Role of Main Air Constituents in the Efficiency of GaN Nanostructures

Nanowires and nanofins have a large surface-to-volume ratio, and they greatly react with the environment. GaN nanostructures also react with major air constituents, and under controlled environments can be used for various applications in areas like environmental sensing, photocatalysis, and light-emitting diodes. 

GaN (Gallium nitride) Nanowires (NW) have garnered interest in the last years as they can be created with high crystal quality. Moreover, GaN NWs show good optical properties like strong luminescence upon electrical or optical excitation; probing changes of the luminescence under controlled environmental conditions is a promising pathway to understand the underlying mechanisms. 

The GaN surfaces react with the environment, and knowledge of the phenomenon could trigger advancements in highly relevant fields like sensing, photocatalysis, or the improvement of nanowire (NW) light-emitting diodes (LEDs). 

Scientists analyzed the influence of GaN NW geometry, different gaseous atmospheres, the pH value, and electrical fields in aqueous solutions on the intensity of the photoluminescence (PL) response and the NW stability. Earlier research shows that anions that accumulate at the NW surface enhance non-radiative recombination and, therefore, reduce the PL intensity.  

The current paper discusses the luminescence behavior and the role of surface-related processes of GaN NWs under different atmospheres with a focus on the main constituents of ambient air, nitrogen (N₂), oxygen (O₂), and water (H₂O) vapor. 

This is in turn combined with the analyzes of the effects of pre-treatments, which can have a significant influence on several surface properties of c- and m-planes of GaN, including the magnitude of the surface band bending (SBB). Various oxide shells are also tested on their capability to stabilize the PL emission of GaN NWs and elucidate the effect of several common solvents on the PL intensity evolution. 

Experimental Details

The GaN Nanostructures (GaN NS) analyzed were grown through plasma-assisted molecular beam epitaxy (MBE) in a selective area growth mode. 

The GaN NWs were grown on n-type Si (111) substrates (iron-compensated Ga-polar GaN templates) under N-rich conditions at a substrate temperature of 875 °C (950 °C) with a Ga flux of 5.7 (4.6) 10⁻⁷ mbar beam equivalent pressure (BEP) and an N₂ flux of 0.363 sccm with an RF plasma power of 425 W for 90 minutes at a background pressure of about 10−0 mbar. The Nano Wires had a uniform shape and similar dimensions. 

Figure 1. (a) SEM image of GaN NWs on a Si substrate and (b) GaN nanofins on an AlN substrate in 45°-tilted view. (c) Sketch of the measurement chamber with gas flow and resistive heater. Image Credit: Kraut et al., 2021 

Three batches of NW arrays were grown on Si to examine the influence of common cleaning treatments of the GaN NW surface on their sensitivity to the environment. NWs labeled “oxidized” are exposed to an oxygen plasma with an RF power of 400 W and an O₂ flow of 0.6 sccm at a temperature of 600 °C in an ultra-high vacuum chamber for 10 minutes. 

The optical investigation and light exposure of the GaN NW surfaces during interaction with their environments was understood by above-band gap light excitation with a quadrupled Nd:YAG laser with a wavelength of 266 nm in continuous-wave operation. 

The excitation intensity was measured by placing a power sensor instead of a sample into the optical path. In ambient air, a 40× UV-enhanced High-Power MicroSpot objective with a working distance of 1 mm and a resulting laser spot size of roughly 2 μm was used. 

Wet environment measurements were taken by bubbling the respective gases through deionized (DI) H₂O until a relative humidity of 100% was reached, while in liquid environment measurements, the sample was mounted into a Petri dish filled with the respective fluid. 

Results and Discussion

PL Intensity in Air

The complexity of the photoluminescence (PL) in response to GaN NWs in the air is depicted in Figure 2(a). In the saturation regime, the speed of the PL enhancement has to be limited by the speed of adsorption or desorption of molecules from the environment to or from the NW surface. 

