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 cm(2) 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.
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 10 8 cm −2 . It increased from 220 to 950 nm with decreasing Mg doping concentration from 3 ϫ 10 19 to 4 ϫ 10 18 cm −3 . For relatively high dislocation density above 10 9 cm −2 , it was less than 300 nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250 nm 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.
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.
Although significant progress has been achieved in the GaN-based high-power/high-frequency electronic devices such as AlGaN/GaN heterostructure field effect transistors (HFETs) [1][2][3], it is necessary to use thinner AlGaN layer for achieving higher transconductance (g m ) and more precise control of threshold voltage in HFETs. To develop normally-off (enhancement) mode devices which are attractive for gaining in flexibility of circuit and/or system design, in addition, very thin AlGaN barrier thickness less than 10nm is required. A gate-recessing process is one of the actual approaches to reduce the effective thickness of a barrier layer.However, Schottky contacts fabricated on GaN and AlGaN still suffer from serious leakage problems [4][5][6][7][8][9]. Although some models associated with the trap-assisted tunneling [10], the defect-related thin surface barrier (TSB) [8] and the dislocation-related hopping transport [11] have been proposed, the leakage mechanism through GaN and AlGaN Schottky interfaces has not yet been clarified, and thereby there is still no solution to suppress leakage currents. For Schottky-gate (SG) structures on AlGaN/GaN HEMTs with thinner AlGaN barrier layers or recessed-gate structures, leakage problems can be enhanced, making the gate control of drain current very difficult.An FET device having an insulated gate (IG) structure is expected to suppress the gate leakage. Moreover, an insulator film can act as a passivation layer, making the surface more stable in the device. An Al 2 O 3 IG structure is very attractive for the application to AlGaN/GaN HFETs [12][13][14], since it has relatively high dielectric constant (~ 9) and a large conduction-band offset at the Al 2 O 3 /AlGaN interface [12]. In fact, the Al 2 O 3 IG AlGaN/GaN HFETs exhibited good gate control of drain current with low leakage currents, and suppressed current collapse under both drain stress and gate stress [12].In this letter, we demonstrate the controllability of an Al 2 O 3 insulated-gate structure in the AlGaN/GaN HFETs having a thin AlGaN barrier layer (7 nm).The Al 0.2 Ga 0.8 N/GaN heterostructures was grown by metal organic vapor phase epitaxy on n-type 6H-SiC substrates, as schematically shown in Fig.1. A very thin AlGaN layer (7 nm) was grown with doping of Si (2 x 10 18 cm -3 ). The electron concentration and mobility of the sample at room temperature (RT) were 4.0 x 10 12 cm -2 and 830 cm 2 /Vs, respectively. The device isolation was carried out by an electron-cyclotron-resonance (ECR) assisted reactive ion beam etching using a gas system consisting of CH 4 , H 2 , Ar and N 2 [15]. As an ohmic contact, a Ti/Al/Ti/Au layered structure was formed on the surface of AlGaN/GaN followed by annealing at 800 o C for 1 min in N 2 ambient. A Ni/Au contact was used as a Schottky gate.The Al 2 O 3 -based surface passivation structure was fabricated through the following in-situ steps [16]: The AlGaN surface was treated in ECR-N 2 plasma at 280 o C for 30 s. Then an Al layer with a nominal thickness of 3 nm was depo...
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