the NW was viewed along growth direction (0001), the periodicity of lattice atoms was observed to be disturbed (shown in red dotted line, Figure 2(g)), probably because of Si vacancies and/or interstitial Si atoms. The inset (left) and (right) exhibited selected area electron diffraction (SAED) patterns and HR-TEM image of edge of as-grown NW, respectively.

(c)

(b)

(d)

(g)

AlGaN

Si

0.2 µm

AlGaN (e)

((f)

Figure 2. (a) Schematic of vertically aligned AlGaN NW grown on a Si (111) substrate. (b) 45° tilted (bird’s eye) view of the ensemble of NWs. (c) Top view of the NWs. (d) TEM image of a single NW transferred on a Cu grid. HR-TEM image of the (e) hexagonal base of the NW, (f) edge of the H3PO4-treated NW, and (g) a region (shown in red dotted lines) of Si

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vacancy and/or interstitial Si atoms. Inset (left); SAED pattern and inset (right); HR-TEM image of edge of as-grown NW.

To estimate the tilt of the ensemble of the NWs grown on the Si substrate, X-ray diffraction (Rigaku) was performed using a double-axis diffractometer (SmartLab). The full width at half maximum (FWHM) values of the rocking curves (ω scan) of the symmetric (0002), (0004), and (0006) reflections are shown in Figure 3(a,b,c). The broadening of the rocking curves of these reflections is attributed to the tilt (out-of-plane misorientation) only in NWs. By employing the Williamson Hall method18, βsinθ/λ can be plotted as a function of sinθ/λ for each reflection and fitted by a straight line, as shown in Figure 3(d). β is the FWHM, 2θ is the scattering angle, and λ is the X-ray wavelength (1.5406 A°). The slope of the fitted experimental data points is referred to as the tilt angle, which was ∼1°. Therefore, it is clear that the ensemble of NWs was highly oriented along the growth direction (c-axis). Information related to separate ω scans was obtained via the 2θ−ω scan of the AlGaN NWs grown on the Si substrate (see supporting information, Figure S1).

FWHM = 1.13°

14

16

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ω (°)

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(c)

(0004)

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(b)

FWHM = 1.06°

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(a)

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ω (°)


FWHM = 1.02°

60

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64

ω (°)

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(d)

βSinθ θ/λ λ (A°-1) × 10-3

Experimetal data Linear fit

(0006)

3

(0004)

2 Tilt = (0.97 ± 0.02)° (0002) 1 0.2

0.3

0.4

0.5

0.6

-1

Sinθ θ/λ λ (A° )

Figure 3. (a, b, c) X-ray rocking curves (ω scans) of the symmetric (0002), (0004), and (0006) reflections. (d) Williamson Hall plot for the symmetric ω scans

The optical properties of the AlGaN NWs were studied using room-temperature (300 K) photoluminescence (PL) spectroscopy with a 325-nm excitation laser source (He-Cd). The laser-illuminated area on the sample was ∼2 × 10-5 cm2 and the density of NWs, as obtained from FEG-SEM, was 6 × 109 cm-2. So, number of NWs excited is approximately 105. However, the NWs were initially dipped into 2% HF solution for cleaning purpose. To see the effect of HF solution on the optical properties of NWs, PL was performed at 300 K, as can be seen in Fig. 4(a). There was not much difference in PL intensity observed between asgrown and HF-treated NWs. These were then treated with H3PO4 solution for a time duration of 5, 10, and 15 mins. As seen in the Fig.4 (b), the NWs for 5 min exhibited maximum PL intensity as compared to the as-grown sample, and hence it was taken for rest of the measurements.

(a)

300 K

(b)

As grown 2% HF

300 K

As grown H3PO4 : 5 min

6.0E5

1.5E5

H3PO4 : 10 min

Intensity (a.u.)

H3PO4 : 15 min

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0E5

5.0E4

4.0E5

2.0E5

7 0.0 3.3

3.4

3.5

Energy (eV)

3.6ACS

0.0

Paragon Plus Environment 3.3 3.4 3.7

3.5

Energy (eV)

3.6

3.7


Figure 4. Room temperature PL spectra of AlGaN NWs treated with (a) 2% HF and (b) H3PO4 solution for a time duration of 5, 10, and 10 mins. As-grown NWs were taken as reference sample.