Figure 2. (a) NBE PL intensity evolution of GaN NWs in ambient air (T = 300 K) normalized to the initial value. (b) Enhancement factor of the PL intensity of oxidized GaN NWs after 10 and 15 min under UV light exposure as a function of the excitation intensity. Image Credit: Kraut et al., 2021 

PL Intensity in Controlled Atmospheres

To understand the interactions at the NW surface, a more accurate controlled experimental environment is needed. Figure 3 shows the normalized saturated PL intensity under continuous UV illumination with an excitation intensity of 18 kW cm⁻² for HCl-etched and oxidized NWs under varied atmospheres and for different temperatures. The PL intensity measured in dry N₂ is used as the reference value for normalization. 

In humid SA, the PL intensity of the oxidized GaN NWs decreases to 60% of the dry N₂ level and 15% for the HCl-etched NWs within 1 min. And as a final step, when the flow-chamber is purged with dry N_2, both samples increase their PL emission back to the initial level within the measurement error, which underlines the reversible nature of the observed effects. 

In SA and N₂ atmospheres, the values obtained for dry and humid environments are similar and while switching back to first wet and later dry N₂ decreases the luminescence to about 3 and 1.5 times the initial value, respectively. 

Figure 3. Variation of the stabilized NBE PL intensity of oxidized and HCl-etched GaN NWs in different atmospheres under continuous illumination. The numbers next to the dotted arrows indicate the typical transition time from one steady-state to the other. An excitation intensity of 18 kW cm⁻² has been used. The given Teff was determined by the evaluation of the NBE peak position of GaN. Image Credit: Kraut et al., 2021. 

The researchers conclude that oxygen alone can form two fundamentally different interacting states at the NW surface.

Figure 4. (a) PL intensity of HCl-etched GaN NWs in dry atmospheres as a function of O₂ concentration in N₂ background for several Teff. (b) Arrhenius plot of the temperature dependence of the PL intensity in dry N₂ and SA atmospheres. For all measurements, an excitation density of 18 kW cm⁻² was used. Dashed lines are Arrhenius fits. Image Credit: Kraut et al., 2021

PL Intensity Evolution After Illumination Onset

The PL intensity evolution immediately after the UV excitation onset probes the light-induced changes of the surface properties in a given ambient. The PL intensity of the oxidized GaN NWs in dry N₂ increases within 1 min to 140% of the initial value, whereas in humid N₂ it reaches only 120%. 

The HCl-etched GaN NWs show almost no change in the PL intensity in dry N₂, but a pronounced quenching of the PL signal to 50% of the initial intensity in humid N₂. The equivalent measurements in dry and humid SA.

Figure 5. Normalized PL intensity evolution of oxidized (red) and HCl-etched (blue) GaN NWs after UV laser illumination onset with an excitation intensity of 18 kW cm⁻² (a) in N₂ (b) in SA. The solid (doubled) lines represent measurements in a dry (humid) atmosphere. (c) PL intensity recovery of HCl-etched GaN NWs in different atmospheres. The red symbols show the PL intensity after switching the laser illumination in humid SA. The black dots (stars) show the recovery behavior in dry SA (N₂). The data are normalized to the initial PL intensity in dry SA before the quenching procedure. Image Credit: Kraut et al., 2021.

Combined PL and Electrical Measurements

Complementary electrical measurements of GaN NFs can provide further insight into the underlying mechanisms. Figure 6(a) depicts the electrical current through GaN NFs under illumination with 266 nm laser light and the simultaneously measured NBE PL in varied dry atmospheres at room temperature. 

Under illumination, the total current (dark and photocurrent) decreases to 88% and recovers to 95%. A possible explanation lies in the location of photo-excitation within the GaN NF, which mainly takes place in the surface region, while the majority of the dark current contribution is through the conductive channel in the center of the NF. 

Figure 6. Normalized NBE PL intensity and normalized current density of GaN NFs on an AlN template under illumination with 266 nm light at (a) 300 K and (c) 375 K. The atmosphere was changed from dry N₂ to dry SA and back under constant applied voltage (1 V) and excitation intensity (18 kW cm⁻²). (b) The normalized current density of GaN NFs at 300 and 375 K in the dark. Due to the sake of argument (c) is discussed later in the text. Image Credit: Kraut et al., 2021 

Generally, changes in PL intensity of NSs under constant excitation originate in changes in the ratio of radiative to non-radiative charge carrier recombination. This is either caused by the creation or passivation of non-radiative recombination channels, for example, unsaturated surface states, or by changing the amount of radiative recombination through spatial separation of electrons and holes, for instance, due to SBB. 

