In-plane anisotropic photoconduction in nonpolar epitaxial a-plane GaN

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In-plane anisotropic photoconduction in nonpolar epitaxial a-plane GaN Rohit Pant, Arjun Shetty, Greeshma Chandan, Basanta Roul, Karuna Kar Nanda, and Saluru Baba Krupanidhi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05032 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Applied Materials & Interfaces

In-plane anisotropic photoconduction in nonpolar epitaxial a-plane GaN Rohit Pant,1 Arjun Shetty, 2 Greeshma Chandan,1 Basanta Roul, 1, 3 K K Nanda*1 and S B Krupanidhi∗1 1. Materials Research Centre, Indian Institute of Science, Bangalore-560012, India. 2. Electrical Communication Engineering, Indian Institute of Science, Bangalore, India 560012. 3. Central Research Laboratory, Bharat Electronics, Bangalore 560013, India.

* ∗

[email protected] [email protected]

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ABSTRACT: Nonpolar a-plane GaN epitaxial films were grown on an r-plane sapphire using plasma-assisted molecular beam epitaxy (PAMBE) system, with various nitrogen plasma power conditions. The crystallinity of the films was characterized by high-resolution X-ray diffraction and reciprocal space mapping. Using the X-ray ‘rocking curve − phi scan’, [0002], [1-100] and [1-102] azimuth angles were identified and interdigitated electrodes (IDEs) along these directions were fabricated to evaluate the direction-dependent UV photoresponses. UV responsivity (R) and internal gain (G) were found to be dependent on the azimuth angle and in the order of [0002] > [1-102] > [1-100], which has been attributed to the enhanced crystallinity and lowest defect density along [0002] azimuth. The temporal response was very stable irrespective of growth conditions and azimuth angles. Importantly, response time, responsivity and internal gain were 210 ms, 1.88 AW−1 and 648.9%, respectively even at a bias as low as 1 V. The results were validated using Silvaco Atlas device simulator and experimental observations were consistent with simulated results. Overall, the photoresponse is dependent on azimuth angles and requires further optimization, especially for materials with in-plane crystal anisotropy. KEYWORDS: Nonpolar, molecular beam epitaxy (MBE), interdigitated electrode (IDE), Gallium Nitride, azimuth angle, UV- detectors.

INTRODUCTION: UV photodetectors find a variety of applications in optical communication, flame detection, ozone monitoring, etc.1–3 The wide band gap materials such as GaN and ZnO are very much suitable in the area of UV detection.1–8 Though ZnO has also been widely studied, the oxygen


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adsorption plays a key role in the conduction process and slows down the response and recovery time. In addition, the exciton binding energy of ZnO (~60 meV) is high as compared to GaN (~21 meV) which is expected to yield low photocurrent.9–11 Furthermore, chemical stability and radiation hardness qualify III-nitrides as a very suitable material in the area of UV detection.1–6 The first UV detector based on GaN was reported by Khan et al in 1992 and subsequently, IIInitride based UV photodetectors witnessed significant growth in the following decade including thin films, nanostructures and hybrid materials adhering to principle of plasmonics.4–6 In spite of all these advantages, GaN suffers from a lack of a suitable substrate and high dislocation densities. Attempts have been made to grow thin films on freestanding GaN crystal, though it is an expensive proposition.12–15 Lattice mismatch between the thin film and the substrate is also one of the factors which decide the quality of the GaN and the performance of subsequent devices. Nonpolar growth of epitaxial GaN appears to be a viable approach, due to the lowest lattice mismatch between nonpolar a-plane (11-20) GaN and r-plane (1-102) sapphire which is around 1.19% along one of the azimuths.16 Nonpolar nitrides have been widely considered and explored due to their advantage over polar nitrides as the latter inherits spontaneous polarization along with piezoelectric polarization. Moreover, nonpolar nitrides have shown better performance than polar in terms of device performance and stability.17,18 Polar and nonpolar nitride UV photodetectors have been extensively studied in the past using different configurations such as Schottky diodes, metal-insulator-metal (MIS), p-n and p-in junction diodes and metal-semiconductor-metal (MSM) structures.1,5,19–21 Among these, MSM type of detectors have been widely used because of their advantages such as simplicity, simple fabrication process and lower capacitance. The MSM structure consisting of Schottky contacts offers advantages such as very low dark current, reduced noise and high internal gain.21,22 There