After Gaussian deconvolution, the overall PL spectrum of the as grown and H3PO4-treted NWs recorded at 300 K comprised two peaks, as can be seen in Figure 5(a,b). The first peak was observed at 3.55 eV and is attributable to the emission due to the recombination of an exciton bound to a shallow neutral donor (D°XA)19. The possible neutral donors are impurities (Al), which may form stable complexes with VN, i.e., Al–VN20. The other peak appeared at 3.57 eV, called the near-band-edge (NBE) emission peak, due to recombination of a free exciton (FE) and was observed to be blue-shifted as compared to 3.47 eV for GaN at 300 K21,22. The Al concentration estimated from the NBE emission was 10% at a band gap bowing of 1.3 eV. The PL intensity of the H3PO4-treated NWs was about four times higher than that of the as-grown NWs because of a reduction in surface states or dangling bonds and found to be almost durable with time (see supporting information, Figure S2). Figure 5(c,d) shows temperature-dependent PL measurements from 19 to 300 K at an excitation power of 9 mW. At a low temperature, donor-bound excitons (D°XA) and free excitons were formed. When the temperature was increased to 300 K, the peak intensity gradually decreased. This indicates that at low temperatures, the nonradiative recombination centers were sufficiently inactive, whereas at higher temperatures, the density of these defect centers increased and hence the optical properties were strongly affected, eventually leading to PL intensity degradation. The overall emission peaks for the as-grown sample remained almost at the same position up to 100 K, after which they gradually red-shifted.


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However, the emission peaks of the treated NWs were constant up to 125 K. Lattice expansion and electron–phonon interactions are responsible for the characteristic red-shift with increasing temperature23. Therefore, the difference in the temperatures up to which the PL emission remains constant for the asgrown and treated NWs is attributed to the difference in their lattice expansion and electron–phonon interactions. The NBE emission of semiconductor nanostructures blue-shifts mainly because of the following three factors: the quantum confinement effect24, temperature reduction of the material25, and alloying with appropriate elements26. Since the Bohr radius of GaN is 2.8 nm27, the effect of quantum confinement in blue-shifting of the NBE emission of AlGaN NWs with an average diameter of 40–100 nm can be excluded. 2.0E5 As grown D°XA

(a)

H3PO4

(b)

300 K

D°XA NBE Cumulative peak

6.0E5

Intensity (a.u.)

Intensity (a.u.)

300 K

NBE Cumulative peak

1.5E5

1.0E5 3.57 eV

5.0E4

4.0E5 3.57 eV

2.0E5

3.55 eV

3.55 eV

0.0 3.3

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3.5

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0.0 3.3

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3.5

Energy (eV) (c)

9 mW

As grown

2.0E6

H3PO4

19 K 25 K 50 K 75 K 100 K 125 K 150 K 175 K 200 K 220 K 240 K 260 K 280 K 300 K

1.0E7 8.0E6 6.0E6 4.0E6 2.0E6 0.0

0.0 3.4

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Energy (eV)

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IQED°°X = 6% A IQENBE = 3% IQEoverall = 4% 0

10

20

(f)

1

D°°XA NBE Overall

0.1

30

1000/T (1/K)

3.6

3.7

Energy (eV)

As grown

(e)

1

0.01

3.7

9 mW

(d)

1.2E7

Intensity (a.u.)

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4.0E6

3.6

Energy (eV)

19 K 25 K 50 K 75 K 100 K 125 K 150 K 175 K 200 K 220 K 240 K 260 K 280 K 300 K

6.0E6

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H3PO4

D°°XA

0.1

NBE Overall IQED°°X = 14% A IQENBE = 7% IQEoverall = 9%

9 ACS Paragon Plus 0.01 Environment

40

50

60

0

10

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30

1000/T (1/K)

40

50

60


Figure 5. Room temperature PL spectra (a,b), temperature-dependent PL spectra (c,d), and integrated PL intensities (e,f) of the as-grown and H3PO4-treated AlGaN NWs.

