High free carrier concentration in p-GaN grown on AlN substrates

The III-nitride material system efficiently provides for a tunable band structure that provides for realizing optoelectronic devices such as light emitting diodes (LEDs) and laser diodes covering from the IR to the UV part of the spectrum.¹ Furthermore, it allows for novel devices such as biosensors and high power switches.^2,3 One requirement for III-nitride-based devices is to have a high free carrier concentration in order to achieve high conductivity and low resistance Ohmic contacts.⁴ Typically, it is relatively easy to achieve a high free electron density in GaN and AlGaN due to the low activation energy of Si and Ge donors.^4–6 However, achieving a high free hole density in GaN and AlGaN is still a challenge.^7–9 This is primarily due to the higher activation energy of the Mg-dopant, which results in poor acceptor ionization. The Mg-acceptor in p-GaN has an activation energy around 150 meV implying that around 1% of the acceptors are ionized at room temperature.¹⁰ In addition, compensation of the Mg acceptor, mainly by N-vacancies, further reduces the achievable free hole concentration.^11,12

Currently, GaN and AlGaN based devices are mostly grown on sapphire substrates. This results in high dislocation density and, consequently, significant non-radiative recombination.¹³ As a recent alternative, native GaN and AlN substrates have been used to reduce dislocation density and improve device performance.¹⁴ However, only very recently, studies on the characteristics of doped layers grown on native substrates have been conducted. Until now, there are no reports on p-GaN grown on single crystal AlN substrates. Since p-GaN is used as the contact layer in III-nitride-based LEDs and laser diodes,¹ achieving a significantly higher free carrier concentration would result in better contact formation and device performance.¹⁵ 

In this work, we report on the MOVPE (metalorganic vapor phase epitaxy) growth of p-GaN layers on single crystal AlN substrates. A high free hole concentration of 5.3 × 10¹⁸ cm⁻³ at room temperature is observed. This is an order of magnitude higher than the free hole concentration typically found in p-GaN grown on sapphire substrates. Atomic force microscopy (AFM) and x-ray diffraction (XRD) analyses show epitaxial p-GaN films grown on AlN substrates, whereas photoluminescence (PL) analysis shows Mg-incorporation without self-compensation. Temperature dependent Hall measurements indicate a lower activation energy in p-GaN films grown on AlN substrates due to screening of Coulomb potential by the free carriers. Thus, a higher doping concentration (> 5 × 10¹⁹ cm⁻³) can be obtained in p-GaN when grown on AlN substrates. Hall measurements also suggest that a free hole concentration of around 1 × 10¹⁹ cm⁻³ could be achieved for devices operating at 100 °C. The achieved free hole concentration is promising for future UV LEDs and laser diodes on AlN substrates.

An MOVPE reactor was employed to grow p-GaN layers on either sapphire or physical vapor transport (PVT) grown single crystal AlN substrates. Excellent crystalline quality of the AlN substrate and homoepitaxial layer is confirmed by the low full width at half maximum (FWHM) of the (0 0.2) AlN omega rocking curve (60 arc sec).¹⁶ III-polar growth on sapphire was achieved by the deposition of a 20 nm thick AlN buffer layer. Next, a 200 nm AlN layer followed by a 200 nm undoped Al_0.7Ga_0.3N layer were grown on both substrates before growing the p-GaN layer. Triethylgallium (TEG), trimethylaluminum (TMA), and Cp₂Mg were used as metal sources and NH₃ as the nitrogen source. H₂ gas was used as diluent and the V/III ratio was fixed to 2000 to minimize the incorporation of compensating defects.^17,18 The thickness of the p-GaN layer was kept at 200 nm, unless otherwise mentioned. The Al_0.7Ga_0.3N interlayer has been adopted in the growth template to match the optoelectronic device design. The doping concentration in p-GaN was determined to be 5 × 10¹⁹ cm⁻³ using secondary mass ion spectroscopy (SIMS). Further details of the MOVPE growth process can be found elsewhere.^16,17

Following the MOVPE growth, the samples were initially cleaned with acetone, methanol, and deionized water for organic removal,¹⁹ followed by the removal of surface oxide using a hot 1:1 HCl:H₂O solution.²⁰ For activating the holes, the activation anneal in oxygen or air ambient has been suggested to remove hydrogen from the Mg-H complex in the form of H₂O.^21,22 Hence, the dopant activation was performed with a 20 minute anneal at 700 °C in air. Standard Ni (20 nm)/Au (40 nm) was deposited as contact metal for the Hall measurements using Van der Pauw geometry. The samples were then annealed at 600 °C for 10 min in air for contact formation.²³ The active area was defined using a mesa etch performed using reactive ion etching. Temperature-dependent Hall measurements were performed to investigate the activation energy of the Mg-dopant. In addition, atomic force microscopy (AFM), x-ray diffraction (XRD), and photoluminescence (PL) analyses were used to study the film properties.

Figure 1(a) shows an AFM topograph of the p-GaN grown on an AlN substrate. Spiral growth with bilayer steps is observed on the grown surface. The RMS roughness was observed to be around 1 nm on a 2 × 2 μm² scan. This value is comparable to that obtained from p-GaN and AlGaN grown on sapphire substrates.²⁴ The observation of spiral growth in p-GaN on AlGaN/AlN indicates the generation of dislocations at the GaN/AlGaN interface. No cracking or significant surface roughening was observed for thicker films.²⁵ Figure 1(b) shows the (0 0.2) ω-rocking curves for the p-GaN layer and the AlN substrate. The FWHM of the GaN reflection is ∼600 arc sec, typical of GaN layers with a dislocation density of ∼10¹⁰ cm⁻². This dislocation density is comparable to GaN films grown on sapphire.¹⁷ For comparison, the FWHM of the AlN substrate has a FWHM of ∼60 arc sec corresponding to a dislocation density of <10⁴ cm⁻². Simulation and analysis of high resolution XRD measurements (not shown) indicate a pseudomorphic Al_0.7Ga_0.3N layer and a nearly fully relaxed p-GaN layer.

