Electron cyclotron resonance etching characteristics of GaN in plasmas with and without hydrogen
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temperature. This circumstance leaves dry etching as the only practical way for reliable pattern definition of GaN. Various systems have been used to dry-etch GaN, including reactive ion etching (RIE), electron cyclotron resonance (ECR) plasma etching, chemically assisted ion beam etching (CAIBE), and magnetron reactive ion etching (MIE). To date, the highest etch rates of GaN are 3500A/min in BC13 MIE [5], 2340A/min in CIH2 /HCH4/ArECR plasma at 170 0 C [6], 2100A/min in C12/Ar CAIBE [7], and 1800A/min in HCI/Ar CAIBE at 300'C [8]. Typically, CAMBE tends to cause more damage to the material than ECR etching because of higher ion energy, and the uniformity of MIE plasma is unsatisfactory. Compared to conventional RIE, ECR plasma etching has the advantage of providing higher ion density (>5x10" cm-3) at lower pressures. These conditions yield high etch rates with large etch anisotropy. With a separately controlled RF power source that controls the ion energy independently of the ion density, ECR obtains a high ion density plasma with a relatively low ion energy that minimizes the ion induced damage to the material. The addition of hydrogen to plasmas containing chlorine has resulted in some of the highest etch rates of GaN by enhancing the removal of N as NH, [6,8]. Achieving high etch rates of GaN is very important for the fabrication of etched laser mirrors. Since GaN grown on sapphire does not have the same orientation as the substrate, cleaved mirrors cannot be expected. Thus, to make diode lasers other than vertical cavity surface emitting lasers (VCSELs), a process for dry-etched mirrors has to be developed. Hydrogen incorporated into semiconductors, however, passivates both donor and acceptor shallow levels causing lower free carrier concentrations [9,10]. For III-V semiconductor power763 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
FET processing, hydrogen compensation during the gate recess would be very undesirable because this step is critical in determining a transistor's performance. Nakamura has shown that N2-ambient thermal annealing can eliminate donor/acceptor-hydrogen complexes in GaN at an optimum temperature of 700'C and restore conductivity. Our own studies of Ti/A1 contacts to undoped GaN grown by metal organic chemical vapor deposition (MOCVD) indicate that the optimum anneal temperature for driving out the hydrogen and maximizing the conductivity is about 800'C. Nakamura has shown, however, that the annealing process causes dissociation of GaN at the surface [11]. This dissociation can have a variety of deleterious effects on device characteristics and processes, e.g., it can cause lower surface carrier concentration which would make low resistance ohmic contacts more difficult to fabricate. In the gate-drain region of FETs, surface dissociation can increase the leakage current and lower the breakdown voltage due to the increased defect density at the surface. The first part of this work investigates a dry-etch process that potentially leaves less damage than CAMBE,
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