Thickness Dependence on Interfacial and Electrical Properties in Atomic Layer Deposited AlN on c-plane GaN
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The interfacial and electrical properties of atomic layer deposited AlN on n-GaN with different AlN thicknesses were investigated. According to capacitance–voltage (C–V) characteristics, the sample with a 7.4-nm-thick AlN showed the highest interface and oxide trap densities. When the AlN thickness was 0.7 nm, X-ray photoelectron spectroscopy (XPS) spectra showed the dominant peak associated with Al–O bonds, along with no clear AlN peak. The amount of remained oxygen atoms near the GaN surface was found to decrease for the thicker AlN. However, many oxygen atoms were present across the AlN layer, provided the oxygen-related defects, which eventually increased the interface state density. The barrier inhomogeneity with thermionic emission (TE) model was appropriate to explain the forward bias current for the sample with a 7.4-nm-thick AlN, which was not proper for the sample with a 0.7-nm-thick AlN. The reverse leakage currents for both the samples with 0.7- and 7.4-nm-thick AlN were explained better using Fowler–Nordheim (FN) rather than Poole–Frenkel emissions.
KeywordsAtomic layer deposited AlN Interface state density Reverse leakage current
Atomic layer deposition
X-ray photoelectron spectroscopy
Because of large bandgap, high electron saturation velocity, and high breakdown field, III-nitride materials are of great interest not only for optoelectronic devices such as blue light emitting diodes (LEDs), laser diodes (LDs), and UV detectors but also for electronic devices such as high electron mobility transistors (HEMTs) and power devices [1, 2, 3, 4]. Realizing high-performance GaN-based devices requires metal/GaN interface with a minimum interface state density, which can act as electron traps or limit to modulate the barrier heights according to metal work function by pinning the Fermi level [5, 6]. For other GaN-based device improvement techniques, some methods such as coalescence overgrowth of GaN nanocolumns, nonpolar m-plane GaN, nanoimprint GaN template, and semi-polar face GaN nanorods have also been demonstrated [7, 8, 9, 10, 11]. Among III-nitride compound semiconductors, aluminum nitride (AlN) can be applied to UV detectors, short-wavelength emitters and detectors, due to its high bandgap (∼ 6.2 eV), high thermal conductivity, high electric resistance, as well as low expansion at high temperatures [12, 13]. In addition, AlN can be deposited in a complementary metal-oxide-semiconductor (CMOS) compatible process by atomic layer deposition (ALD) (~ 300 °C), which is a big advantage. Polycrystalline- and amorphous ALD-grown AlN films can be used as dielectric layer for microelectronic devices . Despite the progress of AlN growth techniques, ALD-grown AlN still reveals non-stoichiometric property which contains a large amount of oxygen-related impurities . The amount of oxygen atoms in AlN can affect strongly the electrical and optical properties of AlN .
High-k dielectric oxides such as Al2O3 and HfO2 have been employed as a passivation layer in AlGaN/GaN high electron mobility transistors (HEMTs) [17, 18]. But the formation of Ga–O bonds at the Al2O3/(Al)GaN interface has been known to produce high density of deep (and slow) interface states . As an alternative passivation material with low interface states, AlN has been considered for GaN-based devices due to its smaller lattice mismatch to GaN [20, 21]. In addition, modulation of electrical properties such as barrier heights in metal/semiconductor (MS) contacts by inserting very thin oxide layer has been reported in GaN [22, 23]. Increase of the barrier height in Pt/HfO2/GaN metal-insulator-semiconductor (MIS) diodes with a 5-nm-thick HfO2 layer was reported . Insertion of a 3-nm MgO layer at a Fe/GaN interface was found to reduce the effective barrier height to 0.4 eV . Still now, however, there is limited number of papers reporting on the engineered contact properties with ALD-grown AlN on GaN. In this work, we deposited AlN layers on n-GaN by ALD with different thicknesses and investigated the properties of AlN/n-GaN interface.
Materials and Device Fabrication
Hydride vapor phase epitaxy (HVPE)-grown, undoped, c-plane (0001) bulk GaN (thickness 300 μm, carrier concentration 5 × 1014 cm−3, threading dislocation density 1.5 × 107 cm−2) purchased from Lumilog was used in this work. After cutting the wafer into small pieces, some of them were loaded into an ALD chamber after cleaning process in a HCl:H2O (1:1) solution. Then, the temperature was ramped up to 350 °C to deposit AlN layer. AlN thin films were deposited by thermal ALD system (manufacturer: CN-1 in Korea; model: Atomic Classic) using trimethylaluminum (TMA) and NH3 as precursors. Three different thick AlN layers (0.7, 1.5, and 7.4 nm) were prepared by varying the number of ALD cycles. The thicknesses of AlN film were measured using a FS-1 multi-wavelength ellipsometers (manufacturer: Film Sense in the USA; model: FS-1). To examine the electrical characteristics of the films, MIS diodes were fabricated with a Pt Schottky electrode (diameter 500 μm, thickness 50 nm) and an Al back contact (thickness 100 nm). As a reference, Pt/n-GaN Schottky diodes (i.e., without AlN layer) were also fabricated.
Temperature-dependent current–voltage (I–V–T) measurements were carried out with a HP 4155B semiconductor parameter analyzer after placing samples on a hot chuck connected with a temperature controller, and capacitance–voltage (C–V) measurements were performed using a HP 4284A LCR meter. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a monochromatic Al Κα X-ray source to observe the formation mechanism at the AlN/GaN interface.
