N-polar InGaN-based LEDs fabricated on sapphire via pulsed sputtering

The growth of group-III nitrides on an N-polar plane has attracted much attention because they enable the growth of high-quality InGaN with high In composition owing to their thermal stability, which is higher than those grown on a Ga-polar plane.^1–3 In addition, in N-polar InGaN/GaN heterostructures, the spontaneous and piezoelectric polarization directions were reversed from those in Ga-polar structures, which could be advantageous for operating green or longer-wavelength InGaN light-emitting devices and solar cells.^4,5 However, metalorganic chemical vapor deposition (MOCVD) growth of N-polar GaN on sapphire typically leads to the formation of a rough surface morphology with hexagonal pyramidal structures and lower crystalline quality than Ga-polar GaN.⁶ Such hexagonal hillocks were also observed in homoepitaxial GaN layers grown on N-polar GaN bulk substrates by MOCVD.⁷ Moreover, the growth on N-polar faces suffers from the significant incorporation of residual impurities such as oxygen, which leads to difficulties in controlling n-type and p-type conductivities.⁸ Hence, few research groups have reported p-type doping in N-polar GaN with MOCVD.⁹ Therefore, molecular beam epitaxy (MBE) groups have mainly made recent progress on the growth of N-polar nitride films and its application to light-emitting devices.^4,10 However, it is well known that MBE growth of nitride materials suffers from many problems such as a low growth rate and a low mass productivity.

On the contrary, we have recently found that using a new growth technique called pulsed sputtering deposition (PSD) leads to the formation of device-quality nitride materials at a low cost and with a high growth rate.¹¹ In fact, high-quality n-type and p-type GaN with room-temperature (RT) carrier mobilities of 1008 cm²V⁻¹s⁻¹ and 34 cm²V⁻¹s⁻¹, respectively,¹² have already been prepared, and various devices such as light-emitting diodes (LEDs),^13,14 solar cells, high-electron mobility transistors,¹⁵ and metal-insulator semiconductor field-effect transistors¹⁶ were fabricated via PSD. The PSD growth mechanism involves the enhanced surface migration of film precursors from the pulsed-supplied group-III metals. We have demonstrated that the growth mechanisms of PSD are inherently different from those behind the MOCVD growth, enabling the growth of high-quality nitride films at considerably lower growth temperatures owing to the enhanced surface migration of adatoms on the growth surfaces.^11–17 These features are also expected to facilitate the growth of high-quality N-polar nitride films because surface roughening in N-polar growth is probably caused by the insufficient surface migration of growth precursors.

In this study, we have grown GaN (000$1¯$⁠) films on sapphire substrates via PSD and investigated their structural and electrical properties. We have also investigated the electrical properties of Mg-doped GaN (000$1¯$⁠) and demonstrated the successful fabrication of N-polar InGaN LED by sputtering growth.

We used 2 inch sapphire substrates (0001) with a miscut angle of 2° toward the m-axis to improve the crystalline quality of the GaN films.¹⁸ The growth of nitride films was performed with pulsed magnetron sputtering sources in an N₂/Ar atmosphere. The growth temperature was monitored using a calibrated pyrometer. To obtain N-polar films, 5-nm-thick AlN initial layers were deposited under N-rich conditions at the growth temperature of 700–800 °C and then GaN layers were deposited under slightly Ga-rich condition at the growth temperature of 650–750 °C with the growth rate of ca. 2 $μm/h$⁠. The thickness of the GaN layers is varied from 0.05 to 6 $μm$⁠. The Si and Mg atoms were used as an n-type and p-type dopant, respectively. The doping concentration was controlled by varying the dopant vapor flux from the solid state source.¹² InGaN LED epitaxial structures comprising a 0.15-$μ$m-thick Mg-doped GaN layer and an InGaN emitting layer are grown on n-type 4-$μ$m-thick GaN with the typical electron concentration and mobility of 2 × 10¹⁸ cm⁻³ and 200 cm²V⁻¹s⁻¹. The growth temperature of InGaN emitting layers was varied from 450 to 650 °C, depending on their In composition. To check their EL characteristics, Pd/Au and In are used as a p-type and an n-type contact metal, respectively.

Figures 1(a) and 1(b) show the RHEED patterns of a 1-$μ$m-thick GaN film prepared on sapphire by PSD with electron beam incidence parallel to [10$1¯$0] and [11$2¯$0], respectively. A clear (3 × 3) surface reconstruction indicated that the surface polarity of this film was N-polarity.¹⁹ An AFM surface image of the GaN (000$1¯$⁠) film in Fig. 1(c) shows atomically flat step and terrace structures even at the film thickness of $1μm$⁠, which indicates the PSD growth of N-polar GaN proceeded in the step-flow growth mode even at the initial stage of the growth. The root mean square of the surface roughness was estimated to be 3.3 nm. Step bunching was observed on the surface due to the large miscut angle of the sapphire substrate. Thus, it is necessary to optimize the sapphire miscut angle to improve its surface flatness further. Although, in Fig. 1(c), hexagonal surface pits with the density of 2 × 10⁸ cm⁻² were also observed at the step edges, no pyramidal hillock was observed unlike conventional MOCVD growth.⁶ 

