III-Nitrides (AlGaInN compounds) have direct bandgap ranging from ultraviolet (6.2 eV) to infrared (0.7 eV). Thanks to robustness and non-toxicity, AlGaInN materials form the semiconductor solution to many devices applicable in military, astronomy and medicine. Some of AlGaInN-based devices are ultraviolet detectors (avalanche photodetectors and single photon detectors), and ultraviolet and visible light emitters (blue/green light emitting diodes/laser diodes) with applications in non-line-of-sight communications, space-to-space communications, biological agent detection/elimination, blue-ray technology, solid-state lighting and hand-held projectors/displays. However, the unavailability of an affordable lattice-matched substrate have been the main bottleneck in the development of these devices. Novel MOCVD techniques such as lateral epitaxial overgrowth have been employed to decrease the dislocation densities, and unleash the full performance of nitride-based devices.

Metalorganic chemical vapor deposition (MOCVD) is a non-thermal equilibrium deposition technique that employs metalorganic (MO) compounds reacting in a chamber. Depending on the material grown, parameters such as reactor pressure, V/III ratio, MO flow, growth temperature and other reactor parameters should be optimized for perfect material quality.

The lack of affordable lattice-matched substrate forces growth on highly lattice mismatched substrate such as sapphire, silicon carbide and silicon. The large lattice mismatch between substrate results in dislocation generation as a means of strain relief mechanism. Thus, given a device design, the growth is vital so as to maximize the device performance.

P-type material is a key in nitride optoelectronics as p-(i)-n structures are common in both detectors and emitters, due to their higher performance than schottky diodes. A common acceptor is magnesium (Mg), however, it has an activation energy of ~200 meV in GaN, and even larger one in AlGaN. This requires incorporation of around two orders of magnitude more Mg into (Al)GaN than the required carrier concentration. Besides, high Mg concentration brings out compensation effects inhibiting p-type carrier concentration. We have realized delta-doping technique to minimize compensation effects and maximize the hole concentration.

In order to optimize the hole concentration and material quality of the -doped p-GaN, first the Mg flow time is varied from 15 to 60 s. The effect of Mg flow time on hole concentration, resistivity, and mobility is shown in Fig. 1(a) for annealed samples. The high hole concentration achieved by -doping suggests successful depassivation of the Mg–H complexes and reduction in self-compensating effects after annealing. Keeping the GaN period at 10 nm, with increasing Mg flow time, hole concentration increases and mobility decreases. The higher hole concentration compensates the mobility decrease leading to a reduction in resistivity with increasing Mg flow time. For comparison, conventional p-GaN is also grown using the same growth conditions by supplying TMGa and DcpMg together. The results of the conventional doping are indicated as dashed-levels at each axis (Fig.1). Comparing the -doping to the conventional doping, around two orders of magnitude higher doping is achieved under the same growth conditions, with four times lower resistivity.

Figure 1: (a) Effect of the Mg flow time on hole concentration, resistivity (), and mobility (µ). (b) Effect of Mg flow time and annealing on (002) (omega/2theta) XRD FWHM. GaN period is 10 nm for both graphs. The results of the conventional doping are indicated as dashed-levels at each axis

In order to assess the structural properties, open detector omega/2theta (002) XRD scans are performed before and after annealing. The XRD FWHM is plotted with respect to the Mg flow time in Fig. 1(b). An increase in the FWHM in Fig. 1(b) suggests increase in crystal imperfections. Increasing nonsubstitutional Mg incorporation, heterogeneous strain or threading dislocations may contribute to broaden the (002) reflection as Mg flow increases. Similar to conventionally doped p-GaN, the annealing decreases the FWHM of -doped p-GaN for the highest hole concentrations, as seen in Fig. 1(b).

P-type doping in the near 10¹⁸ cm⁻³ range using a -doping technique is achieved by systematically optimizing the growth conditions while preserving the material quality. Mg flow time is shown to increase the hole concentration. The GaN period is determined to be crucial for successful Mg–H depassivation and lowering self-compensation effects via annealing. Successful annealing results in a smaller XRD FWHM and a RT Mg-level luminescence at 3.29 eV.

The doping enhancement in the case of -doped p-GaN is due to (1) the decrease in Mg activation energy due to dense packing of Mg dopants and (2) lower self-compensation effects. This technique could be particularly beneficial for p-AlGaN where low pressures are required for avoiding parasitic reactions and where high temperatures are required for high quality.

Growth of III-Nitride materials are challenging due to inherit lattice-mismatch and lack of affordable lattice-matched substrates. In addition, significant material differences between AlN, GaN and InN results comprimises between layer qualities and device performance. The quality of the individual layers are essential as well as overall device performance. Use of nanostructures and device-oriented approach is the one way to ensure high performance devices.