Native AlN Research Articles

Efficient diameter enlargement of bulk AlN single crystals with high structural quality

We present the growth of bulk AlN crystals by physical vapor transport and the structural characterization by various X-ray techniques and defect-selective etching. Starting from native AlN seeds with 8 mm in diameter we show a fast increase of the crystal diameters with expansion angles of about 45°. Only two subsequent grown seeded crystals are required to reach crystals up to 34 mm in diameter. The threading dislocation density is below 104 cm−2. The process outlines a shortcut path to industrially relevant AlN crystal diameters compared to all other published expansion processes for bulk AlN crystals so far.

Step pinning and hillock formation in (Al,Ga)N films on native AlN substrates

The influence of edge-type threading dislocations (TDs) on the epitaxial growth of AlGaN on native AlN substrates was investigated theoretically and experimentally. In the step flow growth regime, we find that pure edge-type TDs cause a pinning of surface steps, resulting in curved step segments. Theoretical calculations reveal that this pinning mechanism is solely mediated by the altered surface potential due the strain field imposed by the TD. Within the curved step segment, the step width is subject to changes resulting in an altered Ga/Al incorporation rate. According to the density functional theory calculation, this effect is related to the different surface diffusion length of Ga and Al and represents a further destabilization mechanism during step flow growth. Another consequence of surface step pinning is the occurrence of areas where the step width is increased. These areas serve as precursors for 2D nucleation due to an increased adatom density. Once nucleated, these nuclei grow along the c-direction via continuous 2D nucleation, while lateral expansion occurs due to adatom incorporation on the m-facets.

Record >10 MV/cm mesa breakdown fields in Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors on native AlN substrates

The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3 μm separation possess a record-high breakdown field strength of ∼11.5 MV/cm for an undoped Al0.6Ga0.4N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9 μm are 850 and 1500 V, respectively.

Influence of the Growth Substrate on the Internal Quantum Efficiency of AlGaN/AlN Multiple Quantum Wells Governed by Carrier Localization

The influence of the growth substrate on the internal quantum efficiency (IQE) of deep ultraviolet light‐emitting diodes is studied. Two nominally identical Al‐rich AlGaN/AlN multi‐quantum‐well (MQW) structures grown by metal–organic vapor phase epitaxy (MOVPE) on different substrates are investigated. The first MQW structure is grown on a native AlN substrate, whereas the second one is deposited on an AlN template on sapphire. By the combination of atomic force microscopy (AFM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy, it is demonstrated that the dislocation‐mediated spiral growth of MQWs on sapphire results in the more efficient localization of carriers. This effect helps to prevent nonradiative carrier recombination at point defects, improving the IQE of the structure.

Effects of Native AlN Particles on Heterogeneous Nucleation in an Al-3Fe Alloy

The naturally formed inclusions in an Al-3Fe alloy were investigated using a pressurized melt filtration technique. Naturally formed AlN particles with a rodlike morphology and a size distribution between 200 nm and 4 µm coexist with the native Al2O3 particles in the Al-3Fe alloy. These native AlN particles were investigated with scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). A well-defined orientation relationship (OR) between AlN and α-Al was found from the AlN particle embedded in α-Al. A tilt angle of 5.8 deg was observed between {111}α-Al and {0001}AlN at the α-Al/AlN interface. The atomic matching for the α-Al/AlN interface was investigated by considering interfacial segregation on the AlN particles. The heterogeneous nucleation potency of the native AlN particles was investigated. Although these native AlN particles can nucleate the α-Al, the contribution is very small. This contribution reveals the heterogeneous nucleation of α-Al on the native AlN particles and the competitive relationship between the native Al2O3 and AlN as the nucleation substrates. The surface modification of the native AlN in Al alloys containing multiple alloy elements and the corresponding effects on the heterogeneous nucleation were preliminarily investigated.

Optically pumped room temperature low threshold deep UV lasers grown on native AlN substrates

We report here an optically pumped deep UV edge emitting laser with AlGaN multiple quantum wells (MQWs) active region grown on AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a high-temperature reactor. The 21 period Al_0.53Ga_0.47N/Al_0.7Ga_0.3N MQWs laser structure was optically pumped using 193 nm deep UV excimer laser source. A laser peak was achieved from the cleaved facets at 280.3 nm with linewidth of 0.08 nm at room temperature with threshold power density of 320 kW/cm². The emission is completely TE polarized and the side mode suppression ratio (SMSR) is measured to be around 14 dB at 450 kW/cm².

The role of transient surface morphology on composition control in AlGaN layers and wells

The mechanisms governing “compositional pulling” during the growth of AlxGa1−xN wells are investigated. Gallium-rich AlxGa1−xN wells grown on high dislocation density AlN/sapphire templates exhibit asymmetric and diffuse composition profiles, while those grown on low dislocation density native AlN substrates do not. Furthermore, strain in all AlxGa1−xN wells is found to be pseudomorphic, ruling it out as the dominating driving force. Rather, the high threading dislocation density of the AlN template is considered to play the defining role. We propose that a transient surface morphology is introduced during dislocation mediated spiral growth, which, in conjunction with process supersaturation, determines the Ga incorporation. These findings provide insights into compositional pulling in high Ga content AlxGa1−xN grown on AlN and provide a route to grow thicker wells with very abrupt interfaces on native AlN substrates.