To investigate the crystalline quality of GaN (000$1¯$⁠) grown on sapphire by PSD, we performed x-ray rocking curve (XRC) measurements. Figure 2(a) shows the film thickness dependence of the full-width at half maximum (FWHM) values of XRCs for 0002 and 10$1¯$2 diffraction. Both FWHM values of 0002 and 10$1¯$2 diffraction monotonically decreased with increasing film thickness. The 0002 and 10$1¯$2 FWHM values reached as narrow as 313 and 394 arcsec, respectively, at the film thickness of $6μm$ as shown in Figs. 2(b) and 2(c). These values are equal to or lower than those of GaN (000$1¯$⁠) on sapphire grown by the state-of-the-art MOCVD technique.^20,21 Such reduction in the dislocation density with increasing film thickness can be explained by the formation of a dislocation loop during the growth.

Next, we investigated the electrical properties of 1-$μm$-thick Mg-doped GaN (000$1¯$⁠) grown on sapphire by PSD. After the growth, the Mg-doped sample showed a bright and streaky RHEED pattern with a (3 × 3) surface reconstruction and no degradation in the surface morphology. As-grown Mg-doped GaN (000$1¯$⁠) films exhibited a clear p-type conductivity without the activation by high-temperature annealing. The fact that PSD gives as-grown p-type materials can be explained by the absence of hydrogen atoms in the growth ambient.¹² Figure 3(a) summarizes the relationship between the RT hole concentration and mobility of the Mg-doped samples, estimated from the Hall measurements with a van der Pauw configuration at RT. As the hole concentration increased from 8 × 10¹⁶ to 2 × 10¹⁸ cm⁻³, the mobility decreased from 9 to 2 cm²V⁻¹s⁻¹. A similar tendency was observed in a conventional p-type GaN (0001). Figure 3(b) shows the temperature dependence of the hole concentration of a Mg-doped sample with a RT hole concentration of 2 × 10¹⁷ cm⁻³. From the curve fitting based on a conventional charge neutrality condition in Fig. 3(b), the estimated effective activation energy of the Mg dopant was 161 meV, which was comparable to that of PSD grown Ga-polar GaN.^12,13 It should be noted that the background electron concentration of undoped GaN (000$1¯$⁠) grown by PSD can be reduced to 8.5 × 10¹⁶ cm⁻³, although residual donor impurities such as oxygen may be incorporated into the nitrogen polar face during growth.⁸ However, judging from the background electron concentration, the oxygen concentration of N-polar GaN may be still higher than that of Ga-polar GaN ([O] < 1 × 10¹⁶ cm⁻³) prepared by PSD under similar growth conditions.¹² 

Based on these achievements, we demonstrated the fabrication of N-polar InGaN LED structures with an In composition of their light emission layers of 0.15, 0.28, and 0.40. A cross-sectional schematic of the N-polar InGaN LED structure is shown in Fig. 4(a). The InGaN emitting layers consisted of 4-period InGaN/GaN multiple quantum wells (MQWs) for the In composition of 0.15 and 0.28 and a 10 nm-thick single InGaN layer for the In composition of 0.40. N-polar InGaN LEDs showed typical diode current–voltage characteristics with a rectifying ratio over 10⁶ at ±5 V. As shown in Fig. 4(b), these LEDs operated under forward bias showed reasonable emission spectra at the wavelengths of 423, 523, and 609 nm, respectively. The successful fabrication of such high In composition InGaN LEDs can be attributed to the use of the N-polar plane and low-temperature PSD growth technique. In fact, the In composition of N-polar InGaN films was higher than that of Ga-polar films grown under the same growth conditions. The details of this comparison will be discussed in a separate paper. Although the internal quantum efficiencies of these LEDs were an order of magnitude lower than those of Ga-polar LEDs prepared by PSD,¹³ these results clearly indicated a strong potential of N-polar nitride devices prepared by PSD.

In conclusion, we have grown device-quality GaN (000$1¯$⁠) films on sapphire by PSD. The crystalline quality of GaN (000$1¯$⁠) was improved as the film thickness increased, and the FWHM values of XRCs of 6 $μm$-thick GaN (000$1¯$⁠) for 0002 and 10$1¯$2 diffraction were as low as 313 and 394 arcsec, respectively. The surface morphology comprised an atomically flat surface with step and terrace structures. Mg-doped GaN (000$1¯$⁠) films grown by PSD showed good p-type conductivity with hole concentration and mobility of 8 × 10¹⁶–2 × 10¹⁸ cm⁻³ and 2–9 cm²V⁻¹s⁻¹, respectively. With the use of these materials, we demonstrated the operation of N-polar InGaN LEDs with a long wavelength up to 609 nm. Based on these results, the PSD growth technique is quite promising for fabricating nitrogen polar devices such as high-efficiency InGaN-based long-wavelength LEDs or solar cells.

This work was partially supported by JSPS KAKENHI Grant No. JP16H06414.