Figure 7(b) summarizes the CPD and SPV results in a schematic drawing of the GaN NS SBB in SA and N₂ atmosphere. In the dark, the SBB in SA is higher than in N₂. This difference increases upon above bandgap illumination as the SBB reduction due to the light is more pronounced in SA compared to N₂. 

This matches well with the increased sensitivity of the current to changing the atmosphere under illumination compared to in the dark (Figure 6): the higher reduction of the SBB under illumination leads to a stronger shrinking of the conductive channel within the NF, which increases the electrical resistivity to a greater extend.

Figure 7. CPD of an m-plane bulk GaN plate in the dark and under 340 nm illumination with an excitation intensity of 6 mW cm⁻². The atmosphere was changed from dry N₂ to dry SA and back. (b) Schematic drawing of the band bending at a GaN NS surface (green line) in SA (left) and N₂ (right) atmosphere at room temperature. The dashed/solid black lines represent the SBB in the dark/under illumination. The red lines illustrate the CPD differences between SA and N₂ atmosphere in the dark (upper part) and under illumination (lower part). The corresponding SPV in SA and N₂ are indicated in blue. Image Credit: Kraut et al., 2021. 

PL Response in Liquid Environments

Figure 8 depicts the PL response of HCl-etched GaN NWs in various liquids on a logarithmic scale. The various magnitude of the PL response in different liquid environments opens a possibility to use GaN NWs to optically identify impurity molecules for, for example, H₂O quality control applications.

Figure 8. (a) Normalized PL intensity evolution of HCl-etched GaN NWs after UV laser illumination onset with an excitation intensity of 9.5 kW cm⁻² in the air (orange), DI H₂O (blue), acetonitrile (black), ethanol (red), and cyclohexane (green). The PL intensity axis is shown with a logarithmic scale. (b) Normalized PL intensity of GaN NWs with several shells at an excitation intensity of 9.5 kW cm⁻² in ambient conditions. For comparison, oxidized NWs are also shown. ZnO and Gax Oy shells are MBE-grown, the 50 nm Al₂O₃ was deposited by ALD. SEM images in a 45° tilted view of exemplary NWs with different shells are depicted at the right of the graph. Image Credit: Kraut et al., 2021. 

GaN-Metal Oxide Core–Shell NWs

The MBE-coated samples (ZnO, Gax Oy) indicate an intensity increase, but the amplitude differs significantly. While the PL signal of the GaN NBE of the oxidized NWs more than doubled within 2 min, for the thickest Gax Oy shell it increases only by 6%. 

The absolute PL intensities (table S5, SI) of the GaN-ZnO core-shell NWs decrease by a factor of almost 25 from the thinnest to the thickest shell due to absorption of the excitation laser in the ZnO. Further, the sample with an Al₂O₃ shell exhibits the highest PL signal and is at the same time the only one exhibiting a decreasing PL intensity over-illumination time. 

Conclusion

Knowledge of the influence of the environment on the optical and electrical properties of nanostructures (NSs) is a matter of high technological importance and scientific interest. The research carried out an extensive and systematic investigation of some major external contributions manipulating the photoluminescence (PL) intensity and the electrical resistance of GaN nanowires (NWs) and GaN nanofins (NFs) on several substrates. 

One major finding is that both H₂O and oxygen show an ambivalent influence with either quenching or enhancing effect, depending on the sample temperature. While for the PL-influencing adsorption of H₂O the ultraviolet (UV) illumination plays a key role, the surface reactions related to oxygen also take place in the dark. 

The results highlight the importance of controlled environmental conditions, surface treatments, and passivation technologies to ensure reliable experimental results and for boosting device performances. 

Journal Reference: 

Kraut, M., Pantle, F., Wörle, S., Sirotti, E., Zeidler, A., Eckmann, F. and Stutzmann, M., (2021) Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins. Nanotechnology, 32(49), p.495703. Available at: https://iopscience.iop.org/article/10.1088/1361-6528/ac1dd1#nanoac1dd1s5

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