A conduction model for contacts to Si-doped AlGaN grown on sapphire and single-crystalline AlN

Ohmic contacts to AlGaN grown on sapphire substrates have been previously demonstrated for various compositions of AlGaN, but contacts to AlGaN grown on native AlN substrates are more difficult to obtain. In this paper, a model is developed that describes current flow through contacts to Si-doped AlGaN. This model treats the current through reverse-biased Schottky barriers as a consequence of two different tunneling-dependent conduction mechanisms in parallel, i.e., Fowler-Nordheim emission and defect-assisted Frenkel-Poole emission. At low bias, the defect-assisted tunneling dominates, but as the potential across the depletion region increases, tunneling begins to occur without the assistance of defects, and the Fowler-Nordheim emission becomes the dominant conduction mechanism. Transfer length method measurements and temperature-dependent current-voltage (I-V) measurements of Ti/Al-based contacts to Si-doped AlGaN grown on sapphire and AlN substrates support this model. Defect-assisted tunneling plays a much larger role in the contacts to AlGaN on sapphire, resulting in nearly linear I-V characteristics. In contrast, contacts to AlGaN on AlN show limited defect-assisted tunneling appear to be only semi-Ohmic.

Stimulated emission at 257 nm from optically-pumped AlGaN/AlN heterostructure on AlN substrate

Optically pumped deep-ultraviolet lasers operating at room temperature are demonstrated from heterostructures consisting of AlxGa1−xN/AlN grown on (0001) AlN substrate. The substantial reduction of the threading dislocation density by using a native AlN substrate over sapphire substrates is critical to the realization of the photo-pumped lasers. The threshold power density was 1.88 MW cm−2. The layers below the active region were Si-doped and had bottom waveguide and cladding layer n-type structures required for diode lasers, thus demonstrating the feasibility of deep UV lasing for the proposed structures on AlN substrates by current injection, provided that effective hole injection layers are included.

Performance and reliability of ultraviolet‐C pseudomorphic light emitting diodes on bulk AlN substrates

AbstractLow dislocation density, epitaxial layers of AlxGa1–xN are grown pseudomorphically on native AlN substrates prepared from high quality, bulk crystals. In addition to low dislocation density, they are atomically smooth and can be doped n‐type to obtain sheet resistances < 200 Ohms/sq/μm. These layers are used as templates for the growth of high quality multiple quantum well (MQW) structures and ultraviolet (UV) light emitting diodes (LED). The pseudomorphic growth and atomically smooth surfaces are achieved for a full device structure. Previously we have demonstrated the high optical quality of these structures through temperature dependant photoluminescence measurements and X‐ray diffraction. Recently full device structures have been fabricated and packaged. These devices, with emission wavelength between 250 nm and 280 nm show linear increase in output power as a function of current up to 150 mA in thermal equilibrium. Additionally output powers up to 1.3 mW have been achieved at 258 nm at 400 mA drive current. Reliability of the devices has been measured for 1000 hours with expected lifetimes greater than 3000 hours for input currents up to 150 mA (with an average current density of approximately 150 A/cm2). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modulated precursor flow epitaxial growth of AlN layers on native AlN substrates by metal-organic chemical vapor deposition

We have investigated the growth of AlN layers on bulk AlN substrates by modulated precursor flow epitaxial growth using metal-organic and hydride precursors as well as by conventional metal-organic chemical vapor deposition growth. A nanopit-containing morphology was observed for an AlN layer grown by conventional growth, while atomically smooth and pit-free surface was achieved for an AlN layer created by modulated precursor flow epitaxial growth. For similar growth set-point temperatures, nanopit-free surfaces were observed for AlN layers created by conventional growth on sapphire substrates. This is believed to be due to the difference in the temperature of the growing surface, as evidenced by finite element method thermal profiling and the morphology change of the AlN layer with decreasing temperature observed for growth of AlN on sapphire substrates. The AlN layers grown by modulated precursor flow epitaxial growth on the AlN bulk substrates also have excellent crystalline qualities with narrow x-ray rocking curve peak linewidths of 36 and 61arcsec for the (002) and (102) diffraction conditions, respectively.

The effect of BCl3 pretreatment on the etching of AlN in Cl2-based plasma

AbstractThe effect of BCl3 and BCl3/Ar pretreatment on Cl2/Ar and Cl2/Ar/BCl3 dry etching of AlN is investigated using inductively coupled plasma reactive ion etching. The native AlN oxide can be effectively removed by a short exposure to BCl3 or BCl3/Ar plasma. Compared to the chlorine based plasma etching, BCl3/Ar is found to have the highest etch rate for both AlN and its native oxide. Following removal of the native oxide, Cl2/Ar/BCl3 plasma etching with 15% BCl3 fraction results in a high etch rate ˜ 87 nm/min and modest increases in the surface roughness.

Bandgap engineering of electronic and optoelectronic devices on native AlN and GaN substrates: A modelling insight

Native GaN and AlN substrates cut out of single-crystal boules provide threading dislocations in on-grown III-nitride materials with the density much lower than that observed in structures on conventional sapphire or SiC wafers. The native substrates are also expected to enable an easy choice of growth surface orientation that controls the crystal polarity and, hence, the distribution of polarization charges in a nitride heterostructure. The impact of these factors on the bandgap engineering of advanced electronic and optoelectronic devices is discussed in this paper in terms of numerical simulation with the focus on high-electron mobility transistors and light-emitting diodes.