are reports in the literature on nonpolar GaN based MSM photodetectors, where results have shown better performance compared to conventional polar MSM photodetectors19,20. In these reports it has been shown that the responsivity (R) of nonpolar a-plane GaN was 0.155 AW−1 at 2.0 V which was much better as compared to polar c-plane GaN (R = 0.00576 AW−1). Similarly, the external quantum efficiency (EQE), response and recovery times of non-polar GaN are better as compared to polar GaN19. Also higher responsivity (R = 0.340 AW−1), response and recovery times have been reported for nearly stress free, non-polar GaN but the bias voltage applied was 5.0 V20. In addition to the stress and bias voltage, in-plane anisotropy of non-polar nitrides is expected to influence the optoelectronic properties and has not been exploited for optoelectronic properties. It would be really interesting to study the UV detection performances along the anisotropic directions in nonpolar based MSM photodetectors. Categorially, these anisotropic directions are known as azimuth angles, and for the nonpolar a-plane GaN these are termed as [0002], [1-100] and [1-102] azimuths Here, we have reported the growth of nonpolar (11-20) aGaN film on (1-102) r-Al2O3 with different plasma powers. Interdigitated electrodes (IDEs) were fabricated along different azimuths by combining sputtering and photolithography. Orientations of the IDEs were carefully chosen with the help of rocking curve analysis such that the carrier transport will be along the [0002], [1-100] and [1-102] azimuth angles. We have also evaluated response and recovery time, responsivity (R) and internal gain (G) along these azimuths and and compared with simulations acquired from Silvaco Atlas device simulator. Interestingly, [0002] azimuth exhibits better photoresponse as compared to [1-100] and [1-102] which has been attributed to the enhanced crystallinity and lowest defect density along [0002] azimuth and well supported by simulation. The spectral response taken along [0002] direction show that all the devices are highly selective for a very narrow UV-A range and has maximum responsivity at 360


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nm (3.44 eV). We strongly believe that our study would pave the way for designing an efficient photoresponsive materials and low power devices for photodetection.

EXPERIMENTAL DETAILS: Epitaxial nonpolar (11−20) a-plane GaN were grown on (1−102) r-plane Al2O3 with different plasma power conditions in plasma assisted molecular beam epitaxy (PAMBE). Prior to the growth, the sapphire substrates were cleaned by a standard procedure which includes boiling for 5 minutes in trichloroethylene followed by rinsing in acetone and then in methanol for 2 minutes each. The above procedure was repeated 3 times. The substrate was then chemically etched with (3:1) ratio of H2SO4:H3PO4 at 300 °C for 20 minutes and rinsed multiple times with deionized water to remove any residue and flushed with nitrogen gun prior to loading into the MBE chamber. Substrates were further thermal cleaned at 860 °C for 30 minutes under MBE pressure ~ 1 × 10−10 mbar. Nitridation was done at 710 °C before the growth of epitaxial GaN films. The epitaxial growth of GaN involves two steps, a ~ 20nm GaN buffer layer growth at 500 oC and a high temperature epitaxial growth of about 130 nm at 760 °C. Effusion cell of Ga was kept at 970 °C with a maintained ‘corresponding beam equivalent pressure’ (BEP) at 6 × 10−6 mbar. The nitrogen flow rate was kept constant at 0.5 sccm, which provides nitrogen (Ga:N [1-100].



GaN350

[1-102]

1.88 1.24 1.15 0.83

5.12 4.10

(c)

80

Time (s) GaN400

120

(d)

[1-102] 0.71 0.57

[1-100]

0.26 0

40

80

120

160

Time (s)

Defect density

1.75

0.96 1.40

[1-100] 0

160

[0002]

[1-102]

2.13 3.36 2.05

[1-100]

40

[0002]

40

11

10 4x 1. 11 10 2x 11 . 1 10 0x 1. 10 10 0x 10 . 8 10 0x 6. 10 0 x1 4.0 10 0 x1 2.0 .0 0 [1-100]

80

120

160

Time (s)

12 GaN350 GaN375 GaN400 Sim 1 Sim 2

[1-100] [1-102] [0002]

2

2.01

10.59

Mobility(cm /Vs)

[0002]

Normalized photocurrent

Current (µA)

2.86

0

GaN375

(b)

Current (µA)

(a)

Current (µ A)

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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10 GaN350 GaN375 8 GaN400 Mobility 6

[1-102]

[0002]

Azimuth Direction

FIGURE 3: (a – c) Temporal response of GaN350, GaN375 and GaN400, along [0002], [1-100] and [1-102] azimuths plotted with ‘y’ offset, and (d) defect density, mobility and the normalized photocurrent with respect to [0002] (inset).

