Effects of Thickness of a Low-Temperature Buffer and Impurity Incorporation on the Characteristics of Nitrogen-polar GaN
© The Author(s). 2016
Received: 15 October 2016
Accepted: 8 November 2016
Published: 18 November 2016
In this study, effects of the thickness of a low temperature (LT) buffer and impurity incorporation on the characteristics of Nitrogen (N)-polar GaN are investigated. By using either a nitridation or thermal annealing step before the deposition of a LT buffer, three N-polar GaN samples with different thicknesses of LT buffer and different impurity incorporations are prepared. It is found that the sample with the thinnest LT buffer and a nitridation step proves to be the best in terms of a fewer impurity incorporations, strong PL intensity, fast mobility, small biaxial strain, and smooth surface. As the temperature increases at ~10 K, the apparent donor-acceptor-pair band is responsible for the decreasing integral intensity of the band-to-band emission peak. In addition, the thermal annealing of the sapphire substrates may cause more impurity incorporation around the HT-GaN/LT-GaN/sapphire interfacial regions, which in turn may result in a lower carrier mobility, larger biaxial strain, larger bandgap shift, and stronger yellow luminescence. By using a nitridation step, both a thinner LT buffer and less impurity incorporation are beneficial to obtaining a high quality N-polar GaN.
KeywordsN-polar GaN Buffer layer Thickness Biaxial strain Impurity incorporation
III-Nitride semiconductors have been used for light-emitting diodes (LEDs), laser diodes, solar cells, and high electron mobility transistors (HEMTs) [1–4]. These nitride devices are generally grown along the polar c-axis (Ga-polar), where the built-in polarization field decreases the overlap of the electron and hole wave functions and leads to quantum-confined Stark effect (QCSE) [1–4]. Meanwhile, the reverse polarization field of Nitrogen (N)-polar III-nitrides along the -c-axis [000-1] can be used for device applications, such as enhancement mode and highly scaled transistors, photodetectors, Zener tunnel diodes, and sensors [5–7]. Due to the reversed direction of the polarization field, a larger forward bias reduces QCSE in N-polar quantum wells (QWs), increasing overlap of electron and hole wave function . By time-resolved electroluminescence measurement, an N-polar LED has been shown to have more efficient carrier relaxation and faster carrier recombination .
The properties of Ga- and N-polar GaN are different in the direction of the polarization field, the incorporation of residual impurities, and the surface morphology [8–11]. Because of the low-quality epitaxial layers of N-polar GaN, the device performance of the N-polar LED is poor compared to that of the Ga-polar LED . The incorporation of oxygen in N-polar GaN during nitridation is much larger than that in Ga-polar GaN [10, 11]. Oxygen incorporates on nitrogen sites by bonding with neighboring Ga atoms and substitutes for nitrogen atoms on the N-polar GaN surface . By calculating the adsorption energy of oxygen on the different surfaces, atoms impinging on a group V site form three bonds to the Ga surface atoms, leading to a stronger bonding of oxygen atoms to the N-polar surface [11, 12].
In addition, hexagonal hillocks on a rough surface are often observed in N-polar GaN [13–15]. Smooth N-polar GaN grown on on-axis c-plane sapphire substrates can be achieved by the following approaches before the main GaN growth: (1) With a nucleation layer deposited at 550 °C, followed by an annealing at 1030 °C ; (2) With a nitridation step on the sapphire substrates ; and (3) By using a low temperature (LT) GaN buffer and optimizing the nitridation temperature of the sapphire substrates from 1130 to 950 °C, atomically smooth N-polar GaN has been achieved and the hillock density has decreased . The GaN nucleation layers were deposited at 600 °C, followed by the main GaN layer at 1055 °C. Furthermore, an improved surface morphology and narrow spectral width can be obtained by growing N-polar GaN on misoriented sapphire, silicon carbide, and silicon substrates . An optimized LT GaN buffer is crucial to improve the epitaxial layer quality of the multiple quantum well structures. However, the effects of the thickness of a LT buffer and impurity incorporation on the characteristics of N-polar GaN with either a nitridation or thermal annealing step were not well studied.