To further investigate the Mg incorporation into GaN, room temperature PL spectra were recorded with a typical PL spectrum shown in Fig. 2. Samples were excited using an ArF laser (193 nm). Only the signal from the top 200 nm thick GaN layer is observed, and no peaks of the underlying substrate or AlGaN layers were found as the excitation volume lies within the GaN layer only. The luminescence spectrum is dominated by a strong near band edge emission (NBE) line around 3.42 eV.²⁶ In addition, a peak related to the donor acceptor pair (DAP) in GaN was observed around 3.27 eV.²⁷ Based on the energetic position of the NBE and the line shape and position of the DAP, the film is not in the self-compensation regime¹¹ which occurs due to over-doping of Mg acceptors. The peak position of the NBE emission can also be used to estimate the strain in the layer. The observed peak position at 3.42 eV correlates with a fully relaxed GaN layer, in agreement with XRD data as discussed above.

In order to study the hole concentration and the thermal activation of holes in p-GaN, temperature dependent Hall measurements were performed. Hole concentration, mobility, and the resistivity values were extracted from the Hall measurements. All these parameters at room temperature for p-GaN grown on sapphire and AlN substrates are summarized in Table I. At room temperature, the free hole concentration in the p-GaN grown on the AlN substrate was observed to be 5.3 × 10¹⁸ cm⁻³, which is about an order of magnitude higher than the p-GaN grown on the sapphire substrate under the same growth conditions. The resistivity was observed to be significantly low (0.7 Ω cm at room temperature) in p-GaN grown on AlN substrates. Typical peak free carrier concentrations and minimum resistivities lie at ∼mid-10¹⁷ cm⁻³ and ∼1 Ω cm, respectively, for p-GaN grown on the sapphire substrate using MOVPE.^12,32 It is known that UV optoelectronic devices work at an elevated temperature due to energy loss through the contacts and high resistivity of AlGaN cladding layers. Thus, a free hole concentration of 1 × 10¹⁹ cm⁻³ is measured to be available at device operation temperature (around 100 °C). Note that the underlying Al_0.7Ga_0.3N layer has oxygen impurities present during the growth.¹⁶ However, oxygen is likely to form V_III-oxygen complexes inside AlGaN.³³ As a result, the underlying Al_0.7Ga_0.3N layer was observed to be semi-insulating and hence does not interfere with the hall measurement of p-GaN. Moreover, a high Mg concentration in the p-GaN layer results in a negligible depletion width (≤10 nm). Figure 3(a) shows the temperature dependent free hole concentration comparison of the p-GaN grown on sapphire and AlN substrates. The acceptor activation energy (E_a) was estimated by fitting the temperature dependence of free hole concentration (log p vs 1/T) using the following equation:²⁹ 

where g is the acceptor degeneracy factor (g = 4 for holes), N_V is the effective density of states in the valence band, N_D is the compensating donor concentration, N_A is the acceptor concentration, k is the Boltzmann constant, and T is the temperature.^28,29 The activation energy of acceptors in p-GaN grown on sapphire substrates is observed to be around 120–200 meV, which compares well with the literature.^28,30 Interestingly, the activation energy follows a power law dependence to the free hole concentration due to screening of the Coulomb potential by the free carriers,²⁸ as shown in Fig. 3(b). A higher ionized acceptor concentration leads to a reduction in the distance between the ionized atoms, thereby reducing the activation energy.^34,35 Consequently, the calculated activation energy of acceptors in p-GaN grown on AlN substrates lies around 60–80 meV. Therefore, a reduction of activation energy due to carrier screening is deemed to be the prime contributor for the higher free hole concentration. It is important to note that SIMS analysis showed a magnesium doping concentration of 5 × 10¹⁹ cm⁻³ for p-GaN grown on AlN substrates (not shown here). The PL spectra and high free carrier concentration observed with Hall measurements confirm that the p-GaN films grown on the AlN substrate are not in the self-compensation regime. For p-GaN films grown on the sapphire substrate, self-compensation occurs at a doping concentration of around 2–3 × 10¹⁹ cm⁻³.³² Thus, the growth of p-GaN films on the AlN substrate apparently pushes the self-compensation regime to higher doping concentrations (beyond 5 × 10¹⁹ cm⁻³). Even though the p-GaN dislocation density is similar on the two substrates, this marked difference in the onset of self-compensation may arise from the nature of the dislocations. The observed increase in free hole concentration and reduction in resistivity for layers grown on AlN are promising for future UV LED and laser devices on AlN substrates.

In this letter, we have investigated the p-doping of GaN grown on a single crystal AlN substrate. About an order of magnitude higher carrier concentration was observed as compared to the p-GaN growth on the sapphire substrate at the same Mg doping level. A lower acceptor activation energy was observed in p-GaN films due to screening of Coulomb potential by free carriers. A doping concentration of 5 × 10¹⁹ cm⁻³ could be achieved in p-GaN on AlN without observing onset of self-compensation, as demonstrated by NBE emission and lack of mid-gap emission. A more than a factor of 2 lower resistivity obtained in p-GaN grown on AlN substrates is attractive for fabricating UV optoelectronic devices.

We sincerely thank the support from NSF (DMR-1312582, ECCS-1508854, ECCS-1610992, DMR-1508191) and ARO (W911NF-15-2-0068, W911NF-16-C-0101) for funding this work.