To further confirm the effect of azimuth angles, we have calculated the response and recovery times along [0002], [1-102] and [1-100] azimuths. Quantification of response and recovery time was performed using following equations: ‫ܫ‬ሺ‫ݐ‬ሻ = ‫ܫ‬ሺௗ௔௥௞ሻ + ‫ ܣ‬൤1 − ݁‫ ݌ݔ‬൜− ‫ܫ‬ሺ‫ݐ‬ሻ = ‫ܫ‬ሺௗ௔௥௞ሻ + ‫ ܣ‬ቂ݁‫ ݌ݔ‬ቄ−

௧ି௧೚ ఛ೏

௧ି௧೚ ఛ೒

ൠ൨

ቅቃ

(1)

(2)


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where ‫ܫ‬ሺௗ௔௥௞ሻ is the dark current, ‫ܫ‬ሺ‫ݐ‬ሻ is the current after time ‘t’, A is scaling constant, ‫ݐ‬௢ is the time when lamp was turned on or off and ߬௚ , ߬ௗ are the response and recovery times, respectively. The response and recovery times for GaN350, GaN375 and GaN400 along the three azimuths have been obtained by fitting above equations in the temporal response (Figure S3) and the values are given in table I. From the values in table I, it may be noted that the response time improves along the azimuths as [0002] > [1-102] > [1-100], which shows the unanimous effect of azimuth angles on nonpolar a-GaN film but recovery time remains constant. TABLE I: Comparison of response and recovery time for all samples. Samples

GaN350

Time (s) ߬௚

߬ௗ

GaN375

߬௚

߬ௗ

GaN400

߬௚

߬ௗ

Azimuth [1-100]

[1-102]

[0002]

0.50

0.40

0.35

1.2

1.2

1.2

0.36

0.27

0.21

1.2

1.2

1.2

0.45

0.43

0.37

1.2

1.2

1.2

R and G are two very important factors of photodetectors which are given by: ܴ = ‫= ܩ‬

ூ೗೔೒೓೟ ష ூ೏ೌೝೖ

(3)

௉ௌ

௛௖ோഊ ௘ఒ

=

௛௖ ΔIഊ

(4)

௘ఒ௉ഊ S

where ‫ܫ‬௟௜௚௛௧ ି ‫ܫ‬ௗ௔௥௞ = ΔIఒ , is the difference between photo and dark current, P is the power of the UV source (0.3 mW/cm2) and S is illuminated device area (1 mm × 1 mm), c is the speed of light and e is the electron charge. Here, we have evaluated these two quantities with rotated IDEs



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along the three azimuths. The comparison of responsivities and internal gains has been tabulated (table II). The earlier reported highest values for R and G for c-GaN were 0.00567 AW-1 and 0.0195, respectively and for a-plane GaN the values were 0.155 AW-1 and 53%.19,20 However, we observed that the maximum values for R and G for GaN375 along [0002] azimuth are 1.88 AW-1 and 649%. It could be established from the values of R and G compared in table II that the values of these two quantities are higher than the earlier reports.19,20 These results clearly show that it is important to align the IDEs in accordance with the azimuth angles to achieve enhanced photoresponse. TABLE II: Comparison of responsivities (R) and internal gain (G) along three azimuth directions ([0002], [1-100] and [1-102]).

[0002] [10-12]

GaN350 0.3269 0.24036

R (AW−1) GaN375 1.8803 0.6638

GaN400 0.2673 0.2407

[10-10]

0.1166

0.4595

0.0867

Azimuth

GaN350 112.81 82.94

G (in %) GaN375 648.9 229.10

GaN400 90.73 83.08

40.2

158.57

29.92

Room temperature spectral response of a-plane GaN taken along [0002] azimuth for GaN350, GaN375 and GaN400 acquired at 1 V is shown in Figure 4. All the GaN films selectively respond to UV-A region while there is very low or no response for solar blind (400 nm). The reason behind the low responsivity at lower wavelength is the reduction in the photopenetration depth and higher recombination of the photogenerated carriers at the surface.29,30 It can also be noted that GaN375 shows better responsivity as compared to GaN350 and GaN400. All three films show maximum response at ~ 360 nm which corresponds to energy of 3.44 eV, this is very near to the band gap of GaN (3.4 eV) and hence, confirms a band to band transition.


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


Responsivity (AW−1)

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2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

GaN350 GaN375 Gan400

200

250

300 350 400

450

500 550

Wavelength (nm) FIGURE 4: Spectral response of GaN350, GaN375 and GaN400 along [0002] azimuth taken at 1 V bias.

In the table III, we have compared the values of response and recovery time of a-plane GaN of this work with the recently reported values, consisting of thin film and nanostructuresbased devices. Generally, nanostructure-based devices have better optoelectronic properties compared to thin films as nanostructures have very less defects density and hence, few recombination centers. If we compare the recent work done exclusively on a-plane GaN19,20 then the rise and recovery times shows significant improvements and the responsivity is many orders higher. It should also be noted that the responsivity in GaN/β-Ga2O331 thin film based photodetector has been taken at 2V which is expected to be much lower at 1V and in this work aplane GaN device performs better than GaN/β-Ga2O3 even at 1 V. On the other hand, response and recovery time of a-plane GaN along [0002] azimuth is either faster or comparable with these nanostructures-based devices. The values has been taken at very high bias (∼5V) compared to this study (1V)32–34, which might be attributing to the high values of responsivity and it is expected that at 1 V bias these values will be lower.