Results and Discussion
Except the sample with a 7.4-thick-AlN, all other samples showed the anomalous peak in the C–V curve with increasing the bias voltage, which were associated with the distribution of deep traps in the gap, the series resistance, and interface states [28, 29]. The frequency dispersion in the accumulation region is associated with the formation of an inhomogeneous layer at the interface. The capacitance of such layer acts in series with the oxide capacitance causing the dispersion in the accumulation . The dispersion in depletion is due to the presence of interface states responding to applied frequency. If the time constants of the interface states are comparable to the frequency of small signal, the interface states make a contribution to the total capacitance such that the threshold capacitance increases with decreasing the frequency .
As shown in Fig. 6c, the peaks at ~ 530.2 and ~ 531.9 eV are attributed to the chemisorbed O and Ga2O3, respectively . In addition, the peak at ~ 532.8 eV is associated with Al–OH . However, no peculiar peak was observed for the sample with a 7.4-nm-thick AlN at the selected depth. Similarly, no peak was observed at the deeper etch depths (not shown). When the AlN thickness is thin (0.7 nm), the chemisorbed oxygen atoms were removed but Al atoms bonded with OH. With increasing the AlN thickness, very little amount of oxygen atoms were present near the GaN surface region, indicating the cleaning up effect. However, large amount of oxygen atoms were present in the overgrown AlN region, provided oxide charges. O 1s core-level spectra at the etch depths where the amount of Ga atoms are negligible (about 0~3 nm from the AlN surface in Fig. 6a) were found to exhibit the dominant peak at ~ 531.8 eV, associated with Al2O3 . This means that some portion of AlN layer is composed of Al2O3. As shown in Fig. 6d, the peak related with AlN is not observed well for the sample with a 0.7-nm-thick AlN. Rather, two peaks are observed at ~ 74.1 and ~ 75.6 eV, associated with AlOx and Al–OH, respectively . These Al–O bond-related peaks such as AlOx and Al–OH can act as defects. The peak at ~ 73.6 eV for the sample with a 7.4-nm-thick AlN is associated with AlN .
Even though Eq. (1) contains no temperature dependence, the obtained barrier heights decreased with increasing the temperature. The slopes were obtained as − 6.67 meV/K and − 1.62 meV/K for the samples with 0.7- and 7.4-nm-thick AlN, respectively. It has been reported in SiO2/4H-SiC structure that the FN tunneling possesses a temperature dependence with a slope of − 7.6 meV/K . The ejected electrons from the Pt electrode followed the Fermi–Dirac distribution , and thus, the reverse leakage current by the tunneling could also increase with temperature. In this case, the increase with temperature would be larger for thinner AlN layer.
Meanwhile, it has been reported that current transport mechanisms at high electric field cannot be explained solely by the FN tunneling [53, 54]. Even including the changes in the charges in the oxide and Fermi level of the substrate, and the electron energy distribution at the SiO2/SiC interface with temperature, the reverse leakage current in SiO2/4H-SiC was not explained satisfactorily . It was proposed that thermally activated PF emission of trapped electrons from the interfacial electron traps contributes significantly to the increase in leakage current . Therefore, reducing such defects in AlN during the ALD process is crucial in the AlN/GaN-based device performance, especially during the high-temperature operation.
As seen from the plot of barrier height vs. AlN thickness in Fig. 2c, Li et al. observed similar behavior in metal/n-Ge contacts with Y2O3 layers . They attributed the reduction in the barrier height to the suppression of the unstable GeOx growth and the passivation of dangling bonds on the Ge surface. Karpov et al. inserted Si3N4 layer into Ni/n-GaN contacts and found that the barrier height decreased from 0.78 to 0.27–0.30 eV with a Si3N4 layer. The results were explained by the dipole formation at the Si3N4/GaN interface . Further, Zheng et al. investigated the contact resistance vs. Al2O3 thickness in Al/n-SiC structure and found that the interface dipole started to form at the thickness of 1.98 nm . Above this thickness, the contact resistance decreased first due to the dipole effect and then increased due to the increased tunneling resistance. According to XPS data in Fig. 6, the formation of AlN layer is unclear for the sample with a 0.7-nm-thick AlN. Hence, the reduction of barrier height with a 0.7-nm-thick AlN is more likely due to the passivation effect rather than the formation of interface dipole.
Dry etching process such as inductively couple plasma (ICP) etching is widely used in GaN-based devices due to the chemical stability of GaN , even though ultraviolet-enhanced wet chemical etching was demonstrated . However, dry etching process can induce damage on the GaN surface, increasing the leakage current and degrading the rectifying behavior. Post etch treatment using thermal annealing and KOH solution after reactive ion etching (RIE) was found to effectively remove the surface damage on GaN . Considering the results so far, we suggest that AlN deposition (larger than 1 nm) can be applied to reduce the damage on the etched GaN surface, which is expected to increase the interface quality and the rectifying characteristics further.
We have investigated the interfacial and electrical properties of atomic layer deposited AlN on n-GaN with different AlN thicknesses. According to capacitance–voltage (C–V) characteristics, the sample with a 7.4-nm-thick AlN showed the highest interface and oxide trap density. According to X-ray photoelectron spectroscopy (XPS) measurements, the sample with a 0.7-nm-thick AlN revealed a dominant peak related with Al–O bonds, with no clear peak associated with AlN. The remained oxygen atoms near the GaN surface were found to be very little for the sample with a 7.4-nm-thick AlN. On the other hand, many oxygen atoms were found to be present across the AlN layer, which provided the oxygen-related defects in the AlN layer. Analyses on the reverse leakage current revealed that Fowler–Nordheim (FN) rather than Poole–Frenkel (PF) emission were more appropriate to explain the current transport for the samples with 0.7- and 7.4-nm-thick AlN.
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03030400).
Availability of Data and Materials
All data are fully available without restriction.
HK supervised the work and drafted the manuscript. HY carried out the ALD growth of AlN films and fabricated the devices. BC helped to guide the experiments and to analyze the experimental results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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