The internal quantum efficiency (IQE) can be estimated from the ratio of the integrated PL intensity recorded at 300 K to that measured at a low temperature (19 K), assuming there is no non-radiative recombination centers present at 19 K. The integrated intensities of the entire spectrum and the fitted peaks for both as-grown and chemically treated NW samples were plotted against the inverse of temperature, as shown in Figures 5(e,f). At an excitation power of 9 mW, for the as-grown NWs, the integrated intensity of the entire spectrum at 300 K remained about 4% of what it was at 19 K. The IQE of the D°XA and NBE emissions was also estimated by comparing the PL intensities at 3.55 eV and 3.57 eV, respectively. At 300 K, the peak integrated intensity of the NBE remained 3% of that at 19 K, whereas that of the D°XA was 7%. For H3PO4treated NWs, the IQEs of the entire spectrum, NBE, and D°XA were increased to 9%, 7%, and 14%, respectively. This indicates that the NBE peak integrated intensity of the treated NWs at 300 K remained 7% of that at 19 K, which is an improvement of more than two-fold compared to that of the as-grown NWs. The high IQE of the treated NWs may be attributed to the reduction in nonradiative recombination centers associated with surface states or defects. For investigating changes in the chemical properties of the AlGaN NWs after chemical treatment, X-ray photoelectron spectroscopy (XPS; Kratos) was performed with a monochromatic Al-α source (hν = 1486.4 eV) at an angle of 54.7° with the analyzer. So, top as well as side walls of NWs were probed by 10 ACS Paragon Plus Environment

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the X-ray source. The binding energy was calibrated based on C 1s peak (284.8 eV) with a standard deviation of approximately ±0.05 eV. The results were fitted with the Lorentzian distribution. Figure 6(a) shows the XPS results of the P 2p spectra of the H3PO4-treated NWs. Spin-orbit components i.e. P 2p3/2 and P 2p1/2, were found to be overlapped for the phosphate (PO4)3-, which is unlikely in phosphides (P3-)28. A strong peak observed at 134.6 eV and another peak located at 129.8 eV can be assigned to the P–O bonds and Si–P, respectively, based on the P 2p1/229. Two additional peaks were also observed in the spectra, which could be due to presence of impurities during growth and/or contamination issue during chemical treatment. Due to spin-orbit interaction, the Ga 2p peak split into two peaks corresponding to Ga 2p3/2 (1118.8 eV) and Ga 2p1/2 (1145.6 eV), as seen in Figure 6(b). After normalization of the Ga 2p3/2, the peak position (linewidth) was found at 1119 (1.78) and 1118.9 (1.48) eV for the as-grown and treated NWs, respectively, (Figure 6(c,d)). The peak position at 1119 eV was referred to Ga 3+ of Ga ̶ O30. Since, there was hardly any change observed in the Ga 2p3/2 peak positions, the oxidation state (3+) of Ga was not changed after the treatment. To illustrate the surface morphology of NWs, particularly the suppression of surface oxidation after H3PO4 treatment, the O 1s peak was focused on, as it is associated with surface oxidation. Figure 6(e,f) shows the O 1s peaks for both the as-grown and chemically treated NWs at 532.5 eV, which can be assigned to Ga–O and/or Ga–OH31,32. The intensity of this peak can be correlated with the thickness of the oxide layer formed in the AlGaN NWs. The surface oxidation was considerably suppressed after H3PO4 treatment, indicated by the significant decrease in the Ga–O and/or Ga–OH peak intensities. The improvement in the O 1s peak spectrum clearly indicates that H3PO4 treatment plays a significant role in reducing chemisorbed −O and −OH bonds in AlGaN NWs. The present study results in a better NW surface than that of the recently reported GaN NWs after EDT treatment, which was apparently effective in reducing surface oxidation33. More useful information 11 ACS Paragon Plus Environment


about the P 2p1/2, Ga 2p3/2, and O1s can be found in the supporting information, Table S1.