In this study, the effects of the thickness of a LT buffer layer and impurity incorporation on the characteristics of N-polar GaN are investigated by atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS), photoluminescence (PL), Raman, and Hall measurements. It is found that the sample with the thinnest LT buffer and a nitridation step proves to be the best in terms of a strong PL intensity, fast mobility, small biaxial strain, and smooth surface. Thermal annealing of the sapphire substrates may cause more impurity incorporation around the HT-GaN/LT-GaN/sapphire interfacial regions, which in turn may result in a lower carrier mobility, larger biaxial strain, larger bandgap shift, and stronger yellow luminescence. By using a nitridation step, both a thinner LT buffer and less impurity incorporation are beneficial to obtaining a high quality N-polar GaN.
The N-polar GaN samples are grown on nominally on-axis c-plane sapphire in a metalorganic chemical vapor deposition reactor (MOCVD). Trimethylgallium (TMGa) and ammonia (NH3) are used as the precursors for Ga and N, respectively. Sapphire was heated up in a mixture of NH3 (3 slm) and N2 (4 slm) to a high temperature for nitridation. One N-polar GaN sample with 20-nm thickness of LT buffer (namely LT-20 sample) was grown on nitridized sapphire at 600 °C and a pressure of 300 mbar in H2, followed by the growth of ~1 μm N-face GaN at 1055 °C and 100 mbar with 0.5 slm NH3 and a TMGa flow of 66 μmol/min. Besides, the other two N-polar GaN samples with 21 and 32 nm thicknesses of LT buffers are prepared (namely LT-21 and LT-32 samples, respectively). The LT-21 and LT-32 sample sapphires did not have a nitridation step but had a thermal cleaning at 1070 °C in H2 for 2 min before the growth of LT-GaN buffer. The thermal cleaning of the sapphire substrates probably caused some issues at the initial growth interface, which accounts for the different properties among the LT-20, LT-21, and LT-32 samples.
The surface morphology was revealed by AFM (Park Systems, XE-70) with a non-contact mode using a silicon tip of curvature less than 10 nm. The samples were placed in a cryostat for temperature-dependent PL measurement with the 325-nm line of a 55-mW He-Cd laser for excitation. Raman spectra were recorded in the backscattering configuration using a Jobin Yvon-Horiba micro-Raman system (model T64000) with a 532-nm laser. Mobility and sheet resistance are measured by Hall/van der Pauw measurement.
Results and Discussion
AFM Images and SIMS Profiles
PL and Raman Measurements
Phonon frequency shift ∆ω, biaxial strain σ, bandgap shift ∆E g , estimated bandgap E ′ g from phonon frequency shift, measured bandgap E g from PL peak position for the LT-20, LT-21, and LT-32 samples
∆E g (eV)
E ′ g (eV) from Raman
E g (eV) from PL peak
Mobility and Sheet Resistance Measurements
PL intensity, mobility (μ) calculated from Hall/van der Pauw measurements, and sheet resistance (R sheet) for the LT-20, LT-21, and LT-32 samples
R sheet (ohm/□)
In summary, we have studied the effects of the thicknesses of a LT buffer and impurity incorporation on the material and optical properties of N-polar GaN samples. The sample with the thinnest LT buffer and a nitridation step proves to be the best in terms of a strong PL intensity, fast mobility, small biaxial strain, and smooth surface. As the temperature increases at ~10 K, the DAP band is responsible for the decreasing integral intensity of the band-to-band emission peak. In addition, the thermal annealing of the sapphire substrates may cause more impurity incorporation around the HT-GaN/LT-GaN/sapphire interfacial regions, which in turn may result in a lower carrier mobility, larger biaxial strain, larger bandgap shift, and stronger yellow luminescence. By using a nitridation step, both a thinner LT buffer and less impurity incorporation are beneficial to obtaining a high quality N-polar GaN.
This research was supported by the Ministry of Science and Technology, Taiwan, under grants NSC 102-2112-M-390-001 and MOST 103-2112-M-390-002.
YYC performed the experiments. QS and JH fabricated the samples. FWY and SWF coordinated the project and drafted the paper. All the authors read and agree the final version of the paper.
The authors declare that they have no competing interests.
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