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TABLE III: The comparison of response and recovery time of a-plane GaN of this study taken along [0002] azimuth with the other previously reported GaN based photodetectors. Materials

20

a-GaN a-GaN19 c-Gan19 c-GaN a-GaN

߬݃ (s)

߬݀ (s)

Applied Bias (V)

GaN thin-film based devices 0.28 0.45 6 2.36 15 2.54 − − 0.21 1.2

5 2 2 − 1.0 5.0 GaN nanostructure-based devices InGaN/GaN MQW-CS 15.4 4 − NB32 GaN NB32 9.4 4 − 31 0.15 0.12 1 GaN/β-Ga2O3 33 GaN nanowire 17.5 6.2 5 Pt-GaN nanowire33 1.1 0.65 5 34 GaN nanowire 0.144 0.256 5

R (AW−1)

0.340 0.155 0.00576 0.12 1.8803 13.024 7.0×103 1.1×105 0.19846 @ 2V 773 6.39×104 1.74 × 107

The experimental observations are compared with simulation using Silvaco Atlas device simulator.35–37 An MSM device structure was created and modelled in 2D Silvaco Atlas and the semiconductor properties for GaN were taken from the inbuilt material library. The effects of orientation along [0002], [1-102] and [1-100] directions were taken care of by using appropriate values of electron and hole mobilities (Figure 3(d)).27,35 Here, the dislocations along [0002] azimuth are less than compared to [1-102] or [1-100] and it affects the electrical properties along that direction, particularly the mobility of charge carriers. Increase of dislocations leads to increase of dangling bonds along the edge dislocation lines. These dangling bonds provide Coulombic scattering centers in GaN and reduce the electron mobility. A trend of increasing photo and dark current is evident from the simulations (Figure 5 (a)) as [0002] > [1-102] > [1-100] and it validates our experimental observations (Figure 5 (b)).


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The experimental results follow the same trend alongside with minor variations. Techniques such as Van der Pauw Hall anisotropic measurements have been used to show that mobility and carrier concentration increases as the direction of current flow changes from [1-100] to [0002] azimuths.27 It is this anisotropy of a-plane GaN which gives rise to enhanced photo and dark currents depending on the azimuth orientation of IDEs. The nature of dark and photo currents measured experimentally matches with simulation results and supports earlier report which demonstrates the anisotropy in electrical characteristics of non polar a-plane GaN.28

(a)10

(b)10

-2

10-3 10-4 10

Current (A)

Current (A)

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


[1-100] dark current [1-100] photo current [1-102] dark current [1-102] photo current [0002] dark current [0002] photo current

-5

10-6

1

2

3

4

Applied Voltage (V)

10-5 10-6 10-7

[1-100] dark [1-100] UV [1-102] dark [1-102] UV [0002] dark [0002] UV

10-8 10-9

10-10 10-11

10-7 0

-4

5

0

2

4

6

Applied Voltage (V)

FIGURE 5: (a) Simulated dark and photoresponse I−V of the photodetector along different azimuth angles and (b) experimental I−V of the fabricated photodetectors along different azimuth angles.

CONCLUSIONS: We have successfully grown epitaxial a-GaN film on r-sapphire in PAMBE. The alignment of the electrodes along three different azimuth angles was carried out by marking the azimuths with the help of azimuth-angle-dependent rocking curve analysis. The results shown by Silvaco simulations and experiments were similar in terms of electrical properties, along all three



azimuths. The variations in electrical properties along three azimuths were attributed to the defect densities which affect the mobility and carrier concentrations along the different azimuths. The response time was fastest for the IDE when the transport was along [0002] azimuth. In addition, R and G are found to be maximum along [0002] azimuth, which is higher than earlier reported values. To test the universality of this phenomenon, we have used three samples grown at different conditions, and similar behavior was observed with respect to the azimuth angles. Overall, our results demonstrate that it would be very important to align the IDEs with the azimuth angles for surfaces with in-plane anisotropy, in order to realize enhanced photoresponse. SUPPORTING INFORMATION: The supporting information contains additional information related to FWHM of HRXRD, asymmetric RSM of the three films along [1000], SEM and optical images of the device have been shown. Fitting of the rise and recovery with respect to UV light and time has been calculated respectively. REFERENCES:

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145x137mm (600 x 600 DPI)


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In-plane anisotropic photoconduction in nonpolar epitaxial a-plane GaN

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