25

80

P 2p

(a)

H3PO4

Ga 2p3/2

Si− −P 70

65

125.6 eV 127.3 eV 129.8 eV 125 130

60

Ga 2p1/2 15

10

140

1120

1130

Binding energy (eV) 1.0

Ga 2p3/2

(c)

1.0 (d)

As grown

1150

Ga 2p3/2

H3PO4

0.8

Intensity (a.u.)

Intensity (a.u.)

1140

Binding energy (eV)

0.8 0.6 0.4 0.2

0.6 0.4 1118.9 eV 0.2

1119 eV

0.0

0.0 1116

1118

1120

1116

1122

1118

(e)

As grown

O 1s

1120

1122

Binding energy (eV)

Binding energy (eV) 15

As grown H3PO4

Ga 2p

5

134.6 eV 135

(b)

20

P− −O

Impurity

Intensity (a.u.)

Intensity (a.u.)

75

(f)

H3PO4

O 1s

13 12

532.4 eV

11

Intensity (a.u.)

14

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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532.5 eV

11

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12

9

526

528

530

532

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532

534

Binding energy (eV)

536

538

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Figure 6. XPS spectra of (a) P 2p, (b) Ga 2p , (c,d) normalized Ga 2p3/2 and (e,f) O 1s peaks of the as-grown (red solid circles) and H3PO4-treated (blue solid circles) AlGaN NWs. Dots and solid lines define the experimental data and fitted curves, respectively. The dotted lines indicate the corresponding binding energy of the fitted curves.

Ultrafast laser spectroscopy was used to investigate the effect of surface states on the photoexcited carrier of the NWs. Repetition rate of injected pulses and average injected power were 1 MHz and 1-2 mW, respectively. Figure 7 shows the time-resolved photoluminescence (TRPL) spectroscopy of the as-grown and chemically treated AlGaN NWs in the spectral window around the NBE emission, i.e., 347 nm. The emission decay signal was best fitted by the double exponential decay functions. The carrier life times, i.e., τ1 and τ2, which are the fast and slow decay constants, were 0.7 ± 0.01 and 2.6 ± 0.05 ns for the asgrown NWs, respectively, and 0.7 ± 0.01 and 3.2 ± 0.12 ns for the treated NWs, respectively. The carrier life time is related to the density of band gap electronic states caused by internal defects and/or surface states36. Therefore, it is clear that the H3PO4-treated NWs have a lower decay constant than the as-grown NWs do, thus indicating a reduction in surface states. Very recently, slow decay constant of the MBE-grown p-i-n AlGaN nanowires has been improved by 1.16 times after KOH treatment34, which is less than 1.23 times of our present study. Hence, we concluded that surface passivation of AlGaN NWs can be realized using H3PO4 treatment.



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τ1 (0.7 ± 0.01) ns

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τ2 (3.2 ± 0.12) ns

As grown τ1 (0.7 ± 0.01 1) ns

5

10

τ2 2.6 ± 0.05) ns

15

20

Time (ns)

Figure 7. TRPL of the as-grown and the H3PO4-treated AlGaN NWs.

Typical unpolarized Raman spectroscopy (Horiba HR 800) was performed for the highly oriented AlGaN NWs before and after chemical treatment by using a 514-nm laser source (Ar+) and a charge-coupled detector to collect the signals. For a hexagonal structure, group theory suggests that eight phonon normal modes can exist at Γ point (k ≈ 0, wave vector of phonons), which can be defined as 2A1 + 2E1 + 2B1 + 2E2. Among them, one each of A1 and E1 phonon modes is acoustic and both B1 modes are silent; therefore, the remaining four modes, A1 + E1 + 2E2, indicate Raman active35. An intense peak appeared at 572 cm-1, attributable to GaN-like E2 (high)AlGaN, which was blue-shifted compared to 567 cm-1 of the stress-free GaN36. This indicates that NWs are under compressive strain because of Al incorporation. In the range 600–775 cm-1, three peaks were observed around 620, 672.5, and 758 cm-1, which were assigned to AlN-like E2 (high)AlGaN, E2 (high)AlN, and A1 (LO)AlGaN37, respectively, (Figure 8(a)). The presence of GaN-like E2 (high) and AlN-like E2 14 ACS Paragon Plus Environment

