Kazuhide Kumakura

Layered boron nitride as a release layer for mechanical transfer of GaN-based devices

Nitride semiconductors are the materials of choice for a variety of device applications, notably optoelectronics and high-frequency/high-power electronics. One important practical goal is to realize such devices on large, flexible and affordable substrates, on which direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques—such as laser lift-off—have been investigated to enable the transfer of nitride devices from one substrate to another, but existing methods still have some important disadvantages. Here we demonstrate that hexagonal boron nitride (h-BN) can form a release layer that enables the mechanical transfer of gallium nitride (GaN)-based device structures onto foreign substrates. The h-BN layer serves two purposes: it acts as a buffer layer for the growth of high-quality GaN-based semiconductors, and provides a shear plane that makes it straightforward to release the resulting devices. We illustrate the potential versatility of this approach by using h-BN-buffered sapphire substrates to grow an AlGaN/GaN heterostructure with electron mobility of 1,100 cm2 V−1 s−1, an InGaN/GaN multiple-quantum-well structure, and a multiple-quantum-well light-emitting diode. These device structures, ranging in area from five millimetres square to two centimetres square, are then mechanically released from the sapphire substrates and successfully transferred onto other substrates.

Minority carrier diffusion length in GaN: Dislocation density and doping concentration dependence

We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p–n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108cm−2. It increased from 220to950nm with decreasing Mg doping concentration from 3×1019to4×1018cm−3. For relatively high dislocation density above 109cm−2, it was less than 300nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length.

Mg-acceptor activation mechanism and transport characteristics in p-type InGaN grown by metalorganic vapor phase epitaxy

The Mg-acceptor activation mechanism and transport characteristics in a Mg-doped InGaN layer grown by metalorganic vapor phase epitaxy are systematically investigated through their structural, optical, and electrical properties. The In mole fraction was from 0 to 0.13, and the Mg concentration varied from 1×1019 to 1×1020 cm−3. X-ray rocking curves for Mg-doped InGaN layers indicate that the structural quality is comparable to that of undoped and Si-doped InGaN layers. Their photoluminescence spectra show emissions related to deep donors emerged at lower energy when Mg doping concentrations are above 2−3×1019 cm−3. The electrical properties also support the existence of these deep donors in the same Mg concentration range because the hole concentration starts to decrease at around the Mg concentration of 2−3×1019 cm−3. These results indicate that self-compensation occurs in Mg-doped InGaN at higher-doping levels. The temperature dependence of the hole concentration in Mg-doped InGaN indicates that the acceptor activation energy decreases with increasing In mole fraction. This is the reason the hole concentration in Mg-doped InGaN is higher than that in Mg-doped GaN at room temperature. In addition, the compensation ratio increases with doping concentration, which is consistent with the deep donor observed in PL spectra. For Mg-doped InGaN, impurity band conduction is dominant in carrier transport up to a relatively higher temperature than that for Mg-doped GaN, since the acceptor concentration for Mg-doped InGaN is higher than that of Mg-doped GaN.

Activation Energy and Electrical Activity of Mg in Mg-Doped InxGa1-xN (x<0.2)

We investigated the electrical properties of Mg-doped InGaN with an In mole fraction of less than 0.2 grown by metalorganic vapor phase epitaxy. We obtained p-type InGaN with a hole concentration above 1018 cm-3 at room temperature. The hole concentrations of Mg-doped In0.04Ga0.96N and In0.14Ga0.86N were 1.2×1018 and 6.7×1018 cm-3, respectively, while that of Mg-doped GaN was 3.0×1017 cm-3 with the same Mg doping concentration. The activation energy of Mg in InGaN, calculated from the temperature dependence of the hole concentration, decreases with the increase in the In mole fraction. Furthermore, the electrical activity of Mg in InGaN increases with the In mole fraction. As a result, higher hole concentrations were obtained at room temperature for Mg-doped InxGa1-xN (x<0.2) with higher In mole fractions.

High current gains obtained by InGaN/GaN double heterojunction bipolar transistors with p-InGaN base

InGaN/GaN double heterojunction bipolar transistors have been fabricated using p-type InGaN as a base layer. The structures were grown on SiC substrates by metalorganic vapor phase expitaxy and defined by electron cyclotron resonance plasma etching. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1×1019 cm−3 corresponding to a hole concentration of 5×1018 cm−3 at room temperature. From their common-emitter current–voltage characteristics, the maximum current gain of 20 was obtained at room temperature.

High-power characteristics of GaN/InGaN double heterojunction bipolar transistors

High-power characteristics have been investigated for GaN/InGaN double heterojunction bipolar transistors (HBTs) on SiC substrates. A base-collector diode showed a high breakdown voltage exceeding 50 V, which is ascribed to a wide band gap of a GaN collector. The maximum collector current is proportional to the emitter size in the emitter-size ranging from 1.5×10−5 to 1.4×10−4 cm2. The corresponding maximum collector current density is as high as 6.7 kA/cm2, indicating the high current density characteristics of bipolar transistors. A 50 μm×30 μm device operated up to a collector–emitter voltage of 50 V and a collector current of 80 mA in its common-emitter current–voltage characteristics at room temperature. The corresponding power density is as high as 270 kW/cm2, showing that nitride HBTs are promising for high-power electronic devices in terms of both the material and the device structure.

Al2O3 insulated-gate structure for AlGaN/GaN heterostructure field effect transistors having thin AlGaN barrier layers

An Al2O3 insulated-gate (IG) structure was utilized for controlling the surface potential and suppressing the gate leakage in Al0.2Ga0.8N/GaN heterostructure field effect transistors (HFETs) having thin AlGaN barrier layers (less than 10 nm). In comparison with the Schottky-gate devices, the Al2O3 IG device showed successful gate control of drain current up to VGS = +4 V without leakage problems. The threshold voltage in the Al2O3 IG HFET was about -0.3 V, resulting in the quasi-normally-off mode operation.

Low-resistance nonalloyed ohmic contact to p-type GaN using strained InGaN contact layer

A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1×10−6 Ω cm2. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN.

Increased Electrical Activity of Mg-Acceptors in AlxGa1-xN/GaN Superlattices

We grew uniformly Mg-doped AlxGa1-xN/GaN superlattices (SLs) by low-pressure metalorganic vapor phase epitaxy and investigated the electrical properties of these SLs parallel to the SL plane. Sheet hole concentration depends strongly on the SL period thickness and Al mole fraction, and the maximum sheet concentration is 8×1012 cm-2 for AlxGa1-xN/GaN (240 A/120 A) SLs in the Al mole fraction range between 0.15 and 0.3, which corresponds to the hole concentration of 3×1018 cm-3. One possible explanation for this high sheet hole concentration is that the strain-induced piezoelectric field greatly increases the electrical activity of the relatively deep Mg-acceptors in the SLs.

High hole concentrations in Mg-doped InGaN grown by MOVPE

We investigated the electrical properties of Mg-doped In x Ga 1 x N (0 ≤ x < 0.25) grown by metalorganic vapor-phase epitaxy with various growth conditions, such as Mg-doping concentration, growth-rate and growth temperature. The hole concentration depends on the growth-rate, the In mole fraction and the crystal quality of the InGaN layers. The hole concentration of Mg-doped In x Ga 1-x N layers below x = 0.15 increased with the In mole fraction, while those above x = 0.15 decreased. We realized the p-type InGaN with the room-temperature hole concentration above 10 18 cm -3 and obtained the maximum hole concentration of 7.8 x 10 18 cm -3 for x x = 0.2 by optimizing the growth conditions.