Band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterostructures measured by X-ray photoemission spectroscopy
© Sang et al.; licensee Springer. 2014
Received: 4 July 2014
Accepted: 26 August 2014
Published: 4 September 2014
The band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are measured by X-ray photoemission spectroscopy. A large forward-backward asymmetry is observed in the non-polar GaN/AlN and AlN/GaN heterojunctions. The valence-band offsets in the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are determined to be 1.33 ± 0.16 and 0.73 ± 0.16 eV, respectively. The large valence-band offset difference of 0.6 eV between the non-polar GaN/AlN and AlN/GaN heterojunctions is considered to be due to piezoelectric strain effect in the non-polar heterojunction overlayers.
During the last decade, group III-V nitrides are very promising semiconductor materials for application in high frequency heterojunction field-effect transistors (HFETs) [1–8]. Large band offsets at the heterojunctions are important to realize these device applications. To understand the band offsets between nitride materials at heterointerface is requested for fabricating devices. The heterojunction formed between GaN and AlN is particularly application-oriented because of their large band gap difference induced by the polarization properties of nitride materials [9–11]. Several groups have reported ΔEV values for GaN/AlN heterojunctions fabricated by different growth techniques and determined the value of ΔEV in a range from 0.5 to 1.4 eV . However, these GaN/AlN heterojunctions reported were nearly deposited all on (0001) orientation substrates. Especially, Martin et al. reported valence-band offsets of GaN/AlN and AlN/GaN heterojunctions on C-plane sapphire substrates were 0.60 ± 0.24 and 0.57 ± 0.22 eV, respectively, both values were almost the same to each other . In recent years, non-polar nitride heterostructure has drawn great interest owing to its potential applications in normally-off HEMT, high-efficiency field-free deep-ultraviolet (UV) light emitting diodes (LEDs) with wavelengths of 200 to 300 nm or sensors and so on. Non-polar nitride can eliminate internal polarization fields because of the absence of spontaneous polarization in the non-polar materials. However, the valence-band offset of non-polar A-plane GaN/AlN heterostructures has been studied by few. In this paper, we studied valence-band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterostructures deposited on R-plane sapphire substrates measured by X-ray photoemission spectroscopy (XPS).
The samples investigated were grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Four samples were used in our XPS experiments, namely, a 1.5-μm-thick GaN layer, 250-nm AlN layer, 5-nm GaN/250-nm AlN heterojunction, and 5-nm AlN/1.5-μm GaN heterojunction. Triethylgallium (TEGa), trimethylaluminum (TMAl), and ammonia (NH3) were used as the sources of Ga, Al, and N, respectively. The carrier gas was high-purity hydrogen. Before growing GaN and AlN layers, R-plane (10 2) sapphire substrates were thermally cleaned in H2 ambient at 1,000°C for 10 min to remove the adsorbed water molecules and activate sapphire surface. Then, sapphire substrates were nitridized for 5 min at 1,000°C under a mixed gas of H2 and NH3. The growth temperature of GaN and AlN layer was 1,000°C and 1,100°C, respectively. The XPS measurements were performed on a PHI Quantro SXM instrument (Physical Electronics, Inc., Chanhassen, MN, USA) with Al Kα radiation (h ν = 1,486.6 eV) at a pressure lower than 2 × 10−9 Torr. Charged displacement was calibrated by C 1 s (approximately 285 eV) photoelectron peak from contamination to compensate the charge effect.
Results and discussion
Binding energies (in eV) of the XPS peaks and VBM for GaN, AlN, GaN/AlN, and AlN/GaN samples
19.70 ± 0.03
20.70 ± 0.03
2.06 ± 0.03
73.85 ± 0.01
75.05 ± 0.01
2.74 ± 0.09
73.36 ± 0.01
18.56 ± 0.03
19.96 ± 0.03
73.43 ± 0.01
74.63 ± 0.01
19.23 ± 0.03
20.23 ± 0.03
Using room temperature band gaps for GaN and AlN (3.39 and 6.2 eV, respectively), ΔEC in the non-polar GaN/AlN and AlN/GaN heterojunctions are calculated to be 1.48 ± 0.16 and 2.08 ± 0.16 eV, respectively, and the ratio of ΔEC:ΔEV is close to 11:10 and 57:20, respectively.
From the calculated results, we can see that there exists a large band offset difference of 0.6 eV between the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions, which may be due to the strain-induced piezoelectric fields in the non-polar films [14, 15]. There is no spontaneous polarization but only piezoelectric polarization exists in the non-polar films. The heterojunction underlayers are thick enough to relax the strain caused by the lattice mismatch between GaN and AlN layers, but the heterojunction thin overlayers are only 5-nm thick and at least partially strained. The strain induces static electric fields via the piezoelectric effect. The strain-induced piezoelectric fields tend to decrease the apparent valence-band offsets for nitride materials .
where Cij is the elastic stiffness coefficient.
This paper reports the study of valence-band offsets in the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions evaluated by XPS technique. The valence-band offsets in the non-polar GaN/AlN and AlN/GaN heterojunctions are predicted to be 1.33 ± 0.16 and 0.73 ± 0.16 eV, respectively. There exists a large valence-band offset difference of 0.6 eV between the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions, which is most likely caused by the strain-induced piezoelectric fields in the heterojunction overlayers.
This work was supported by the National Basic Research Program of China (Nos. 2012CB619306 and 2012CB619303), NSFC (Nos. 61225019, 11023003, 10990102, 61404004, and 61076012), the '863' Program of China (Grant Nos. 2011AA050514, 2011AA03A103, and 2011AA03A111), and the Research Fund for the Doctoral Program of Higher Education. This project was supported by the National Natural Science Foundation of China under Grant No.50990064. This work was supported by the National Science Foundation of China (No. 91233111, No. 61274041, No. 11275228, No. 61006004, and No. 61076001) and by the Special Funds for Major State Basic Research Project (973 program) of China (No. 2012CB619305) and also by the 863 High Technology R&D Program of China (No. 2011AA03A101).
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