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(high) phonon modes indicated the two-mode behavior of the AlGaN nanowires, whereas the A1 (LO) mode indicated one-mode behavior. E2 (high)AlN appeared in the spectra because of the formation of cubic AlN (see supporting information, Figure S3). E2 (high) is non-polar and indicates strain in NWs, whereas A1 (LO) is polar and is affected by both strain and coupling with a free-electron cloud i.e., plasmons formed by Si doping38. Kriste et al. reported that Si doping in AlGaN layer leads to the relaxation of compressive strain39. It also causes a shift and broadening in A1 (LO) compared to that of undoped layer, attributable to enhanced plasmon–phonon interaction and lattice distortion caused by Si incorporation, respectively. A similar trend was previously observed in doped AlN and GaN40. Figures 8(b) and inset show the A1 (LO) phonon mode in the Raman spectra. In H3PO4-treated NWs, additional strain relaxation occurred as E2 high shifted to a lower energy (see supporting information, Figure S4). Broadening (∼8 cm-1) in the A1 (LO) phonon mode frequency was caused by relatively less crystallinity at the edges of the NWs compared to that of the as-grown samples, as shown in Figure 2(f). Si phonon modes were also observed (see supporting information, Figure S5).

300 K

AlN-like E2 (high)AlGaN E2 (high)AlN

H3PO4

A1 (LO)AlGaN

550

600

650

700

Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak sum

8.

763.7 cm-1 754.9 cm-1 761 cm-1

745

760

765

770

775

-1

-1

755.2 cm

761 cm

750

-1

766 cm -1

751.5 cm

745

755

-1

-1

750

750

Raman shift (cm )

763.7 cm

755

760

765 -1

-1

Figure

As grown Peak 1 Peak 2 Peak 3 Cumulative peak

300 K

(b)

As grown H3PO4

Intensity (a.u.)

(a)

Intensity (a.u.)

GaN-like E2 (high)AlGaN

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Raman shift (cm )

Raman shift (cm )


770

775


Raman spectroscopy of (a) as-grown and H3PO4-treated AlGaN NWs recorded at 300 K. Zoomed section of the A1(LO)AlGaN phonon mode of the treated NWs (b) and as-grown NWs (b) inset.

CONCLUSION: Using PA-MBE, a high density of catalyst-free AlGaN NWs were obtained with a tilt angle of 1° with respect to the Si (111) substrate. Since surface states cause poor device performance, their adverse effects in the NWs during growth and subsequent exposure to air were addressed. The IQE of the H3PO4-treated NWs, which exhibit NBE emission at 3.57 eV, was better than that of the as-grown NWs. –OH and –O bonds were evidently suppressed in the treated NWs. Direct evidence of elimination of surface states by H3PO4 treatment was observed through TRPL measurements, which showed that the treated NWs had improved carrier life time. Crystallinity loss at the edges of the NWs was caused by strain relaxation, which resulted in broadening of the A1(LO)AlGaN phonon mode. Supporting Information: Additional HR-XRD 2θ-ω patterns and Raman spectroscopy of AlGaN NWs Author Information: Corresponding author *E-mail: [email protected] ORCID Id: Subhananda Chakrabarti: 0000-0002-8459-0890 Notes: The authors declare no competing financial interests. 16 ACS Paragon Plus Environment

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Acknowledgements: We thank the Department of Science and Technology (Grant No. SB/S3/EECE/ 0106/2014)) for the financial support. We also acknowledge the nanofabrication facility, central facility, and sophisticated analytical instrument facility at IIT Bombay for helping with the HR-XRD, FEG-SEM, FEG-TEM, XPS, and Raman spectroscopy measurements. The TRPL measurements were carried out at Prof. A. Dutta’s laboratory at IIT Bombay.



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300 K

H3PO4-treated nanowires

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 nm As-grown nanowires

3.2

3.3

3.4

3.5

3.6

Energy (eV)


3.7

Passivation of Surface States of AlGaN Nanowires using H3PO4

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